Potatoes:Diagnostics

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POTATO PESTS AND NUTRITIONAL DISORDERS


Contents

AIR POLLUTION PHOTOCHEMICAL

‘‘‘Air Pollution Photochemical: ‘‘‘ Potato injury by photochemical oxidants such as ozone, probably peroxyacetyl nitrate, and related compounds has recently been recognized. Upper leaf surfaces are stippled by darkly pigmented spots, sometimes with yellowing and often with a bronzed appearance. Lower leaf surfaces may be light in color occasionally with a glazed or silvery sheen. Except for severe reduction in yield associated with very early senescence, tuber symptoms have not been reported. Symptoms become evident within 24 hours following heavy exposure, but symptoms of advanced drying and yellowing may require 10-14 days. Palisade mesophyll cells are first affected, becoming water-soaked and later dry out. Photochemical oxidants accumulate when relatively large areas of high atmospheric pressure are present or when air masses stagnate under a layer of warm air over cool land surfaces. Field exposures of approximately 0.15 ppm ozone for a day or two are usually sufficient to injure exposed foliage.


AIR POLLUTION SULFUR OXIDES

‘‘‘Air Pollution Sulfur Oxides: ‘‘‘ Although potato leaves are relatively resistant to injury by sulfur oxides, they respond with interveinal dry areas that are light tan to white, and yields may be reduced.


ALFALFA MOSAIC (AMV)

‘‘‘Alfalfa Mosaic (AMV) ‘‘‘ Alfalfa mosaic virus is found worldwide but is generally considered of little economic importance in potatoes. The disease may induce predominantly calico symptoms on foliage or dry symptoms in tubers. Tuber drying, usually visible by harvest, begins just beneath the epidermis at the stolon attachment and later spreads throughout the tuber, leaving scattered dry, corky areas. Phaseolus vulgaris and Vigna sinensis generally show local lesions and/or systemic infection and serve as diagnostic and assay plants. Vicia faba and Pisum sativum exhibit black, dry, local lesions, stem drying and plants die. In Chenopodium amaranticolor and C. quinoa, local lesions and systemic yellow and dry flecks are produced. Nicotiana tabacum is a good propagation species, with dry or yellow local lesions and systemic spotting.


ALTERNARIA ALTERNATA (ALTERNARIA ALTERNATA)

‘‘‘Alternaria alternata (Alternaria alternata) ‘‘‘ Alternaria alternata infects potato and other solanaceous crops, forming lesions on potato leaves similar to those of early blight.


ALUMINUM

‘‘‘Aluminum: ‘‘‘ Aluminum toxicity causes roots to become short and stubby with few branches. Aluminum solubility is often high in soils below pH 5.0.


ANDEAN POTATO LATENT (APLV)

‘‘‘Andean Potato Latent (APLV) ‘‘‘ Andean potato latent virus is common throughout the Andean region at 2,000-4,000 meters. Primary infection is often symptomless but may cause mosaics and/or yellow netting of minor leaf veins. The virus resembles members of the turnip yellow mosaic group and is therefore considered a strain of eggplant mosaic virus. Andean potato latent virus produces symptoms and multiplies best under cool conditions, found occurring naturally only in potato. In some varieties, mosaic and characteristic netting of minor leaf veins develops with all isolates, and most isolates also cause faint to distinct local lesions in inoculated leaves. Other varieties exhibit faint dry or yellow spots or rings on inoculated leaves with most isolates.


ANDEAN POTATO MOTTLE (APMV)

‘‘‘Andean Potato Mottle (APMV) ‘‘‘ The Andean potato mottle virus is present in Peru, Bolivia, and probably throughout the Andean region at elevations of 2,000-4,000 meters. Primary symptoms are usually mild patchy spots and, in sensitive cultivars, strong spotting, leaf deformation, systemic drying, and/or stunting. It produces symptoms and multiplies best under cool conditions and can be transmitted mechanically only to solanaceous hosts.


APHIDS

‘‘‘Aphids: ‘‘‘ Several potato virus diseases are transmitted by aphids, and identification of the vector involved is often necessary. Aphid species differ not only in morphology and ability to transmit potato viruses but also in form, life cycle, and behavior, depending on the environment (temperature, relative humidity, photoperiod, and host plant condition) to which the aphid or its mother is exposed. Where winter conditions are severe, most potato-infesting aphids overwinter as sexually produced eggs laid on the rough bark of a woody host or on the crown and leaves of an herbaceous biennial or perennial plant. In mild climates, sexual aphids and eggs do not occur, and parthenogenetic, viviparous females are produced throughout the year.


ASTER YELLOWS & STOLBUR

‘‘‘Aster Yellows & Stolbur: ‘‘‘ Aster yellows and its allied diseases occur worldwide. Upper leaflets roll and develop purple or yellow pigmentation. At harvest, an affected hill usually has some normally mature and some immature tubers. When the causal agents survive storage in seed tubers, plants in the second year develop symptoms. Mycoplasma-like organisms occur in phloem sieve cells and occasionally in phloem parenchyma cells of infected plants. The principal leafhopper vector of aster yellows overwinters on weeds, grasses, and small grains. All pathogens are transmitted by and propagative in leafhoppers.


BLACK DOT (COLLETOTRICHUM ATRAMENTARIUM)

‘‘‘Black dot (Colletotrichum atramentarium) ‘‘‘ The pathogen is common in many areas of the world, but the relative importance of the disease has not been well documented. Black dot describes abundant, dotlike, black sclerotia on tubers, stolons, roots, and stems above and below ground. Severe rotting of belowground plant parts and early death of the plant cause reduction in tuber size. It appears on a variety of media, including potato-dextrose agar, as a white, superficial mycelium. Mycelium and sclerotia are commonly associated with cortical and vascular tissue below the ground and at the base of the aboveground stem within several centimeters of the soil line. Overwintering is by sclerotia on the surface of tubers or in plant debris in the field. In addition to potato, the fungus occurs on tomato and other plants in the Solanaceae (eggplant, pepper, tomato), on weed hosts such as Physalis peruviana, and, with inoculum increase, on Datura stramonium, a common weed.


BLACKHEART

‘‘‘Blackheart: ‘‘‘ Blackheart results from inadequate oxygen supply for respiration of internal tuber tissue. Blackheart symptoms consist of black to blue-black discoloration in irregular patterns in the central portion of the tuber.


BLACKLEG BACTERIAL SOFT ROT (ERWINIA CAROTOVORA)

‘‘‘Blackleg Bacterial Soft Rot (Erwinia carotovora) ‘‘‘ Blackleg affects stems and may produce soft rot in tubers. Stems, petioles, and leaves may also become infected through wounds such as petiole scars, hail, or wind damage. Tubers produced by infected plants may show symptoms ranging from slight vascular discoloration at the stolon end to soft rot of the entire tuber. Soft rotted tissues are wet, cream to tan, with a soft, slightly granular consistency. The disease is easily cultured and produces deep pits, or craters, on selective media that contain polypectate and forms acid from maltose. Bacteria may move for some distance in the soil water and contaminate developing daughter tubers of adjacent plants. Erwinia cells released into the soil survive for varying periods of time, depending upon soil temperature and, to a lesser extent, soil moisture. Tubers produced under warm (23-25 °C or higher), dry conditions are less likely to be contaminated because the pathogens are less likely to survive, and they spread through the soil for shorter distances than when the soil is cool and moist. Cool wet soils at planting time followed by high temperatures after plant emergence favor postemergence blackleg expression and higher soil temperatures favor seed piece decay and preemergence death of shoots. Invasion of seed pieces by Fusarium species may predispose tissues to soft rot and favor blackleg development. Anaerobic conditions resulting from poor aeration, flooding of soil, or the presence of a water film on tubers after washing favor disease development.


BLACKSPOT

‘‘‘Blackspot: ‘‘‘ Blackspot is always caused by bruising injury, either from impact during harvest, handling, and grading, or from pressure during storage. Blue-gray to black discolored areas develop just beneath the tuber skin. Tubers with low turgor pressure are more likely to have severe blackspot. Mature tubers are more susceptible than immature tubers, and the stolon end is more susceptible than the apical end. Tubers harvested from soils deficient in potassium tend to be more susceptible to bruising and blackspot development. Nitrogen fertilization, ethylene concentrations, and soil carbon dioxide levels have been reported to affect blackspot susceptibility in some growing areas.


BORON

‘‘‘Boron: ‘‘‘ In plants with boron deficiency, growing points die, lateral buds become active, internodes are shortened, leaves thicken and roll upward, and the plant assumes a bushy appearance. Applications of boron should be made cautiously because it is toxic to potatoes in relatively small amounts and deficiency is rare.


BROWN ROT (PSEUDOMONAS SOLANACEARUM)

‘‘‘Brown Rot (Pseudomonas solanacearum) ‘‘‘ Brown rot, also known as bacterial wilt or southern bacterial wilt, affects potatoes in almost every region in the world. Field symptoms are wilting, stunting, and yellowing of the foliage. Underground stems, stolons, and roots of plants with initial foliage symptoms show few advanced symptoms of infection. When tubers are cut in half and light pressure is applied, grayish white droplets of bacterial slime ooze out of the vascular ring. In tropical and semitropical regions (southeastern Asia, Central and South America, and parts of Africa and Australia) the pathogen can be borne by tubers and quarantines exist against importation of seed potatoes in some areas. Temperature plays an important role in the geographic distribution of the organism, which is rare where mean soil temperatures are below 15 °C. The disease occurs in soil types ranging from sandy to heavy clay and over a wide range of soil pH. Important economic hosts of the disease include tobacco, tomato, pepper, eggplant, peanut, banana, and a number of ornamentals and weeds. At least three dominant and independent genes control resistance in potato to certain strains of bacterial wilt. Immunity or high levels of resistance have not been identified.


CALCIUM

‘‘‘Calcium: ‘‘‘ Calcium deficient plants are spindly, with small, upward rolling, crinkled leaflets having yellow margins that later become dry. Tubers on calcium deficient plants develop diffuse brown drying in the vascular ring near stolon attachments, and later similar flecks form in the pith. Seed tubers in the soil remain hard and produce relatively normal roots. Symptoms are most severe on sandy soils below pH 5.0, where symptoms of manganese or aluminum toxicity may also be present. Transfer of calcium from old leaves to young leaves and from the top of the plant to the tubers is limited.


CERCOSPORA LEAF BLOTCHES (MYCOVELLOSIELLA CONCORS)

‘‘‘Cercospora Leaf Blotches (Mycovellosiella concors) ‘‘‘ The disease is reported from cool and temperate climates of Europe and Russia and from the eastern part of the United States, where it is not considered an important disease. First symptoms on lower leaves are small yellow to purplish lesions that increase from 0.2 to 1 cm in size. Cercospora has dark spores formed on densely branched sporophores that emerge through stomata.


CHARCOAL ROT (MACROPHOMINA PHASEOLI)

‘‘‘Charcoal Rot (Macrophomina phaseoli) ‘‘‘ The fungus is worldwide but economically important only in warm regions where soil temperatures exceed 28 °C. Under hot conditions, the pathogen can attack potato stems and cause a sudden wilt and yellowing. The fungus maintains itself saprophytically on unthrifty or senescent plant parts and survives unfavorable periods as microsclerotia. Tubers are predisposed to infection at temperatures of 32 °C or higher. The fungus has been found on underground parts of an extremely wide range of plants, both cultivated and wild.


CHEMICAL INJURY

‘‘‘Chemical Injury: ‘‘‘ A wide range of chemicals accidentally or improperly applied can cause divergent symptoms on foliage and in tubers, with severity depending upon the nature of the chemical, its dosage, environmental factors, and plant maturity and variety. Growth-regulating herbicides for weed control in potatoes or herbicides airborne from nearby areas may cause leaf distortion superficially suggesting virus infection. In storage, netting of tuber surfaces and dehydration have followed foliage application of maleic hydrazide and abnormal sprouting has been associated with other compounds. Improper application of fertilizer to foliage or application too close to the seed piece in the soil causes foliage or seed tuber drying, followed by decay, poor stands, and low plant vigor.


CHOANEPHORA BLIGHT (CHOANEPHORA CUCURBITARUM)

‘‘‘Choanephora Blight (Choanephora cucurbitarum) ‘‘‘ Choanephora blight is only known so far in hot, moist, tropical sites in Peru where potato has recently been introduced.


COILED SPROUT

‘‘‘Coiled Sprout: ‘‘‘ Underground sprouts lose their normal negative geotropic habit and coil, sometimes rather tightly, with the curved portion of the stem often swollen and sometimes fasciated or split. Coiling is believed to be the result of overmature seed, soils resistant to sprout penetration and emergence, or infection by a fungus. Low soil temperatures, pre- sprouting in light, long sprouts at the time of planting, overmature seed tubers with long sprouts that may form tubers before sprouts emerge from the soil, and deep planting in compacted soil have been associated with the disorder.


COMMON RUST (PUCCINIA PITTIERIANA)

‘‘‘Common Rust (Puccinia pittieriana) ‘‘‘ The disease occurs in restricted mountain valleys of the cool highlands of Mexico, Costa Rica, Venezuela, Colombia, Ecuador, and Peru, and possibly in Bolivia and Brazil. Round and occasionally elongate lesions usually develop on the underside of leaves as small greenish white spots, 3-4 mm in diameter, although the longer axis of some oval lesions may reach 8 mm. Elongated and irregular lesions also occur on petioles and stems. Average temperatures around 10 °C with 10-12 hours of free moisture on leaves are necessary for development and spread.


COMMON SCAB (STREPTOMYCES SCABIES)

‘‘‘Common Scab (Streptomyces scabies) ‘‘‘ This disease is a major production problem that affects grade quality and has only a small effect on total yield or storing ability. Tuber lesions are usually circular, 5-8 mm in diameter, but they may be irregular in shape and larger when infections group together. Brown to tan stem and stolon lesions originate at lenticels as elongate lens-shaped lesions or at other natural wounds as approximately circular lesions. The organism causing scab occurs on the fleshy roots of other plants such as beets (red and sugar), radish, rutabaga, turnip, carrot, and parsnip. Actively growing tubers are infected through young lenticels and probably also through stomata of the epidermis before the periderm differentiates. Scab lesions may be deep or shallow. Continuous crops of potatoes generally increase severity of scab. Maintaining adequate soil moisture during tuber set and enlargement is critical in controlling the extent of scab. Chemical treatment of soil depends on the proper incorporation of chemicals into the soil and should be coordinated with other prevention methods.


CUCUMBER MOSAIC (CMV)

‘‘‘Cucumber Mosaic (CMV) ‘‘‘ Cucumber mosaic virus causes yellowing and a blistering spotting of leaves. It is usually not transmitted by tubers and has not become an economic problem.


DEFORMING MOSAIC

‘‘‘Deforming Mosaic: ‘‘‘ Relatively little is known about this disease, although it is economically important in Argentina. Secondary symptoms appearing soon after plant emergence are severe mosaic and irregular occurrence of yellowish green patterns. Tubers are symptomless, internally and externally. The disease is graft-transmitted and then readily transmitted to potato. Early roguing of diseased plants in seed fields is recommended.


DEFORMING RUST (AECIDIUM CANTENSIS)

‘‘‘Deforming Rust (Aecidium cantensis) ‘‘‘ This disease has been reported primarily from Peru, on the Pacific watershed range and at high altitudes overlapping those of common rust but generally somewhat lower. Symptoms usually develop in mid to late growing season. Rust pustules, consisting of crowded groups of aecia up to 100 mm across, are circular on the underside of leaf lamina and elongated along veins, petioles, and stems. The source of inoculum is probably cultivated potato, which is grown under irrigation in the drier season. Potato is the only well verified host.


EARLY BLIGHT (ALTERNARIA SOLANI)

‘‘‘Early Blight (Alternaria solani) ‘‘‘ This disease is found worldwide wherever potatoes are grown. Initial infection is most frequent on lower, older leaves. Tuber lesions are dark, sunken, circular to irregular in shape, and often surrounded by a raised border of purplish to gun metal color. Depending upon the location, the disease persists in crop debris, soil, infected tubers, or other solanaceous hosts. Immature tuber surfaces are easily infected, whereas those of mature tubers are much more resistant. Most rapid progress of the disease occurs during periods of alternating wet and dry weather. Field resistance to foliage infection is associated with plant maturity. The fungus is pathogenic on tomato and other solanaceous crops and has been reported on other genera such as Brassica species.


FALSE ROOT KNOT NEMATODES (NACOBBUS ABERRANS)

‘‘‘False Root Knot Nematodes (Nacobbus aberrans) ‘‘‘ Information on distribution of false root-knot nematodes is incomplete. No specific aboveground symptoms are diagnostic, although infected plants are stunted and tend to wilt under moisture stress as a result of poor root growth and/or root damage. Individual gall size depends upon nematode density, root size, and the race of the nematode. The disease on potatoes has a disease cycle somewhat similar to that on sugar beets. False root-knot nematodes have wide temperature adaptability, surviving and reproducing most rapidly at a temperature range of 20-26 °C. False root-knot nematodes have a relatively wide host range.


FOLIAGE FUNGICIDE APPLICATION

‘‘‘Foliage Fungicide Application: ‘‘‘ In most parts of the world, sprays have superceded dusts in control of foliage diseases of potato. With few exceptions, the fungicides used to control foliage diseases of potato are protectant in their action. Traditional hydraulic boom sprayers that apply large volumes of dilute spray under high pressure provide good horizontal and vertical distribution of fungicides. Spraying by aircraft has several advantages such as speed, saving of labor, ability to be used in fields too wet for ground equipment, absence of yield loss due to sprayer tracks, reduced spread of virus diseases, and low water requirements. To provide as complete a protective blanket as possible, fungicide sprays must be applied at regular intervals to protect newly-expanded foliage and to supplement fungicide activity lost to dilution, photodegradation, oxidation, etc. Use of the correct amount of fungicide per unit area is vital to effective disease control. In summary, the four basic requirements for success are to apply the right chemical in the right amount at the right time in the right way to obtain maximum coverage.


FUSARIUM DRY ROTS (FUSARIUM SOLANI AND ROSEUM)

‘‘‘Fusarium Dry Rots (Fusarium solani and Roseum) ‘‘‘ Fusarium dry rots are found on potatoes worldwide. This disease affects tubers in storage and planted seed tubers. After about one month of storage, tuber lesions at wounds are visible as small brown areas. Internal dry areas are shades of brown from fawn to dark chocolate, with the advancing margin faint for lighter shades and distinct for darker shades. Whole-tuber seed becomes infected through wounds during storage or preparation for planting. In the field, the shriveling of infected seed tubers and pitting of infected pieces may not be evident. Fusarium can survive for several years in field soil, but the primary inoculum is generally borne on seed tuber surfaces. Tubers are tolerant to infection when harvested. Wound healing can reduce infection. Dry rot develops most rapidly in high relative humidity and at 15-20 °C. If the soil temperature and moisture are suitable for rapid sprout growth and emergence, seed tuber or piece decay after planting may be of little consequence.


FUSARIUM WILTS (FUSARIUM SPP)

‘‘‘Fusarium Wilts (Fusarium spp.) ‘‘‘ These diseases are widespread and most severe where potatoes are grown at relatively high temperatures or when seasons are hot and dry. On vines, symptoms include cortical decay of roots and lower stems, vascular discoloration or rot in the lower stem, wilting, yellowing, or bronzing of foliage, rosetting and purpling of aerial parts, aerial tubers in leaf axils, and premature death of the plant. Tubers are sunken at the stolon attachment, with brown drying extending into the tuber to various depths. Tuber infection through wounds or possibly lenticels causes circular lesions and a dry rot in storage. Tubers show dry stem end rot and vascular tissue that is discolored brown, lacks the water-soaked border of eumartii but is characteristically dry, and may be gray to pink. Tuber infection follows wound infection and differs from other wilts in lacking typical vascular discoloration. The four pathogens associated with the Fusarium wilts are F. eumartii, F. oxysporum , F. avenaceum, and F. solani. Isolations of F. eumartii are readily obtained from roots and less readily from stems. F. oxysporum is easily isolated from roots and lower stems and, with difficulty, from stored tubers. F. avenaceum is successfully isolated from vascular discolored stem tissue below or close to the soil line and from discolored vascular tuber tissue. F. solani is readily isolated from discolored stem tissue. Diseases caused by the four wilt fungi are essentially similar. Fusarium wilts are typically soilborne, and the disease is transmitted with varying degrees of effectiveness from inoculum within and on seed tubers. F. eumartii survives in field soil for long periods without noticeable reduction in pathogenicity when potatoes are again planted in the field. Planting potatoes in artificially infested soil or placing inoculum on freshly cut seed efficiently establishes the disease. Wilts are most severe at high temperatures and particularly when plants are under stress in dry, hot growing conditions. F. eumartii is capable of infection at lower soil temperatures (20 and 24 °C), whereas F. oxysporum and F. avenaceum are more pathogenic at 28 °C. The cultivated potato is the only known natural host for the several Fusarium species causing wilt of the crop.


GANGRENE (PHOMA EXIGUA)

‘‘‘Gangrene (Phoma exigua) ‘‘‘ Gangrene The pathogen Phoma exigua is prevalent in most northern European countries and parts of Australia. Small dark depressions develop in the tuber skin, usually at wounds, eyes, or lenticels, and may enlarge to form thumb-mark or larger, irregularly shaped, sharp-edged lesions, the surface area of which is often unrelated to rot depth. Infected or contaminated seed tubers produce diseased stems, in which infection remains latent during the growing season unless the stems become moribund. The disease occurs on various parts of a wide range of plants.


GENETIC ABNORMALITIES

‘‘‘Genetic Abnormalities: ‘‘‘ Somatic mutations are of economic importance in clonally propagated potato because they may modify foliage type, tuber shape, or color of plant parts, delay maturity, and reduce crop yield. These differ from normal plants by low growth, close bushy habit, numerous thin stems, reduced numbers of leaflets, large rounded terminal leaflets, almost complete absence of flowers, and increased numbers of stolons with numerous small tubers that produce many weak sprouts during storage. Feathery wildings bear no resemblance to true wildings except that, compared to normal plants, they produce more thin stems and many more small tubers. Plants have greater height and stronger, more vigorous vines, with leaflets smaller and often coarser and thicker than those of normal plants. Although giant hill occurs in most commercial cultivars, it can be rogued with ease. Giant hill and tall types are more frequent in the long days of higher latitudes than in lower latitude short days. Many other abnormal types of plants or plant parts too numerous to detail may occur in commercial crops.


GRAY MOLD (BOTRYTIS CINEREA)

‘‘‘Gray Mold (Botrytis cinerea) ‘‘‘ The fungus is found on a wide range of plants throughout the world but in potato the disease is usually considered of minor economic importance. Symptoms become apparent on foliage toward the end of the growing season. Lower leaves that have become yellow from shading break down with a slimy rot. Botrytis fruits profusely on affected tissue, producing a fuzzy appearance. Tuber infection, which is uncommon, is not apparent at digging but develops during storage and may become severe. Infection, sometimes initially latent, becomes apparent on senescent plant parts under stress from shading or excessive humidity. High levels of potassium and nitrogen fertility reduce the percentage of tuber infection.


HAIL INJURY

‘‘‘Hail Injury: ‘‘‘ Hail tears and often perforates leaves. Yield reduction varies with severity of injury, time of injury, and cultivar.


HAIR SPROUT

‘‘‘Hair Sprout: ‘‘‘ Tubers with hair or spindle sprout germinate early, sometimes even before harvest, producing thin sprouts as small as 2 mm in diameter. Virus infection has not been consistently associated with spindle sprouts.


HIGH TEMPERATURE FIELD INJURY

‘‘‘High Temperature Field Injury: ‘‘‘ Stems may be injured at the soil line by high soil temperatures, particularly when plants are small and leaves are not large enough to shade the soil at the base of the plants. Tubers exposed to sunlight as they lie in the field after digging may be injured and thereby predisposed to rot in transit or storage without immediate external symptoms except possibly for watery exudates from lenticels.


HOLLOW HEART

‘‘‘Hollow Heart: ‘‘‘ Hollow heart is associated with excessively rapid tuber enlargement. Usually one cavity forms near the center of the tuber. Before hollow heart develops, central tissue may be water-soaked or translucent and a brown dry patch appears early in tuber formation in some cultivars. The types of origin are 1) a dry patch, up to 1 cm in diameter, composed of many single cells or small groups of cells, becoming enclosed by periderm, then turn brown, shrinks, and collapses to produce a cavity, 2) dry starch-free cells differentiate, causing a brown spot approximately 1 mm in diameter, often in the center of very small tubers, and produce a cavity that enlarges with tuber growth and is surrounded by a partially suberized cambium layer, and 3) internal tissue tensions cause splitting, which results in a lens-shaped cavity not preceded by cell drying. Hollow heart is most severe during growing seasons or under cultural practices favoring rapid tuber enlargement. Hollow heart is frequently severe in fields with poor stands where plants are irregularly spaced.


INTERNAL HEAT NECROSIS

‘‘‘Internal Heat Necrosis: ‘‘‘ Symptoms do not develop in vines. Losses can be severe because of buyer discrimination against internal discoloration. A somewhat similar disorder, present in Israel, produces dry spots in the cortex near the vascular ring and may produce interior cavities. Suberin develops in walls of affected pith parenchyma cells. Internal necrosis becomes progressively more severe during the growing season and is most severe during hot, dry years in light soils of sand, gravel, muck, or peat. Discoloration does not increase and may decrease in storage if affected tubers are not predisposed to storage rots.


INTERNAL SPROUTING

‘‘‘Internal Sprouting: ‘‘‘ Sprouts that develop during storage may become ingrown by penetrating into the tuber. The disorder has been known for over a century. Internal sprouting was recently associated with sprout inhibitors used in concentrations below those required for complete sprout inhibition. Drying at or slightly below the sprout apex is common on the external sprouts of tubers containing internal sprouts, and apicies of internal sprouts become similarly dry when they emerge from the tuber.


LATE BLIGHT (PHYTOPHTHORA INFESTANS)

‘‘‘Late Blight (Phytophthora infestans) ‘‘‘ Late blight is probably the single most important disease of potatoes worldwide. Leaf lesions are highly variable, depending on temperature, moisture, light intensity, and host cultivar. In the field, plants severely affected with late blight give off a distinctive odor. Positive identification of late blight requires confirmation of sporangia and sporangiophores either on lesions in the field under moist conditions or on leaf or tuber lesions incubated in a moist chamber. On susceptible cultivars, exteriors of infected tubers show irregular, small to large, slightly depressed areas of brown to purplish skin. In tropical areas where the crop is grown all year, overwintering of the disease is not an important consideration. Tubers on cull piles frequently sprout and form dense masses of succulent tissue that are easily infected by spores from diseased tubers. Tubers, particularly those inadequately covered by soil, may be infected in the field by spores that have been washed from infected leaves into the soil by rain or irrigation. Field infection is most successful under cool, moist conditions. Systems for forecasting late blight and for timing fungicide applications rely on records of temperature and rainfall or temperature and relative humidity and predict the probability of late blight development, assuming the presence of inoculum. Late blight often severely affects tomatoes and occasionally affects eggplant and many other members of the Solanaceae. Resistance is recognized as specific resistance and general resistance.


LEAK (PYTHIUM SPP)

‘‘‘Leak (Pythium spp.) ‘‘‘ Leak, also called watery wound rot, may occur sporadically wherever potatoes are grown. Only tubers are affected. The fungus lives in the soil and can enter tubers only through wounds. Species of Pythium are pathogenic on an extremely wide range of hosts, including many market-garden crops, causing damping-off, root rot, or soft rot disease.


LESION NEMATODES (PRATYLENCHUS SPP)

‘‘‘Lesion Nematodes (Pratylenchus spp.) ‘‘‘ High populations of lesion nematodes cause areas of poor growth, plants are less vigorous, turn yellow, and cease to grow. In general, it is primarily a root pathogen, whereas other species can cause serious tuber damage. Lesion nematodes are often associated with wilt-causing fungi such as Fusarium and Verticillium. The first molt occurs in the egg, and the second-stage larvae emerges from the egg. Soil temperature requirements vary greatly with species. Damage to potatoes by lesion nematodes is usually associated with coarse-textured soils. Soil moisture influences movement and other activities of Pratylenchus spp. Species of Pratylenchus that attack potatoes have wide host ranges. A high degree of resistance in potato has not been identified, although Peconic and Hudson have some resistance.


LIGHTNING INJURY

‘‘‘Lightning Injury: ‘‘‘ Lightning injury frequently accompanies severe thunderstorms. Within a few minutes to a few hours following lightning, stems collapse and tops of plants irreversibly wilt. Leaf petioles in contact with soil are often collapsed. Belowground portions of stems and roots frequently escape injury. Injured tubers have brown to black skin drying with some cracking. A field may show various patterns of injury such as areas in which all plants are killed adjacent to clearly defined areas of healthy plants, areas with dead plants at the center and progressively reduced injury toward the periphery, poor defined areas with no focus of injury, in which plants with varying degrees of injury are scattered among dead plants and unaffected ones, and a number of scattered, relatively small foci containing several plants in various stages of injury. These variations result from differences in intensity of electrical discharge and from variations in soil hydration.


LOW TEMPERATURE FOLIAGE INJURY

‘‘‘Low Temperature Foliage Injury: ‘‘‘ Certain symptoms of nonlethal low temperature foliage injury may be confused with virus symptoms or herbicide damage. Frozen leaves rapidly wilt, collapse, and when thawed, become water-soaked. Temperatures at or near 0 °C selectively injure leaf and stem primordia and possibly cell organelles. Yellowing in diffused areas, in spots, or in portions of veins may be seen and spotted patterns may be present with or without leaf distortion following nonlethal low temperatures. Low temperature injury is usually most severe in low-lying areas of fields. Because leaf surfaces are frequently well hydrated and often wet with dew, wet bulb temperatures should be more reliable than dry bulb temperatures in determining critical temperatures for leaf injury. Plants on which some leaves have been frozen recover from injury slowly, suggesting more damage than that of the tissue actually destroyed.


LOW TEMPERATURE TUBER INJURY

‘‘‘Low Temperature Tuber Injury: ‘‘‘ Low temperature tuber injury may range from outright freezing and killing of some or all of the tuber to gradations of injury following prolonged exposure to temperatures slightly above freezing. The line of demarcation between frozen and unfrozen tissue is usually distinct. Low temperature surface injury occurs in diffuse patches as a brownish black metallic discoloration. Effects of low temperature storage are primarily internal. Chilling causes formation of reducing sugars in stored tubers, resulting in a sweet flavor when cooked. Chilling injury may also take the form of net drying, in which phloem tissue is selectively killed because it has greater sensitivity to cold than do the surrounding parenchyma storage cells. Following severe injury, blackish patches or blotches may develop near the vascular ring, which may also be partially or completely blackened. Internal mahogany browning is a different low temperature response, in which diffuse brownish red to black discoloration is present, usually in the central part of the tuber. Because of hardening or acclimatization, tubers that have been stored at low temperatures are less injured by a sudden drop in temperature than are those stored at higher temperatures. Low temperatures for a few hours or temperatures just below freezing for a short time can lower internal quality, shorten storage life, and impair suitability of the tuber for processing without leaving visible evidence. Alternating temperatures during the storage season avoids chilling injury and its associated problems.


MAGNESIUM

‘‘‘Magnesium: ‘‘‘ Magnesium deficiency is one of the most commonly encountered nutritional problems and occurs on sandy acid soils that are readily leached, but may occur on heavier soils. Magnesium may be supplied as magnesium sulfate in fertilizer or dolomitic limestone or as a 2% magnesium sulfate foliage spray.


MANGANESE

‘‘‘Manganese: ‘‘‘ Deficiency symptoms develop on the upper parts of the plant as loss in luster. Manganese deficiency is possibly the most common micronutrient problem for potatoes grown on muck, sandy muck, or depressional soils in central and eastern coastal areas of the United States. Manganese toxicity, to which the potato is especially sensitive, has been called stem streak, stem streak drying, land streak, or stem break. Leaves lose their typical bright green color and show a pale, yellow-green interveinal yellowing that becomes progressively severe, often with marginal drying. Symptoms have not been described in tubers except that yield may be severely impaired. Manganese increases in solubility as the soil becomes more acid and toxicity occurs on light acid soils at pH 5.0 and below.


MISCELLANEOUS DISEASES

‘‘‘Miscellaneous Diseases: ‘‘‘ In Germany and Italy, Clonostachys araucariae tuber rot occasionally causes severe losses in storage following bad weather during harvest. Armillaria dry rot is a minor problem in northern areas where potatoes are produced on recently cleared land. Xylaria tuber rot occurs in calcareous marl soils of Florida. Gilmaniella humicola causes small brownish dry spots, 2-6 mm in diameter, around lenticels and eyes. Cylindrocarpon tonkinesis dry rot develops during the rainy season in India as brown patches on skin, later with white mycelium in or on affected tissue. Heterosporium, usually found on potato leaves, also causes lesions on tubers. Periconia, Leptosphaerulina, and Didymella are simultaneously present in leaf spots, severely defoliating potatoes in Peru. Chaetomium leaf spot, superficially resembling early blight but without the targetlike markings, is reported in South Dakota.


MYCOPLASMAS

‘‘‘Mycoplasmas: ‘‘‘ The yellows types of disease, characteristic of mycoplasma infections that have been studied in some detail possess rather broad host ranges. None of these diseases is contact-transmissible. Symptoms may be suppressed by antibiotics of the tetracycline group, and individual plants may be cured by heat therapy.


MYCORRHIZAL FUNGI

‘‘‘Mycorrhizal Fungi: ‘‘‘ The role of these obligate symbionts in potato growth and particularly their relation to tuberization has been extensively investigated.


NEMATICIDES

‘‘‘Nematicides: ‘‘‘ Control of nematodes in soil can be achieved through use of nematicides, of which only a limited number are presently available. Dispersion through the soil and activity of most soil-applied nematicides is enhanced when soil tilth, moisture, and temperature are in the proper range. Some nematicides are fumigants, which volatilize in soil and become gases that move through soil.


NEMATODES

‘‘‘Nematodes: ‘‘‘ Nematodes pathogenic to potatoes occur in all climates and cause serious crop losses, but much of this damage is unrecognized or attributed to other causes. Confining nematode populations to areas where they already exist by restricting movement of infected seed tubers and plants may be the most effective way of preventing loss of productive land.


NITROGEN

‘‘‘Nitrogen: ‘‘‘ Adequate nitrogen in the presence of sufficient phosphorus and potassium stimulates apical and lateral meristems and thus increases leaf development. Speckle leaf, brown to black spots about 1 mm in diameter that may group together on lower leaves of some early cultivars, is particularly severe following heavy rainfall or irrigation and is alleviated by nitrogen side dress. When nitrogen toxicity occurs, yields are reduced, root development is poor, and leaves may roll upward or be deformed as mouse ear. Nitrogen toxicity can result from the form of nitrogen available to the plant. Surface applications of urea, especially when banded at high rates, can cause damage from ammonia volatilization.


NONVIRUS LEAFROLL

‘‘‘Nonvirus Leafroll: ‘‘‘ Leafrolling is a symptom with several unrelated causes. Leafrolling may also be genetic. Certain nutritional soil conditions, such as nitrogen toxicity, also cause non-virus leafroll. Toproll affects the plant's apical leaves.


NUTRIENT IMBALANCE

‘‘‘Nutrient Imbalance: ‘‘‘ Nutrient deficiencies or excesses are frequently difficult to diagnose without analysis of the plant and may be confused with other environmental stresses. Potatoes grow well in soils above pH 5.0.


OXYGEN DEFICIT

‘‘‘Oxygen Deficit: ‘‘‘ Oxygen requirements of underground parts of the potato during plant development are high. Although soil compaction exerts various stresses upon underground parts of the plant, oxygen deficit may be one of the most important, resulting in delayed plant emergence, moderate to severe yield reductions, and frequently, but not always, abnormal tuber shapes.


PHOMA LEAF SPOT (PHOMA ANDINA)

‘‘‘Phoma Leaf Spot (Phoma andina) ‘‘‘ Small leaf spots develop, up to 1 cm but mostly less than 2.5 mm in diameter, have concentric rings and are similar to early blight lesions except that the lesions are not depressed into the leaf tissue. The disease has light-colored pycnidia containing a distinct ostiole surrounded by 2-3 rows of brown cells. Cultivated and wild species of potato are as yet the only known hosts.


PHOSPHORUS

‘‘‘Phosphorus: ‘‘‘ Phosphorus is essential early in plant growth and later in tuberization. Roots and stolons are reduced in both number and length. Deficiency occurs on a wide range of soil types including calcareous soils, peat or muck, light soils with low initial phosphorus content, and heavy soils in which phosphorus is fixed. Where phosphorus levels are very high, especially in alkaline soils, the uptake and/or utilization of zinc or iron may be reduced.


PINK EYE (PSEUDOMONAS FLUORESCENS)

‘‘‘Pink Eye (Pseudomonas fluorescens) ‘‘‘ The disease is of minor importance, and little is known about factors influencing tuber infection and disease development. Pink areas around the eye later turn brown. Symptoms are most conspicuous at harvest, particularly following high soil moisture levels during tuber formation. The red xylem symptom follows infection of the stolon end, causing a scar at the stolon attachment or reddish brown vascular discoloration.


PINK ROT (PHYTOPHTHORA ERYTHROSEPTICA)

‘‘‘Pink Rot (Phytophthora erythroseptica) ‘‘‘ Wilting, which is sometimes the initial symptom, may occur at any time but generally occurs late in the season. Tubers generally become infected through diseased stolons, but some infections appear to occur at buds or lenticels. The disease is soilborne and endemic in many soils. Disease develops in soils approaching saturation from poor drainage or excessive precipitation or irrigation. Although it has been reported as a pathogen only on potatoes and tulips, it has been recovered from the roots of 17 nonsolanaceous plants, including wheat and rye.


PLEOSPORA HERBARUM (PLEOSPORA HERBARUM)

‘‘‘Pleospora Herbarum (Pleospora herbarum) ‘‘‘ The disease is often found associated with potato plants poorly adapted to warm conditions or to other environmental stresses. Conidia are olive to brown and oblong, with three transverse septa and 1-3 longitudinal septa.


POTASSIUM

‘‘‘Potassium: ‘‘‘ Potassium is essential for normal growth and is highly mobile within the plant. Early appearance of unusually dark green, bluish green, or glossy foliage is a dependable symptom of potassium deficiency. Roots are poorly developed and stolons are short. Potassium deficiency predisposes to black spot and is most common on light, easily leached, sand, muck, or peat soils.


POTATO AUCUBA MOSAIC (PAMV)

‘‘‘Potato Aucuba Mosaic (PAMV) ‘‘‘ Potato aucuba mosaic virus is found worldwide but is not common. Foliage symptoms may be lacking in the second year after infection and later. The virus has filamentous particles, 580 nm long and 11-12 nm wide and is strongly immunogenic. On Capsicum annuum, brown, irregular concentric local lesions appear after 8-10 days, followed by systemic symptoms such as vein clearing, deformation, and severe drying and sometimes by complete killing of the plants. Nicotiana glutinosa exhibits mottle and vein banding. Some strains produce small round yellow spots on lower leaves of Lycopersicon esculentum.


POTATO CYST NEMATODES (HETERODERA SPP)

‘‘‘Potato Cyst Nematodes (Heterodera spp.) ‘‘‘ Cyst nematodes do not cause specific aboveground symptoms of diagnostic value, but root injury causes infected plants to seem to be under stress from water or mineral deficiency. Some varieties become round cysts upon maturity. In the spring, over 50% of the second stage larvae inside eggs within a cyst are stimulated to hatch. The female feeds near the vascular cylinder, resulting in multinucleate units called syncytia (giant cells) near the nematode's head. Syncytia may be formed in the cortex, endodermis, pericycle, and parenchyma of the central vascular strand. Syncytia usually are elongate, with ends merging with normal tissue, and each syncytium is generally associated with one larvae. Ingrowths or protuberances develop next to xylem vessels; boundary formations and microtubules are associated with the ends of these protuberances. Although populations of cyst nematodes do not increase as rapidly as do fungal and bacterial pathogens of potatoes, once well established in a potato-growing area, they are, with present technology, impossible to eradicate. Cyst nematodes develop well in soils suited for survival and movement of wormlike stages, such as medium to heavy clay soils and well-drained and aerated sands, silts, and peat soils with a moisture content of 50-75% of water capacity. Encysted eggs withstand desiccation and can remain viable 20 years or more in soil under severe environmental extremes. These include tomato, eggplant, and a number of Solanaceous weeds. Resistant cultivars and nonhost crops cause an average of 95 and 50% reduction in populations, respectively.


POTATO LEAFROLL VIRUS (PLRV)

‘‘‘Potato Leafroll Virus (PLRV) ‘‘‘ Potato leafroll, an aphid-transmitted disease, is one of the most serious in potato and is responsible for high yield losses throughout the world wherever potatoes are grown. Primary symptoms follow transmission by aphids. Secondary symptoms become evident when an infected tuber produces a plant. In S. tuberosum andigena cultivars, secondary symptoms tend to be somewhat different from those in S. tuberosum, consisting of a marked upright habit of growth, stunting, and a marginal and interveinal yellowing of leaflets, especially of dwarfed upper leaves. Internal net drying, visible to the unaided eye when the tuber is cut, is particularly marked in certain cultivars. Transmission is successful only by aphids and grafting. From the moment of symptom appearance, infections are always accompanied by phloem drying, which consists of thickening of the walls of the primary phloem cells in the stem and petioles. The virus is tuberborne and is also efficiently transmitted in a persistent manner by aphids that colonize potatoes, Myzus persicae being the most efficient. Several plant species, mostly in the Solanaceae, are known as hosts.


POTATO MOP TOP (PMTV)

‘‘‘Potato Mop Top (PMTV) ‘‘‘ Potato mop-top virus occurs in the Andean region of South America, in Northern and Central Europe, and probably also in other parts of the world where the powdery scab fungus, Spongospora subterranea, occurs. Primary infection from soil to tuber rarely spreads to the rest of the plant. Particles are elongated, have a hollow core, and are normally defective because of terminal uncoiling of the protein helix. The virus survives inside resting spores for several years and is transmitted to roots by zoospores. The most important means of long-term survival in infested land is in dormant resting spores. Symptom production is especially sensitive to light and temperature conditions. The disease is known naturally only in potato but can be transmitted mechanically to species of the Solanaceae, Chenopodiaceae, and Aizoaceae. In Chenopodium amaranticolor, diagnostic, spreading, concentric necrotic rings and local lesions form in shaded inoculated leaves at about 15 °C. In Nicotiana debneyi, dry or yellow ringspots occur in inoculated leaves.


POTATO ROT NEMATODES (DITYLENCHUS DESTRUCTOR)

‘‘‘Potato Rot Nematodes (Ditylenchus destructor) ‘‘‘ Ditylenchus destructor is primarily an important potato pathogen in the temperature regions of Europe and, especially, in Russia, probably due to its inability to withstand drying rather than to a direct temperature relationship. No specific aboveground symptoms exist. Nematode inoculum may survive in the soil, on fungi, or on weed hosts, or it may be introduced by planting diseased seed tubers. The potato rot nematode survives in soils as low as -28 °C.


POTATO SPINDLE TUBER VIROID (PSTV)

‘‘‘Potato Spindle Tuber Viroid (PSTV) ‘‘‘ Reliable reports of potato spindle tuber viroid causing spindle tuber in commercial plantings have been made from the United States, Canada, and Russia. Vine symptoms are seldom evident before blossom time. Tubers are elongated, round in cross section, and tend in some cultivars to have pointed ends. Transmission is largely mechanical, principally by man himself and to a lesser extent by chewing insects. About two to three weeks after inoculation of Lycopersicon esculentum with severe strains, new leaflets show marked rugosity, epinasty, and down-curling. Scopolia sinensis responds with dark brown, dry local lesions in two or three weeks and later with systemic drying.


POTATO VIRUS A (PVA)

‘‘‘Potato Virus A (PVA) ‘‘‘ Potato virus A which causes mild mosaic is widespread in most potato-growing areas. The mild mosaic symptom induced in leaves of many potato cultivars is a yellow spotting, sometimes severe, in which yellowish or light-colored irregular areas alternate with similar areas of darker than normal green. Infected plants usually seem open because stems bend outward. The virus is transmissible by at least seven species of aphids in the nonpersistent manner, by grafting, and by inoculation with sap.


POTATO VIRUS M (PVM)

‘‘‘Potato Virus M (PVM) ‘‘‘ Potato virus M is found worldwide in potato cultivars. Aboveground symptoms range from very slight to severe and include spotting, mosaic, crinkling, rolling of leaves, stunting of shoots, leaflet deformation and twisting, and some rolling of the top of the plant. The particles are straight to slightly flexuous rods 650 X 12 nm. Transmission by mechanical inoculation with infective sap or by tuber or stem grafting can be achieved with ease. The virus infects mainly the Solanaceae but also members of the Chenopodiaceae and Leguminosae.


POTATO VIRUS S (PVS)

‘‘‘Potato Virus S (PVS) ‘‘‘ Potato virus S occurs worldwide wherever the potato is grown and was first detected not through symptoms but through serology during efforts to produce an antiserum to potato virus A. It is virtually symptomless in most of the common potato cultivars. The particles are straight to slightly curved filaments, approximately 650 X 12 nm. The virus is tuber-perpetuated, readily transmitted mechanically by infective sap, and reputed to be spread in nature primarily by contact with diseased plants. In Nicotiana debneyi, systemic vein clearing occurs after 20 days, spreading from the leaf tip toward the base. Solanum rostratum and Saracha umbellata (affected by some strains only) show dry spotting on inoculated leaves in 20 days, and later, on systemically invaded leaves. Well-defined mature plant resistance is present in potato, so transmission must occur relatively early in the season if tubers are to become infected. Certain cultivars are moderately resistant to reinfection.


POTATO VIRUS T (PVT)

‘‘‘Potato Virus T (PVT) ‘‘‘ Potato virus T occurs in Peru, Bolivia, and probably elsewhere in the Andes and produces no obvious symptoms in several tuberosum and andigena types but may produce mild spotting or slight vein drying and yellow spots, and some andigena types may develop top drying after grafting. The virus is readily transmitted mechanically by tubers and sap. Phaseolus vulgaris varieties are good local lesion hosts when heavily shaded after inoculation, later developing systemic drying followed by plant recovery.


POTATO VIRUS X (PVX)

‘‘‘Potato Virus X (PVX) ‘‘‘ Potato virus X occurs wherever potatoes are grown. It may be latent, without foliage symptoms or apparent effect on plant vigor except when closely compared to virus-free stocks, or it may show mild spotting to severe or rugose mosaic, with dwarfing of the plant and reduced leaflet size. Symptoms in most plants are enhanced by low temperatures of 16-20 °C and are mild or may be masked at temperatures above 28 °C. Cultivars may be freed from the virus by meristem culture of sprout tips grown at 32-36 °C. Isolates cause widely different symptom severity ranging from mild to severe. In tobacco, infection is systemic, with spotting or ring spots. Datura stramonium and D. tatula show systemic infection, latent to severe spotting, and some leaf drying.


POTATO VIRUS Y (PVY)

‘‘‘Potato Virus Y (PVY) ‘‘‘ Potato virus Y causes rugose mosaic. Symptoms in potato vary widely with virus strain and potato cultivar, ranging in severity from weak symptoms to severe foliage drying to death of infected plants. Primary symptoms depending on the potato cultivar, are drying, spotting, or yellowing of leaflets, leaf dropping, and sometimes premature death. Plants with secondary infection are dwarfed, and leaves are spotted and crinkled. The virus evokes stipple-streak symptoms in several cultivars. A correlation generally exists between symptoms in the foliage and those in tubers. It is a virus with flexuous, helically constructed particles, 730 X 11 nm. Spread of the disease depends mainly on the presence of winged aphids and is considered one of the most damaging potato viruses in causing yield depression. Many plant species, mostly in the Solanaceae, but also in the Chenopodiaceae and Leguminosae are hosts of the disease. In tobacco, most strains produce vein clearing followed by spotting. Mature plant resistance may be of importance in seed-potato production procedures. In areas where virus sources are limited and vectors are present only in certain periods of the growing season, seed with a tolerable percentage of infection can be produced. Preventing spread of the virus strains by chemical control of aphid vectors, including the use of systemic insecticides, is not possible because aphids have short periods of acquisition and infection feeding.


POTATO YELLOW DWARF (PYDV)

‘‘‘Potato Yellow Dwarf (PYDV) ‘‘‘ Potato yellow dwarf virus occurs in Canada and in the United States in Michigan, New York, and Wisconsin. Vines from infected seed pieces are dwarfed and brittle, and the entire plant has a yellowish cast. Tubers are usually few, small, and deformed. The virus is bacilliform, with particles measuring about 380 X 75 nm. It is the only known virus borne by leafhoppers that is also mechanically transmissible. High temperatures enhance vine symptoms and reduce plant emergence from infected tubers, whereas low temperatures increase plant or sprout emergence and suppress vine symptoms. In addition to solanaceous plants, vectors have transmitted the virus to members of Compositae, Cruciferae, Labiatae, Leguminosae, Polygonaceae, and Scrophulariaceae.


POTATO YELLOW VEIN (PYVV)

‘‘‘Potato Yellow Vein (PYVV) ‘‘‘ Vein yellowing, caused by the potato yellow vein virus is very common in the highlands of Ecuador and southern Colombia. Newly developed symptoms are bright yellow veins and interveinal yellowing. The virus particle is isometric and 26 nm in diameter.


POWDERY MILDEW (ERYSIPHE CICHORACEARUM)

‘‘‘Powdery Mildew (Erysiphe cichoracearum) ‘‘‘ Powdery mildew can be an important foliage disease in arid or semiarid climates. Elongated, light brown stipples, 0.5-2 mm in length, may appear on stems and petioles of infected plants. Conidia form in chains on unbranched conidiophores, 7-13 X 36-50 µm.


POWDERY SCAB (SPONGOSPORA SUBTERRANEA PV SUBTERRANEA)

‘‘‘Powdery Scab (Spongospora subterranea pv subterranea) ‘‘‘ Although powdery scab develops best under cool, moist conditions, it is found in practically every potato-producing area in the world. Tuber infection in lenticels, wounds, and less frequently in the eyes is evident as purplish-brown pustules, 0.5-2 mm in diameter, extending laterally under the periderm and forming a raised or pimplelike lesion. Wound periderm forms beneath the lesion, which gradually darkens and decays, leaving a shallow depression filled with a powdery mass of dark brown spore balls. In storage, powdery scab may lead to a dry rot or to more warts or cankers. Powdery scab lesions may serve as infection courts for late blight and a number of wound pathogens. Infection on roots and stolons parallels that on tubers, with small dry spots developing into milky white galls varying in diameter from 1 to 10 mm or more. The fungus survives in soil in the form of spore balls made up of resting spores. Inoculum is spread by soil and by tuberborne resting spores. The time from tuber and root infection to gall formation is less than three weeks at a temperature of 16-20 °C.


PSYLLID YELLOWS

‘‘‘Psyllid Yellows: ‘‘‘ This disorder results from insect feeding, and no infectious microorganism is involved. On vines, young leaves, which are often red or purple, become erect and have cupped basal portions. Border parenchyma surrounding the phloem is first affected, and later tissue breakdown extends laterally, causing phloem drying. Few if any tubers are set on plants attacked in early development. Abnormally small tubers sprout without a dormant period. The disorder is not tuber-transmitted. The disease results from toxic substances introduced during feeding of nymphs of the tomato or potato psyllid also known as the jumping plant louse. Similarity between psyllid yellows, the mycoplasma disorder haywire, leaf rolls of various types, and Rhizoctonia has caused some confusion in diagnosis.


RHIZOCTONIA CANKER (RHIZOCTONIA SOLANI)

‘‘‘Rhizoctonia Canker (Rhizoctonia solani) ‘‘‘ Rhizoctonia canker, commonly called black scurf, is present in all potato growing areas. Black or dark brown sclerotia develop on surfaces of mature tubers. Plants are most severely damaged in the spring shortly after planting and killing of underground sprouts delays emergence, especially in cold, wet soils. Reddish brown lesions on stolons cause stolon pruning or tuber malformation. The perfect stage of the pathogen occurs on stems just above the soil line as a whitish gray mat on which basidiospores are formed, giving the surface a powdery appearance. A type of tuber malformation, incompletely understood and not directly linked to Rhizoctonia infection, is frequent when Rhizoctonia is severe on tubers. The mycelium is generally tan to dark brown and hyphae are rather large (generally 8-10 µm in diameter). Rhizoctonia produces a growth-regulating toxin that may be partially responsible for tuber malformation. The pathogen overwinters as sclerotia on tubers, in soil, or as mycelium on plant debris in the soil. Rhizoctonia populations may increase in soils where little or no rotation is practiced. R. solani is a pathogen of numerous crops and weed hosts throughout the world.


RHIZOPUS SOFT ROT (RHIZOPUS SPP)

‘‘‘Rhizopus Soft Rot (Rhizopus spp.) ‘‘‘ This fungus occurs throughout the world, but the disease is important chiefly in the tropics. Water-soaked lesions on skin, initially small, enlarge rapidly and become a soft, watery rot extending into the flesh. Rhizopus species are typically saprophytes but may also be wound parasites on a wide range of fleshy storage organs of fruits and vegetables.


RING ROT (CORYNEBACTERIUM SEPEDONICUM)

‘‘‘Ring Rot (Corynebacterium sepedonicum) ‘‘‘ Plant symptoms begin with wilting of leaves and stems after midseason. This disease derives its name from the characteristic internal breakdown in the vascular ring of an infected tuber cross-sectioned at the stem end. The organism overwinters primarily in infected tubers, either those in storage or those that survive the winter in the field. Conditions for dissemination of the pathogen are most favorable in the spring when infected seed tubers are warmed before planting, thus increasing bacterial activity. Disease-resistant cultivars have been developed, but immunity is as yet unknown.


ROOT KNOT NEMATODES (MELOIDOGYNE SPP)

‘‘‘Root Knot Nematodes (Meloidogyne spp.) ‘‘‘ One or more species of Meloidogyne are known to attack almost all major crop plants and many weeds species. Root-knot nematodes are worldwide in distribution but are limited in specific areas by temperature and cropping practices. Aboveground symptoms are not diagnostic. Knots or galls of varying sizes and shapes are present on the roots. The disease cycle of root-knot nematodes on potato is similar to that on other crops and plants. In general, root-knot nematodes reproduce most rapidly, survive longer, and cause the most damage in coarse-textured soils. Because potatoes are predominately grown in the cooler climates, root knot of potato is not a major economic problem.


ROSELLINIA BLACK ROT (ROSELLINIA SPP)

‘‘‘Rosellinia Black Rot (Rosellinia spp.) ‘‘‘ The disease is prevalent in the tropics, where temperate and moist climates are found during the growing season. Plants become stunted and wilted. Rosellinia black rot can be distinguished from stem rot because the fungal strands from Rosellinia may be present on all parts below soil level, whereas Sclerotium rolfsii affects only the parts close to the soil surface. The disease occurs in level or moderately sloping fields where water accumulates. The fungus affects carrots, beets, and members of the Brassicae and of the genera Amaranthus, Rumex, and Polygonum.


SECOND GROWTH & JELLY END ROT

‘‘‘Second Growth and Jelly End Rot: ‘‘‘ Second growth may be of several types including 1) deformed tubers with protruding eyes, lateral buds (knobby tubers), or apical buds (dumbbells or elongated tubers), 2) gemmation-secondary tubers on a stolon extension of the original tuber, or 3) recently formed tubers that, before normal harvest, produce either a sprout or a leafy aboveground plant. Second growth is commonly attributed to high field temperatures and drought and is usually stimulated by soil temperatures of 27 °C and above, although some develops at lower temperatures. Second growth and jelly end rot are interrelated because jelly end rot is prevalent in abnormally shaped tubers, particularly those with second growth. Translucent end or sugar end refers to incipient symptoms visible at harvest or developing in storage. Stolon ends of tubers with jelly end rot become translucent to glassy, lack normal starch content, have reduced specific gravity, shrivel, and collapse into a wet jellylike substance. Jelly end rot is more prevalent in tubers of long-tubered cultivars, particularly those with second growth such as spindle shape (pointed ends) or dumbbell shape. No pathogen has been consistently demonstrated in jelly end rot tissue, although populations of secondary organisms, particularly bacteria, are usually high. The stolon end of the tuber normally has the highest starch content. Incidence of jelly end rot is strongly influenced by seasonal conditions, being a severe problem in some years and not a problem in others.


SECONDARY TUBERS

‘‘‘Secondary Tubers: ‘‘‘ Tubers sprout either in storage or in the field, producing new tubers directly without forming a normal plant.


SEPTORIA LEAF SPOT (SEPTORIA LYCOPERSICI)

‘‘‘Septoria Leaf Spot (Septoria lycopersici) ‘‘‘ The disease is present in Central and South America. Lesions on leaves are round to oval and have concentric rings of raised tissue when viewed from the upper surface. Septoria leaf spot is similar on tomato and potato plants but exhibits differences when isolates from the two hosts are grown on artificial media. The disease is present in regions characterized by cool, moist weather during the growing season.


SILVER SCURF (HELMINTHOSPORIUM SOLANI)

‘‘‘Silver Scurf (Helminthosporium solani) ‘‘‘ Silver scurf is probably present in all of the major potato-growing areas. Small, localized, light brown, circular spots with indistinct borders frequently enlarge to cover a considerable area of the tuber. Black dot and silver scurf produce similar blemishes on the tuber surface and may occur together. Silver scurf has a hyaline mycelium that is septate, branched, and turns brown with age. Transmission of the fungus is largely from infected seed pieces and soil transmission may also occur to a lesser extent. High humidity is necessary for disease development. Some varieties may be more susceptible than others. The disease has never been found on any other host and only infects the tubers of potatoes.


SKIN SPOT (POLYSCYTALUM PUSTULANS)

‘‘‘Skin Spot (Polyscytalum pustulans) ‘‘‘ The disease is common in northern Europe and also occurs in North America and Australia. Following infection, roots, stolons, and stems belowground develop discrete, light brown lesions that enlarge, darken, and crack transversely. Conidia germinate and infect tubers through lenticels, eyes, and skin abrasions. The disease is prevalent in cool wet seasons and is more severe in crops grown on heavy clay loam than in those grown on sandy or organic soils. Skin spot develops only on potatoes, but brown lesions can develop on roots of other Solanaceae.


STEM END BROWNING

‘‘‘Stem End Browning: ‘‘‘ Stem-end browning describes an internal, brown discoloration of tuber tissue near the stem end or stolon attachment. The disorder may be confused with virus leafroll net necrosis. Recently, stem-end browning has been associated with early season leafroll virus infection, in which a limited amount of tissue is affected and the virus does not always establish infection. Stem-end browning has no apparent effect on yield, although affected tissue is believed to be sterile. Stem-end browning is distinguished from Fusarium or Verticillium wilts by culture techniques and from Verticillium infection or defoliants by darker color and coarser strands.


STEM ROT (SCLEROTIUM ROLFSII)

‘‘‘Stem Rot (Sclerotium rolfsii) ‘‘‘ Stem rot affects potatoes in the tropical and subtropical regions. Plant stems are infected at or below the soil surface. Tubers become infected through the stolons of diseased plants and through lenticels from mycelia growing over tuber surfaces. Natural infection of seed tubers in the field occasionally causes seed decay and reduced stands. Dead or dying plants devoid of fungus signs can often be diagnosed by placing them in a moist chamber for a few days and abundant mycelia grow from them. The mycelium is white when young becoming tan as it gets older. Hyphae are both intracellular and intercellular and are constricted at the point of penetration of the cell wall. The fungus is soilborne as sclerotia or mycelia on decaying vegetable matter. Germination of sclerotia and mycelial growth are favored by aerobic conditions, high temperatures (28-30 °C), and high relative humidity. Stem rot infects cultivated and noncultivated plants such as ferns, certain mosses, gymnosperms, grasses, cereals, banana, and many dicotyledonous plants including certain woody trees.


STEMPHYLIUM CONSORTIALE (STEMPHYLIUM CONSORTIALE)

‘‘‘Stemphylium Cosortiale (Stemphylium cosortiale) ‘‘‘ The disease causes lesions occurring with and superficially resembling those caused by Alternaria solani, except that lesions lack the concentric markings of early blight and are lighter brown.


STUBBY ROOT NEMATODES (PARATRICHORDORUS SPP)

‘‘‘Stubby Root Nematodes (Paratrichordorus spp.) ‘‘‘ Stubby-root nematodes have a very wide host range in the temperate regions, and they transmit virus in many different types of plants. No diagnostic aboveground symptoms exist except stunting. Eggs are deposited in the soil. Feeding activity of the stubby-root nematode occurs principally at root tips. Tobacco rattle virus is probably not readily spread for long distances by virus-infected plant material because the strain of the virus would probably not be suitable to the nematode population of the new location.


SUGAR BEET CURLY TOP (BCTV)

‘‘‘Sugar Beet Curly Top (BCTV) ‘‘‘ Curly top, caused by the sugar beet curly top virus, moves slowly through the potato plant and is apparently now of little importance. Current season symptoms consist of dwarfing, yellowing, elongation, and upward rolling of the midrib of terminal leaflets. Transmission in nature is by the leafhopper, Circulifer (Neoliturus) tenellus, in which the virus is circulative.


SULFUR DEFICIENCY

‘‘‘Sulfur Deficiency: ‘‘‘ Sulfur deficiency, as reported in several locations in Wisconsin on Planefield loamy sand, is a general yellowing with a slight upward rolling of leaflets.


SURFACE ABRASIONS

‘‘‘Surface Abrasions: ‘‘‘ Immature tubers that are mechanically injured during harvest before the periderm is mature exhibit shreds of loose skin exposing underlying flesh. Mature tubers may be skinned by rough handling during harvesting and grading operations, thus providing infection courts for wound pathogens. Tuber cracking is of four types including growth cracks from internal pressure, growth cracks from virus infection, mechanically produced cracks, and harvest cracks. Growth cracking or bursting usually follows the long axis of the tuber and results from internal pressure exceeding the tensile strength of surface tissues during tuber enlargement. Growth cracks may also develop in tubers of plants with the yellow dwarf virus, potato mop-top virus, or certain strains of the spindle tuber viroid. Mechanical cracking during harvesting may follow sudden impacts. Harvest cracks are crescent shaped, resembling cracks made with a thumbnail.


THECAPHORA SMUT (ANGIOSORUS SOLANI)

‘‘‘Thecaphora Smut (Angiosorus solani) ‘‘‘ Thecaphora smut is found in Mexico and the northern regions of South America. Usually no aboveground symptoms are found. Angiosorus solani has locular sori, 1-1.2 mm in diameter, that are surrounded by a periderm six to eight cells deep and contain globose-ovoid spore balls 15- 50 X 12-40 µm in diameter. Tumor growth, caused by hypertrophy of the outer phloem and parenchyma of the stem and stolon, consists largely of enlarged parenchymatous cells. Smut is favored by high soil moisture and possibly high soil salinity. Spores are believed to be long-lived in the soil.


TOBACCO MOSAIC (TMV)

‘‘‘Tobacco Mosaic (TMV) ‘‘‘ Tobacco mosaic virus is not a problem in potato production. Mechanical inoculation of commercial cultivars results in localized infection, local lesions or blotches, drop of inoculated leaves, and rarely with systemic invasion in the current year. Green strains may be almost latent or cause symptoms suggestive of rugose mosaic virus, with leaflet twist, curl, and malformation. Although resistance is present in commercially available cultivars of S. tuberosum, the virus is a potential threat. Severity of reaction is greater at 24-28 °C than at lower temperatures.


TOBACCO NECROSIS (TNV)

‘‘‘Tobacco Necrosis (TNV) ‘‘‘ Only tubers react to infection from tobacco necrosis virus. The virus has a wide host range, and little is known about the disease of potato. It rarely becomes systemic, is transmitted naturally to plant roots by zoospores of Olpidium brassicae, infects both monocotyledonous and dicotyledonous plants, and after mechanical inoculation typically produces local lesions on leaves.


TOBACCO RATTLE (TRV)

‘‘‘Tobacco Rattle (TRV) ‘‘‘ The tobacco rattle virus causes stem spotting of potato foliage and spraing of tubers on potatoes throughout Europe and in widely separated areas of North America and on hosts other than potato in Asia and South America. Stem mottle primary symptoms occur on foliage following feeding by large numbers of viruliferous nematodes on emerging stems. Spraing symptoms on tubers vary in type and severity with the degree of nematode infestation, time of infection, strain of the virus, potato cultivar, environmental conditions such as soil moisture and temperature, and type of infection (primary or secondary). Secondary tuber symptoms tend to be malformation and internal flecking. Stem mottle is more important in seed production, and spraing is more serious with table stock. Isolation of the disease from potato is usually difficult, and difficulty increases with time after harvest. Incidence of spraing is highest in sandy soils. Primary tuber infection follows nematodes feeding directly on tubers. Although the disease can be transmitted through seed tubers, transmission in this manner is infrequent, and the virus is ultimately self-eliminating. In Chenopodium amaranticolor, the virus produces dry local lesions, some spreading, but the virus is not systemic. In Cucumis sativus, yellow or dry local lesions occur. On Nicotiana clevelandii, inoculated leaves are symptomless or have yellow or dry lesions.


TOBACCO RINGSPOT (TRSV)

‘‘‘Tobacco Ringspot (TRSV) ‘‘‘ Andean potato calico caused by tobacco ringspot virus has been confirmed only in Peru, but preliminary tests indicate its presence in other Andean countries. Bright yellow areas on the margins of middle and upper leaves gradually increase in size to form large patches or even to affect the whole leaf. The virus is probably introduced to uninfested soil when infected seed tubers are planted. Cool temperatures favor disease development in potato plants. In Nicotiana tabacum, it produces local and systemic yellow and dry ringspots and line patterns, followed by systemic symptomless infection. Vigna sinensis exhibits reddish dry lesions in inoculated leaves, followed by systemic apical drying. In Cucumis sativus, symptoms are yellow or dry lesions in inoculated leaves, systemic yellow spots, and apical distortion.


TOMATO BLACK RING (TBRV)

‘‘‘Tomato Black Ring (TBRV) ‘‘‘ Tomato black ring virus affects potato in Northern and Central Europe but normally is uncommon. Primary systemic symptoms include dry spots and rings on leaves that develop in only a few plants. Particles are isometric and about 30 nm in diameter. The disease is soilborne and transmitted by nematodes. The most important means of survival in infested land is in infected weeds and their seeds. Cool climates favor development of disease in potato plants. Chenopodium amaranticolor and C. quinoa exhibit yellow or dry spots and rings in inoculated leaves and systemic dry or yellow spots. Nicotiana rustica and N. tabacum show dry or yellow spots or rings in inoculated leaves and systemic spots, rings, and line patterns with variable amounts of drying. Cucumis sativus is a useful bait plant for virus detection in soil samples, producing yellow or dry local lesions and systemic spotting.


TOMATO SPOTTED WILT (TSWV)

‘‘‘Tomato Spotted Wilt (TSWV) ‘‘‘ The tomato spotted wilt virus occurs worldwide but is more frequent in subtropical and temperate regions of the African, American, and Australian continents than in Europe and Asia. Primary symptoms are dry spotting of leaves, stem drying, death of the top of one or more stems, and occasionally death of the whole plant. Local yellow or dry lesions may appear at points of thrip feeding. Tubers produced by infected plants may appear normal or they may be malformed, with cracks and internal rusty or dark dry spots. Secondary symptoms on shoots from infected tubers may include drying, early death, varying degrees of stunting, or a rosette type of growth with coarse, dark green leaves. The spotted wilt virus is vectored by thrips. Transmission is unlikely through true seed of potato, although some transmission through seed of other plants has been reported. The spotted wilt virus can be transmitted mechanically with relative ease if infected leaves are triturated with a reducing substance. Mechanical transmission in potato apparently does not occur naturally. Spotted wilt in potato is seasonal. Most field infection occurs as viruliferous vectors move into potato from outside sources. In Australia, thrip populations increase when the rainfall is satisfactory without being excessive and when the temperature steadily rises as summer approaches. The disease infects dicots and monocots of more than 30 families, annuals and perennials, and many crop, weed, and ornamental plants. Some field resistance is available in selected cultivars.


TUBER GREENING & SUNSCALD

‘‘‘Tuber Greening & Sunscald: ‘‘‘ When tubers are exposed for some time to light in the field or after harvest, chlorophyll forms in the leucoplasts and tuber tissue turns green. Green tissue may extend 2 cm or more into the tuber and is often accompanied by purple pigmentation. Sunscald injury develops in tubers exposed to intense sunlight as restricted areas with almost-white skin, often covering a sunken dry area. Certain potato cultivars have a tendency to set tubers near the soil surface. Table stock potatoes should be stored in the dark.


TUBER SEED TREATMENT

‘‘‘Tuber Seed Treatment: ‘‘‘ Chemical treatment of seed tubers before planting is neither a cure-all nor a replacement for the use of high quality seed that has been properly stored and handled. Possible benefits of seed treatment are: 1) control of storage diseases such as Fusarium dry rot, 2) control of seed piece decay when cut seed has to be held for an extended period before planting, 3) control of seed piece decay in the soil when planting is done under adverse soil conditions (cool and wet) that impair suberization of the cut surfaces, and 4) partial control of diseases such as stem canker, scab, Verticillium wilt, and blackleg. Prestorage treatment of seed usually involves application of fungicides (e.g., thiabendazole) in the form of a fine mist as tubers enter the storage structure. When whole seed is planted, or when cut seed is planted in a warm, moist seedbed promptly after cutting, whole tubers may be surface disinfested by dusting or dipping. Cut seed, whether treated or not, should be planted immediately.


ULOCLADIUM BLIGHT (ULOCLADIUM ATRUM)

‘‘‘Ulocladium Blight (Ulocladium atrum) </b></S></CF> This disease attacks potatoes in high elevations and causes the foliage to turn black and necrotic. Small, dark lesions with irregular margins form on healthy leaves. The disease is often associated with hail storms that cause the leaves to be torn due to attack from the hail, making them more subject to the disease.


VERTICILLIUM WILT (VERTICILLIUM ALBO-ATRUM)

‘‘‘Verticillium Wilt (Verticillium albo-atrum) ‘‘‘ The disease apparently occurs wherever potatoes are grown, although it may be confused with other diseases that cause early maturity. Verticillium wilt causes early senescence of plants. During the growing season, plants may lose their turgor and wilt, especially on hot, sunny days. Tubers from infected plants, but not necessarily all tubers, usually develop a light brown discoloration in the vascular ring and severe vascular discoloration may extend over halfway through the tuber. Verticillium wilt can develop septate, resting dark mycelium on stems in the field and also in culture or may form dark mycelial strands with black, thick-walled pseudosclerotia, also called microsclerotia. Both types may be present within a single potato plant. Infection is through root hairs, wounds (including those at points of emergence of adventitious roots), and through sprout and leaf surfaces. Both species are poor competitors and survive poorly in soil in the absence of suitable hosts. Inoculum in field soil or in soil adhering to the surface of potato tubers is more important in initiating wilt symptoms than is inoculum from seed tubers with vascular discoloration. Inoculum can be distributed long distances by contaminated soil adhering to seed surfaces and from field to field by contaminated equipment or irrigation water. Varieties carrying different levels of resistance have been identified within S. tuberosum ssp. tuberosum.


VIOLET ROOT ROT (HELICOBASIDIUM PURPUREUM)

‘‘‘Violet Root Rot (Helicobasidium purpureum) ‘‘‘ Although infrequent, violet root rot has been reported from most of the major potato-growing areas of the world. Aboveground symptoms are not distinctive. Violet root rot has young hyphae that are light violet, becoming more intensely violet with age. The fungus overwinters in the soil as sclerotia. The fungus parasitizes a wide range of hosts, the most important being carrot, alfalfa, asparagus, and sugar beet.


WART (SYNCHYTRIUM ENDOBIOTICUM)

‘‘‘Wart (Synchytrium endobioticum) ‘‘‘ Wart has been recorded in Africa, Asia, Europe, and North and South America. Warty outgrowths or tumorous galls, pea-sized to the size of a man's fist, develop at the base of the stem. The fungus exists in a number of physiologic races, making evaluation of resistance difficult. Sori of sporangia develop in epidermal cells of meristematic tissues of growing points, buds, stolon tips, or young leaf primordia. From the initial infection, summer sporangia form as membranous sacs, producing motile zoospores. The fungus is most active where susceptible tissue is present in growing sprouts, stolons, buds, and eyes.


WHITE MOLD (SCLEROTINIA SCLEROTIORUM)

‘‘‘White Mold (Sclerotinia sclerotiorum) ‘‘‘ White mold is a cool temperature disease occurring in the Andes and in temperate zones nearly everywhere that potatoes are grown. Water-soaked lesions covered by a cottony mycelial mat and sclerotia are most frequent on the main stem at the soil line or on lateral branches in contact with the soil. Tubers near the soil surface may be infected, beginning with small depressed areas sometimes located near the eyes and having a distinct demarcation between affected and healthy tissues. Soilborne sclerotia near the surface germinate, forming either an apothecium or, if enough moisture and organic matter are available, a mycelial mat. Cool temperatures (16-22 °C) and high relative humidity (95-100%) favor disease development. S. sclerotiorum has a wide host range, attacking many dicotyledonous crops and weeds.


WIND INJURY

‘‘‘Wind Injury: ‘‘‘ Wind injury is evident on upper surfaces of leaves that, through wind movement, have been rubbed by other leaves, usually those directly above the affected area. Leaf injury may be more severe at the edges of the field. During harvest, tubers in sacks in the field may be damaged by drying wind.


WITCHES BROOM

‘‘‘Witches Broom: ‘‘‘ Witches' broom, also called northern stolbur and dwarf shrub virosis, has been reported from North America, Europe, and Asia but is of minor economic importance. Mycoplasmas of witches' broom are tuber-perpetuated, in contrast to those of aster yellows, which usually are not. Pathogens are mycoplasma-like organisms, such as those that cause aster yellows. The pathogens are transmitted by leafhopper vectors which are propagative. Leafhoppers are unable to acquire the pathogen from potato. Lycopersicon esculentum, Cyphomandra betacea, and Nicotiana tabacum have been used to distinguish between strains. Medicago sativa, Melilotus alba, or Lotus corniculatus serve as natural reservoirs and may be used as indicator plants.


ZINC

‘‘‘Zinc: ‘‘‘ Zinc deficiency causes stunting of plants and upward rolling of young, yellow leaves suggestive of early virus leafroll symptoms, with terminal leaves being somewhat vertical. Zinc deficiency in recently developed land, alkaline soils, or soils with excessively high phosphorus results in severe stunting, leaf malformation, and indistinct bronzing or yellowing around leaf margins. Foliage applications of zinc chloride or zinc sulfate alleviate deficiency symptoms. Speckle bottom (small to large dry spotting and yellowing of basal leaves, which progress upward) has responded to applications of zinc. In laboratory studies, zinc toxicity develops as general stunting, with a slight yellowing at tips and margins of upper leaves and purple coloration on the undersides of lower leaves.