Corn:Diagnostics

CORN PESTS AND NUTRITIONAL DISORDERS

2,4-D INJURY

‘‘‘2,4-D Injury: ‘‘‘ Applications of 2,4-D may cause an onion leafing or a buggy-whip condition in which leaves remain wrapped in a tall spike.

AFLATOXIN DETECTION

‘‘‘Aflatoxin Detection: ‘‘‘ Methods for aflatoxin analysis fall into two categories: rapid screening procedures determining the presence or absence of aflatoxin (fluoromotric iodine rapid screening and minicolumn tests), and laboratory procedures quantifying the actual amounts of toxin (thin-layer chromatography, gas-liquid chromatography, or high-pressure liquid chromatography tests).

AFLATOXINS AFLATOXICOSES

‘‘‘Aflatoxins: ‘‘‘ Aspergillus flavus and A. parasiticus are ubiquitous in soil and on decaying plant material, cause heating and decay of stored grain, and, under certain conditions, commonly invade corn. Aflatoxicosis in domestic animals results in lesions in the liver and other tissues, reduced body weight gains and poor growth, and mortality. In poultry, besides fatty liver and kidney disorders, leg and bone problems as well as coccidiosis outbreaks can also result.

AIR POLLUTANTS

‘‘‘Air Pollutants: ‘‘‘ Air pollutants cause substantial annual losses by visibly injuring leaves or suppressing plant growth without leaving visible injury. In general, the extent and nature of air pollution injury depend on genetic and meteorological factors, as well as on the concentration and duration of the exposure to various pollutants singly or in combinations. In examining plants for air pollution damage, consider the possible source of pollutants, terrain, previous weather patterns, abnormal symptoms on other vegetation in the area, time of symptom onset, as well as the possibility of mimicking symptoms due to living pathogens, insects, abnormal nutrition, or various environmental stresses. Sweet maize is much more sensitive to air pollutants than field maize.

ALTERNARIA LEAF BLIGHT (ALTERNARIA ALTERNATE)

‘‘‘Alternaria Leaf Blight (Alternaria alternate) ‘‘‘ Chlorotic streaks, which later become necrotic, form on maize leaves of all ages. Colonies usually black or olivaceous black. Various forms of tissue damage, such as insect feeding scars and embedded sand grains, are apparently essential to infection that develops during periods of heavy dew.

ANHYDROUS AQUA AMMONIA DAMAGE

‘‘‘Anhydrous Aqua Ammonia Damage: ‘‘‘ Short or long skips in rows scattered through the field may indicate anhydrous ammonia damage.

ANTHRACNOSE (COLLETOTRICHUM GRAMINICOLA)

‘‘‘Anthracnose (Colletotrichum graminicola) ‘‘‘ Anthracnose has increased in importance since 1971 around the world. Symptoms vary greatly, depending on the genotype, age of the leaf, and environment. On PDA the mycelium is septate, sparingly branched, hyaline, and granular. The fungus overseasons as a saprophyte on maize crop residue and seed as spores and mycelia. The fungus affects a large number of grasses including the small grains, sorghums, bermudagrass, Dallis grass, Guineagrass, Bungalow grass, crabgrass, Guatemala grass, molasses grass, orchardgrass, Paragrass, redtop, red fescue, and Merker grass. Anthracnose is generally favored by high temperatures and extended periods of cloudy weather. Resistant hybrids and varieties to leaf blight are not very highly correlated with resistance to anthracnose stalk rot. Crop rotation and clean plowdown of crop residues. Balanced soil fertility.

ASCOCHYTA LEAF AND SHEATH SPOTS (ASCOCHYTA SPP)

‘‘‘Ascochyta Leaf and Sheath Spots (Ascochyta spp.) ‘‘‘ Lesions are brown to reddish purple and ellipsoidal, becoming elongated and somewhat irregular with brown margins. Ascochyta leaf and sheath spots have been reported from the USA.

AWL NEMATODES (DOLICHODORUS SPP)

‘‘‘Awl Nematodes (Dolichodorus spp.) ‘‘‘ These generally ectoparasitic nematodes (Dolichodorus spp.) cause devitalized, coarse roots and stunting similar to sting nematode injury.

BACTERIAL DISEASES

‘‘‘Bacterial Diseases: ‘‘‘ Bacteria comprise a group of some 4,000 procaryotic species, most of which lack chlorophyll and are saprophytic.

BACTERIAL LEAF BLIGHT (PSEUDOMONAS AVENAE)

‘‘‘Bacterial leaf blight (Pseudomonas avenae) ‘‘‘ Bacterial leaf blight has been reported throughout the world. Water-soaked linear lesions occur on the leaves as they emerge from the whorl. The pathogen is a Gram-negative, polar-flagellated rod approximately 0.6 X 1.6 µm. The bacterium attacks several other plants, including oats, foxtails, and Vasey grass.

BACTERIAL STALK ROT (ERWINIA CHRYSANTHEMI PV ZEAE)

‘‘‘Bacterial Stalk Rot (Erwinia chrysanthemi pv zeae) ‘‘‘ Primary symptoms generally appear in midseason when plants suddenly fall over. The bacterium is a short, Gram-negative rod, 0.6-0.9 X 0.8 - 1.7 µm, actively motile with peritrichous flagella, usually in pairs and rarely in short chains, noncapsulated, and nonacid-fast. The bacterium lives saprophytically on crop residue in the soil and invades maize plants through stomata, hydathodes, or wounds in the leaves or stalks. Bacterial stalk rot is most prevalent and destructive in areas with high rainfall, where plants are watered by sprinkler irrigation, and on land subject to flooding. Resistant hybrids and varieties. Good cultural management to avoid flooding.

BACTERIAL STRIPE AND LEAF SPOT (PSEUDOMONAS ANDROPOGONIS)

‘‘‘Bacterial Stripe/Leaf Spot (Pseudomonas andropogonis) ‘‘‘ Typical primary symptoms are amber-colored to olive-colored, oil- soaked, translucent lesions with parallel sides that tend to elongate and coalesce. The bacterium is a short rod with rounded ends, 0.5-0.8 X 1.5-2.5 µm, Gram-negative, nonacid-fast, and motile with a polar flagellum. Like other bacterial diseases, bacterial stripe and leaf spot becomes more severe under extended periods of warm, wet weather.

BANDED LEAF AND SHEATH SPOT (RHIZOCTONIA MICROSCLEROTIA AND PELLICULARIA FILAMENTOSA)

‘‘‘Banded Leaf/Sheath Spot (Rhizoctonia microsclerotia and Pellicularua filamentosa) ‘‘‘ Banded leaf and sheath spot produce large, gray, tan, or brown discolored areas alternating with dark bands on infected leaves and leaf sheaths.

BARREN STALKS AND RED LEAF

‘‘‘Barren Stalks/Red Leaf: ‘‘‘ Low fertility, drought, high plant populations, aphid or herbicide damage, chewed off silks, or poor timing of silking and pollen shed may all contribute to barren stalks or cobs without grain. Red leaf may be caused by a broken midrib or the accumulation of sugars and other photosynthetic products in leaves and sheaths.

BIRD INJURY

‘‘‘Bird Injury: ‘‘‘ This is more common in hybrids and varieties with erect ears and having exposed tips.

BLACK BUNDLE DISEASE (CEPHALOSPORIUM ACREMONIUM)

‘‘‘Black Bundle Disease (Cephalosporium acremonium) ‘‘‘ Infection becomes apparent when maize has reached the dough stage. The spores are nearly hyaline, one-celled, elliptical or oblong, straight or curved, 3.2-9.6 x 1.5-3.2 µm, and borne in groups in a slimy matrix at the apex of short conidiophores. The fungus is seedborne and soilborne.

BLACK KERNEL ROT (BOTRYODIPLODIA THEOBROMAE)

‘‘‘Black Kernel Rot (Botryodiplodia theobromae) ‘‘‘ During early stages of infection, kernels show a brown discoloration. Botryodiplodia theobromae Pat.

BORON DEFICIENCY

‘‘‘Boron Deficiency ‘‘‘ Boron (B) is rarely deficient in maize. Boron deficiency is most likely to occur on sandy, low-organic-matter soils. Drought enhances boron deficiency, and some symptoms of drought and boron deficiency, such as delayed ear-shoot and tassel emergence and poor pollination, may occur simultaneously and are often confused. Deficiency appears first on newly formed leaves as elongated water-soaked or transparent stripes scattered between leaf veins. Leaves become brittle, and the growing point often dies. Upper internodes fail to elongate. Because boron plays an important role in pollination, deficiency is manifest in poorly filled ears with missing rows. Boron deficiency is best corrected by broadcast applications of soluble boron salts. Rates are generally 0.5 to 1.0 lbs boron/acre. Corn is very sensitive to excess boron and band applications are not recommended.

BROWN STRIPE DOWNY MILDEW (SCLEROPHTHORA RAYSSIAE)

‘‘‘Brown Stripe Downy Mildew (Sclerophthora rayssiae) ‘‘‘ This local lesion disease can be very destructive and has been reported in limited maize-growing areas. Initially, lesions develop on the leaves as narrow, chlorotic or yellowish stripes, 3-7 mm wide, with well-defined margins and delimited by the veins. Sclerophthora rayssiae Kenneth, Kaltin, and Wahl var. zeae Payak & Renfro The oogonia are subglobose, thin-walled, hyaline to light straw- colored, and 33-44.5 µm in diameter. S. rayssiae overseasons as oospores in infected debris in the soil. The severity of brown stripe downy mildew is influenced by temperature and moisture. Cultural practices; plant before the rainy season begins. Resistant hybrids and varieties. Fungicides are promising as foliar sprays after symptoms appear or applied as a soil drench.

BUGGY-WHIP

‘‘‘Buggy Whip: ‘‘‘ The characteristic symptom of this disorder is the slight or complete fusion of the upper part of the plant into a tight tube that encloses the upper leaves and tassel. The disorder appears to be more prevalent in certain inbred lines grown in the central Corn Belt of the USA but is generally found only in small amounts in the field. Buggy-whip may be caused by some herbicides such as 2,4-D.

BURROWING NEMATODE (RADOPHOLUS SIMILIS)

‘‘‘Burrowing Nematode (Radopholus similis) ‘‘‘ This nematode (Radopholus similis) is widely distributed in tropical and subtropical regions around the world.

CALCIUM DEFICIENCY

‘‘‘Calcium Deficiency ‘‘‘ Calcium (Ca) is rarely deficient in maize, but may occur on acid soils and/or soils of very low cation exchange capacity. Deficiency is favored by soil pH below 5.5. Deficiency is characterized by stunted and distorted young leaves. Leaf tips stick to next lower leaf giving ladder-like appearance. The growing tip may dieback. Young leaves may show spotted necrotic areas. Root growth is greatly reduced and growing points on roots may be deformed and discolored. Calcium deficiency is normally corrected by liming acid soils. Where lime is not available or pH adjustment is not required, soluble calcium salts such as calcium chloride, calcium sulfate, or calcium nitrate may be applied. Calcium fertilization has been shown to enhance nitrogen efficiency when applied with ammonium nitrogen sources such as urea or UAN solutions. Calcium fertilization can reduce the effects of aluminum and manganese toxicity in acid soils when lime is not available.

CERCOSPORA AND GRAY LEAF SPOT (CERCOSPORA ZEAE-MAYDIS AND SORGHI PV MAYDIS)

‘‘‘Cercospora/Gray Leaf Spot (Cercospora zeae-maydis and sorghi pv maydis) ‘‘‘ This disease occurs in warm-to-hot, humid areas. Lesions on maturing maize leaves are pale brown or gray to tan, long (0.5 to 5 cm), narrow and rectangular, being characteristically restricted by the veins. The conidia are straight to slightly curved, hyaline, slender, and multiseptate. The causal fungi also attack barnyardgrass and Johnsongrass and other Sorghum spp. Hybrids differ greatly in their tolerance.

CHARCOAL ROT (MACROPHOMINA PHASEOLINA)

‘‘‘Charcoal Rot (Macrophomina phaseolina) ‘‘‘ Charcoal rot commonly attacks seedlings or plants approaching maturity. The minute sclerotia are black, numerous on diseased plant parts, and globular to irregular in shape. M. phaseolina overwinters and is disseminated as sclerotia. Soil temperatures near 37 °C are favorable for disease development, while either low soil temperatures or high soil moisture decreases severity. In irrigated areas, charcoal rot can be minimized by maintaining moist soils during dry periods after tasseling. Resistant hybrids and varieties. Balanced soil fertility; avoid high levels of N and low levels of K. Lower plant populations.

CHLORINE (CHLORIDE)

‘‘‘Chlorine (chloride) ‘‘‘ Chlorine (Cl) is an essential plant element but it exists in nature primarily as the chloride ion. Chloride is most-often deficient is soils that are not fertilized with chloride salts such as potassium chloride. Soils in coastal areas are rarely chloride deficient. Symptoms of chloride deficiency are not well defined for corn, but it is usually characterized by general yellowing accompanied by a wilted appearance. Chloride deficiency is corrected by application of soluble chloride salts such as potassium chloride, calcium chloride, or magnesium chloride at rates of about 20-60 lbs chloride/acre according to soil test.

CHOCOLATE SPOT (PSEUDOMONAS ATROFACIENS PV ZEAE)

‘‘‘Chocolate Spot (Pseudomonas atrofaciens pv zeae) ‘‘‘ This leaf blight disease occurs sporadically and has been reported in limited areas such as Wisconsis (USA). The bacterium is a Gram-negative rod, approximately 0.6 X 1.0-2.7 µm that may form short chains, produces fluorescent pigment in King's B medium containing glycerol, is motile by one to five polar flagella, and has an oxidative mode of glucose metabolism. Test soil for K-deficiency. Apply a K-containing fertilizer based on the soil test.

CLADOSPORIUM ROT (CLADOSPORIUM HERBARUM)

‘‘‘Cladosporium Rot (Cladosporium herbarum) ‘‘‘ Cladosporium kernel or ear rot is identified by dark, greenish- black, blotched or streaked kernels scattered over the ear. Conidiophores are erect, up to 250 µm long, smooth, sparsely branched, and pale olive-brown to brown.

COLLETOTRICHUM/ANTHRACNOSE TOP-DIEBACK/STALK ROT

‘‘‘Colletotrichum/Anthracnose Top-Dieback/Stalk Rot: ‘‘‘ Corn anthracnose has become much more prevalent. Stalk infections become evident at various stages of growth depending on the susceptibility of the inbred, hybrid, or variety. Occasionally, portions of the plant above the ear blanch and die four to six weeks after pollination while the lower portions of the plant remain green. Stalk symptoms generally appear after tasseling as narrow, vertical or oval, water-soaked lesions in the rind. The causal fungus overseasons as conidia in acervuli on maize leaf and stalk debris and as dormant resting mycelial cells in stalk debris. Resistant hybrids and varieties. Crop rotation, clean and deep plowdown of crop residues, and careful use of minimum or conservation tillage. Balanced soil fertility.

COMMON MAIZE RUST (PUCCINIA SORGHI)

‘‘‘Common Maize Rust (Puccinia sorghi) ‘‘‘ Pustules (sori) may appear on any aboveground part, being most abundant on the leaves. The uredospores are cinnamon brown, mostly spherical to broadly ellipsoidal, and 21-30 X 24-33 µm. Teliospores germinate in the spring in certain areas of the world to form basidia on which small, thin-walled, hyaline, haploid basidiospores are produced. Cool temperatures (16-23 °C) and high relative humidity (100%) favor rust development and spread. Aecial infection occurs infrequently in temperate regions around the world on Oxalis spp. Resistant hybrids and varieties. Fungicide applications, starting when pustules first appear on the leaves, may be feasible, especially in seed-production fields.

COMMON SMUT (USTILAGO MAYDIS)

‘‘‘Common Smut (Ustilago maydis) ‘‘‘ Common smut or boil smut occurs worldwide wherever maize is grown. Losses from this disease vary greatly from one area to another and may range from a trace to (rarely) 10% or more. All aboveground parts of the plant are susceptible, particularly young, actively growing, or meristematic tissues. The chlamydospores overwinter and germinate under favorable conditions to produce sporidia carried by air currents or splashed by water to young, developing tissues of maize. Development of common smut is favored by dry conditions and temperatures between 26 and 34 °C. Somewhat resistant hybrids and varieties. Avoid mechanical injuries to plants during cultivation and spraying. Maintain well-balanced soil fertility. In home gardens, remove and burn galls from infected plants before they rupture. Ears, tassels, leaves with gray gnarled growths that become powdery. Remove and destroy growths as soon as noticed; keep black powder in galls from getting into soil. Plant early; more common problem in later harvests. Resistant varieties available.

CONTROL OF NEMATODE DISEASES

‘‘‘Control of Nematode Diseases: ‘‘‘ Resistance or tolerance in maize is becoming available for only a few forms of nematodes. Most nematodes that attack maize have extremely wide host ranges, thus complicating control by crop rotation. Soil fumigants are sometimes economically feasible with certain high-value crops in light, well-aerated soils, particularly where damage is severe and crop rotation cannot be practiced.

CONTROL RECOMMENDATIONS/VARIOUS VIRAL DISEASES

‘‘‘Control Recommendations/Various Viral Diseases: ‘‘‘ Rotate crops excluding grasses and cereals. Use herbicides to eliminate Johnsongrass and other overwintering hosts. Apply timely insecticide applications to reduce vector populations. Plant resistant or tolerant lines (inbreds), hybrids, and varieties where viral diseases are troublesome. Plant early to avoid insect transmission of certain viruses, e.g., MDMV.

COPPER DEFICIENCY

‘‘‘Copper Deficiency ‘‘‘ Copper (Cu) is rarely deficient in corn. Deficiency usually occurs in organic, or peat and muck, soils and very sandy soils of low organic matter content. Copper deficiency is exhibited as yellowing of young leaves. Tips of leaves may wilt and die. Leaf streaking similar to iron chlorosis may be observed, and some necrosis of leaf margins similar to potassium deficiency may occur. Stalks may become soft and limp. Copper deficiency is prevented by application of soluble copper fertilizers, such as copper sulfate or copper EDTA. Fertilizer rates are generally about one lb Cu/acre. Rates of 2-3 lbs copper/acre may be effective for several years, but these rates should be broadcast not banded. Foliar sprays of copper sulfate or copper EDTA are also effective, but repeated applications may be necessary.

CORN EARWORM

‘‘‘Corn earworm: ‘‘‘ Range in color from green to almost black with lengthwise stripes of various colors. Worms up to 13/4-inches long eating down through kernels of corn. Prior to tasseling, worms found in whorl of plant, feeding on developing tassel. With a medicine dropper, apply mineral oil to the silk just inside the tip of each ear, 3 to 7 days after silks first appear. Use 20 drops per ear or Break off and discard wormy end of ear, Insecticides will not control worms that are inside the ear. Preventive treatments to silks are intended to kill worms before they enter ears.

CORN LETHAL NECROSIS

‘‘‘Corn Lethal Necrosis: ‘‘‘ Corn Lethal Necrosis has only been detected in limited areas with in the United States, namely Kansan and Nebraska, but has potential to become widespread. Corn lethal necrosis results from the synergistic interaction of maize chlorotic mottle virus (MCMV) in combination with different strains of either maize dwarf mosaic (MDMV-A and -B) or wheat streak mosaic virus (WSMV). Leaf necrosis progresses inward from the margins. Vectors include various beetle species for MCMV, aphid species for MDMV, and the wheat-curl mite for WSMV.

CORN STUNT AND MAIZE STUNT

‘‘‘Corn Stunt/Maize Stunt: ‘‘‘ This major disease limits production of maize throughout the tropical lowlands. The first symptoms are chlorosis of the margins of the whorl leaf followed by reddening of the tops of older leaves. A helical, motile, cell wall-free procaryote not yet assigned a Latin binomial but clearly a member of the genus Spiroplasma. In infected plants, the spiroplasma cells are found only in phloem tissue. Maize and its wild teosinte relatives (Zea perennis and Z. mays mexicana) are the only known hosts for CSS. Development of resistant hybrids and varieties appears to offer the best hope for control. Resistance is polygenic and additive in nature.

CRAZY TOP (SCLEROPHTHORA MACROSPORA)

‘‘‘Crazy Top (Sclerophthora macrospora) ‘‘‘ Crazy top occurs in most areas with temperate or warm-temperate climates and rarely in tropical areas. Symptoms vary greatly with time of infection and degree of host colonization by the fungus. Sporangia are hyaline, lemon-shaped, operculate, 60-100 X 30-65 µm, and attached to short, simple, hyphoid sporangiophores emerging from stomata. The mycelium of S. macrospora can be detected in brace roots, stalk tissue, leaves, and floral structures of affected plants by staining with zinc chloriodide. The oospores germinate in saturated soil to produce sporangia from which zoospores escape that penetrate host tissues. Unlike other sclerophthoras, the crazy top fungus develops systemically where soils have been flooded shortly after planting or before plants are in the four-leaf to five-leaf stage. S. macrospora has been reported on more than 140 species of the Gramineae. Provide adequate soil drainage or avoid planting in low, wet spots.

CURVALARIA LEAF SPOTS (CURVALARIA SPP)

‘‘‘Curvalaria Leaf Spots (Curvalaria spp.) ‘‘‘ Lesions are at first small (1-2 mm), straw-colored, circular to oval, with reddish-brown or dark brown margins. Curvahelminthophorium laria spp. Conidiophores are brown, septate, straight or flexuous, often bent, simple, or branched, and bear conidia sympodially. C. lunata (Wakker) Boed. var. aeria (Cochliobolus lunatus Nelson & Haasis) is by far the most prevalent species.

DAGGER NEMATODES (XIPHINEMA SPP)

‘‘‘Dagger Nematodes (Xiphinema spp.) ‘‘‘ In North America, Xiphinema americanum is one of the most common nematodes feeding on maize roots.

DIAGNOSIS MYCOTOXINS

‘‘‘Diagnosis mycotoxins: ‘‘‘ Few mycotoxins produce clinical signs in animals so characteristic that they permit unequivocal diagnosis.

DIDYMELLA LEAF SPOT (DIDYMELLA EXITIALIS)

‘‘‘Didymella Leaf Spot (Didymella exitialis) ‘‘‘ Didymella exitialis (Mor.) Muller causes small, elongate or elliptical leaf spots with brownish margins.

DIPLODIA EAR ROTS (DIPLODIA MAYDIS AND MACROSPORA)

‘‘‘Diplodia Ear Rots (Diplodia maydis and macrospora) ‘‘‘ The husks of early infected ears appear bleached or straw-colored. The causal fungus overseasons as conidia in acervuli on maize leaf and stalk debris and as dormant resting mycelial cells in stalk debris. Dry weather early in the season followed by wet conditions just before and after silking favors ear infection. Resistant hybrids and varieties. Early harvest. Proper storage (below 18% moisture initially for ears, 15% for shelled grain) will prevent further growth of the pathogen, but is not a control measure in the field.

DIPLODIA LEAF SPOT (DIPLODIA MACROSPORA)

‘‘‘Diplodia Leaf Streak (Diplodia macrospora) ‘‘‘ Diplodia macrospora Earle occurs as an ear-rotting and stalk-rotting pathogen, but in warm humid weather it can attack the leaves producing brown streaks, with characteristic yellow margins evident in the early stages.

DIPLODIA STALK ROT (DIPLODIA MAYDIS)

‘‘‘Diplodia Stalk Rot (Diplodia maydis) ‘‘‘ The disease commonly appears several weeks after silking. The fungus produces globose or flask-shaped pycnidia containing olive-colored to brown, elliptical, two-celled, straight to slightly curved spores, 5-6 X 25-30 µm in size. D. maydis overseasons as spores in pycnidia on debris and as spores or mycelium on the seed. Dry conditions early in the season and warm (28-30 °C), wet weather two to three weeks after silking favor development of Diplodia stalk rot. Resistant hybrids and varieties. Balanced soil fertility; avoid high levels of N and low levels of K. Lower plant populations.

DOWNY MILDEWS

‘‘‘Downy Mildews: ‘‘‘ Maize is attacked by at least nine species of three genera (Peronosclerospora, Sclerophthora, or Sclerospora). Symptoms caused by these fungi are somewhat similar and include chlorotic streaking, mottling, stunting, malformation of ears and tassels, and excessive tillering.

DROUGHT

‘‘‘Drought: ‘‘‘ Lack of moisture causes the upper leaves of young maize plants to roll upward and inward and appear dull.

EAR AND KERNAL ROTS

‘‘‘Ear/Kernal Rots: ‘‘‘ Maize is susceptible to a number of ear and kernel rots, some of which are widely distributed.

EARWIGS

‘‘‘Earwigs: ‘‘‘ Feed on silk and prevent pollination and thus lolling of kernels. Ears only partly filled, shortened silks, presence of earwigs on silks. Traps; check daily for earwigs and destroy.

EFFECTS OF ENVIRONMENTAL FACTORS

‘‘‘Effects of Environmental Factors: ‘‘‘ Conditions of light, temperature, humidity, soil moisture, and nutrition optimal for plant growth are also optimal for sensitivity to air pollutants.

ERGOT (CLAVICEPS GIGANTEA)

‘‘‘Ergot (Claviceps gigantea) ‘‘‘ This disease, also known as horse's tooth (diente de caballo), occurs in certain humid valleys of central Mexico. Sclerotia that replace the kernels are first white to cream-colored, 5-8 x 2-5 cm in size, soft, sticky, and hollow. The sclerotia are white-to-cream becoming grayish brown with age and have pink to lavender centers. The fungus overwinters as sclerotia on the ground or mixed with seed. Ergot is endemic in certain high, cool, humid valleys of central Mexico

EXCESS MOISTURE

‘‘‘Excess Moisture: ‘‘‘ The lower leaves of young plants in waterlogged soils turn yellow and die due to oxygen starvation when the roots are exposed to excess water.

EXPOSED EAR TIPS

‘‘‘Exposed Ear Tips: ‘‘‘ This condition is most common where extreme drought occurs at kernel set, followed by abundant rainfall and good growing conditions.

EXTRACTION AND IDENTIFICATION OF TOXINS

‘‘‘Extraction/Identification of Toxins: ‘‘‘ Evidence that a mycotoxin is responsible for illness in animals that consumed the feed requires that the toxin or toxins be isolated, purified, and quantified.

EYESPOT (KABATIELIA ZEAE)

‘‘‘Eyespot (Kabatielia zeae) ‘‘‘ Reported as brown-spot, eyespot has been found in several countries. Early symptoms consist of small, translucent, circular to oval lesions (1-4 mm in diameter) with yellowish halos that may fuse to form large necrotic areas. Epidemiology Only maize has been reported as a host for A. zeae. Certain inbred lines and hybrids appear to be less susceptible to the disease. Less susceptible hybrids and varieties. Clean plowing or crop rotation, or both, has reduced early disease infections.

FALSE SMUT (USTILAGINOIDEA VIRENS)

‘‘‘False Smut (Ustilaginoidea virens) ‘‘‘ False smut has been reported worldwide. False smut is characterized by galls (sclerotia) similar to ergot that replace the flowers on the tassels. Mature sclerotia are spherical, 4-15 mm in diameter, olive-green to black, and velvety with white interiors. Similar to ergot. False smut is favored by hot, wet weather.

FEEDING HABITS

‘‘‘Feeding Habits/Nematodes: ‘‘‘ All nematodes that attack maize are obligate parasites that must feed on living plants to complete their life cycle. Plant-parasitic nematodes are divided into two major groups according to their parasitic behavior. Ectoparasitic nematodes (e.g., Xiphinema and Macroposthonia) spend their life cycles outside the host and feed by inserting their stylets to varying depths within the root tissues. Nematodes damage the roots of maize in two major ways.

FERTILIZER BURN

‘‘‘Fertilizer Burn: ‘‘‘ Excessive amounts of soluble N or K fertilizer placed close to the seed may prevent germination.

FLUORIDES

‘‘‘Fluorides: ‘‘‘ Sweet maize exposed to fluorides shows a chlorotic mottle or flecking along the leaf margins and tips. Zinc or K deficiency, mite injury, genetic variation, or normal senescence cause symptoms resembling fluoride injury.

FROST

‘‘‘Frost: ‘‘‘ Young maize plants are sometimes killed by a hard frost or freeze.

FUNGAL DISEASES

‘‘‘Fungal Diseases: ‘‘‘ Fungi comprise a diverse group of lower plants that lack chlorophyll, usually have chitinous cell walls (cellulose in the oomycetes), and grow as saprophytes or parasites. Most fungi produce branched filaments (hyphae) up to 100 µm wide. Fungal hyphae invade a maize plant through wounds made by man, hail, wind, blowing sand, insects, nematodes, or other fungi; through natural openings; or by direct penetration using pressure or enzyme action or both. Fungi cause the majority of infectious diseases of maize including the rusts; smuts; downy mildews; most rots, spots, and blights; and deformations.

FUSARIUM KERNAL AND EAR ROT (FUSARIUM MONILIFORME AND MONILIFORME PV SUBGLUTINANS)

‘‘‘Fusarium Kernal/Ear Rot (Fusarium moniliforme and moniliforme pv subglutinans) ‘‘‘ A salmon-pink to reddish-brown discoloration first appears on the caps of individual kernels or groups of kernels scattered over the ear. The fungi develop on crop residue in or on the soil surface. Disease development and spread are favored by dry, warm weather. Resistant hybrids and varieties. Balanced soil fertility; avoid high levels of N and low levels of K. Lower plant populations.

FUSARIUM LEAF SPOT

‘‘‘Fusarium Leaf Spot: ‘‘‘ Field symptoms appear as water-soaked spotting mainly on the whorl leaves.

FUSARIUM STALK ROT (FUSARIUM MONILIFORME)

‘‘‘Fusarium Stalk Rot (Fusarium moniliforme) ‘‘‘ This rot is difficult to distinguish from Gibberella stalk rot. Both fungi produce asexual spores on mycelium, which appears as a cottony-pink growth on the leaf sheaths and at the nodes. The perithecia are globose, smooth, and blue-black. The macroconidia of F. moniliforme var. subglutinans are slightly less curved than those of F. moniliforme and usually are three-septate. The fungi develop on crop residue in or on the soil surface. Dry conditions early in the season and warm (28-30 °C), wet weather two to three weeks after silking favor development of Fusarium stalk rot. Resistant hybrids and varieties. Balanced soil fertility; avoid high levels of N and low levels of K. Lower plant populations.

GENERAL EPIDEMIOLOGICAL FEATURES AND FUNGAL STALK ROTS

‘‘‘General Epidemiological Features/Fungal Stalk Rots: ‘‘‘ Fungal stalk rots are generally more severe when N is in excess in relation to K. Leafblights, hail, or insect damage reduce leaf area and increase the potential for stalk rot. Evidence shows that maize plants with two ears are the first to be infected. Insects (e.g., European corn borer, southern and northern corn rootworms) may aid in the development of stalk rots by carrying inoculum into tissues, causing wounds through which pathogens enter stalks or roots, and reducing the photosynthetic area. Genetic resistance involves multiple genes, some with major effects. Early maturing hybrids are generally more susceptible than full-season varieties. High plant populations and narrow rows increase stalk rot potential. Severe stalk rots may involve several organisms. Select resistant hybrids and varieties adapted to the area. Plant sound, disease-free seed treated with a fungicide. Attain balanced soil fertility, especially between N and K, by applying fertilizer based on soil test results. Adjust the plant Population to the particular hybrid or variety, the fertility level, soil type, and available soil moisture in the field. Crop rotation and a clean plowdown of crop residues may reduce stalk rot in some cases. Control root-attacking and stalk-attacking insects using approved resistant varieties, cultural practices, and chemicals where needed and feasible. Harvest when the crop is mature (30% grain moisture) to prevent losses from lodging. Where practical, irrigate during droughts until 50-55 days after flowering.

GENERALIZED LIFE CYCLE

‘‘‘Generalized Life Cycle: ‘‘‘ Life cycles of most plant-parasitic nematodes, although varying in details, are simple and direct.

GENETIC STRIPE LEAF SPOTS FLECKS

‘‘‘Genetic Stripe Leaf Spots Flecks: ‘‘‘ Bright yellow to white stripes with smooth margins running the length of the leaf signify genetic stripe.

GIBBERELLA OR RED EAR ROT (GIBBERELLA ZEAE)

‘‘‘Gibberella/Red Ear Rot (Gibberella zeae) ‘‘‘ A reddish mold, often beginning at the ear tip, is the characteristic sign of Gibberella ear rot. The perithecia are bluish black, spherical, and borne superficially on diseased stalks. The macroconidia of F. roseum 'Graminearum' are hyaline, curved, and tapering toward the tips, three- to five-septate, and 4-6 x 30-60 µm. Perithecia on infected maize stalks mature under warm, wet conditions. Cool, wet weather within three weeks of silking favors development of this disease. Resistant hybrids and varieties. Early harvest. Proper storage (below 18% moisture initially for ears, 15% for shelled grain) will prevent further growth of the pathogen, but is not a control measure in the field.

GIBBERELLA STALK ROT (GIBBERELLA ZEAE)

‘‘‘Gibberella Stalk Rot (Gibberella zeae) ‘‘‘ Leaves on early infected plants suddenly turn a dull grayish-green while the lower internodes soften and turn tan to dark brown. The perithecia are bluish black, spherical, and borne superficially on diseased stalks. The macroconidia of F. roseum 'Graminearum' are hyaline, curved, and tapering toward the tips, three- to five-septate, and 4-6 x 30-60 µm. Perithecia on infected maize stalks mature under warm, wet conditions. Dry conditions early in the season and warm (28-30 °C), wet weather two to three weeks after silking favor development of Gibberella stalk rot. Resistant hybrids and varieties. Balanced soil fertility; avoid high levels of N and low levels of K. Lower plant populations.

GOSS' BACTERIAL WILT OR BLIGHT

‘‘‘Goss' Bacterial Wilt/Blight: ‘‘‘ This disease is sometimes called leaf freckles and wilt and has been found only in the United States and has resulted in up to 50% loss in yield. The gross morphological symptoms (wilt and leaf blight) are indistinguishable from Stewart's bacterial wilt. The bacterium is a nonmotile, nonflagellate, Gram-positive, pleomorphic rod, averaging 0.5-2.5 µm. The pathogen overwinters in infected maize leaves, stalks, cobs, and ears on or near the soil surface. The pathogen infects dent, sweet, flint, and popcorn maize varieties and has been isolated from natural infections of green foxtail and shattercane. Resistant dent and sweet maize hybrids are available. Crop rotation. Selection of seed from noninfested areas. Deep-plowing of infected maize debris, preferably after harvest.

GRAMINICOLA DOWNY MILDEW (SCLEROSPORA GRAMINICOLA)

‘‘‘Graminicola Downy Mildew (Sclerospora graminicola) ‘‘‘ This disease, also called green ear, occurs on various grasses throughout the world. Grayish blotching and mottling of leaves with a white, arachnoid to downy growth developing on discolored areas. Sclerospora graminicola (Sacc.) Schroet. Sporangiophores are long (av. 268 µm), nonseptate, and stalklike with a slightly bulbous foot. Primary infection occurs from oospores that overwinter in the soil. Temperatures of 24-32 °C for two days after sowing are conducive to heavy infection.

GRAY EAR ROT (BOTRYOSPHAERIA ZEAE)

‘‘‘Gray Ear Rot (Botryosphaeria zeae) ‘‘‘ The grayish-white mold on and between the kernels near the ear base resembles that of Diplodia ear rots. Perithecia and pycnidia occur in large lesions on corn leaves and occasionally on tassel necks and under the uppermost leaf sheath. The pycnidia of M. zeae, which may develop in the same lesion with the perithecia, are submerged, black, globose, and protruding with short necks. The fungus overseasons on infected leaves, and the spores mature the following growing season. Extended periods of warm-to-hot, wet weather for several weeks after silking favor disease development. Adapted, resistant hybrids and varieties.

HAIL

‘‘‘Hail: ‘‘‘ Hail damage is easily recognizable by shredded leaves hanging in tatters.

HEAD SMUT (SPHACELOTHECA REILIANA)

‘‘‘Head Smut (Sphacelotheca reiliana) ‘‘‘ Head smut occurs periodically in the deltas and intermountain valleys around the world. Head smut first appears when ears and tassels are formed. Sphacelotheca reiliana (Kuhn) Clint. (Syn. Sorosporium reilianum (Kuhn) McAlp., Ustilago reiliana Kuhn) S. reiliana is primarily soilborne. The infection level is related to the concentration of teliospores in the soil. Resistant hybrids and varieties. Chemical treatment of soil in rows at or before planting time may be feasible in seed-production fields. Sanitation and crop rotation. Seed treatment.

HELMINTHOSPORIUM AND NORTHERN LEAF SPOT (HELMINTHOSPORIUM CARBONUM)

‘‘‘Helminthosporium/Northern Leaf Spot (Helminthophorium carbonum) ‘‘‘ Lesions of Race 1 are tan, oval to circular, usually with concentric zones and measure 1.2 x 2.5 cm. The conidia are golden yellow to dark olive-brown, curved, long- elliptical to spindle-shaped, with rounded ends, 2 to 12 septate, 7- 18 X 25-100 µm. Similar to northern and southern leaf blights. Moderate temperatures and high relative humidity favor the disease. There are at least three races of H. carbonum.

HELMINTHOSPORIUM LEAF DISEASE OR ROSTRATUM (HELMINTHOSPORIUM ROSTRATUM)

‘‘‘Helminthosporium Leaf Disease/Rostratum (Helminthosporium rostratum) ‘‘‘ Lesions appear on mature plants as small, pale yellow, elongated spots 1-2 x 2-5 mm that gradually extend to form longitudinal bands or stripes between the veins. Helminthosporium rostratum Drechs. (Syn. Exserohilum rostratum (Drechs.) Leonard & Suggs amended Leonard, Bipolaris rostrata (Drechs.) Shoemaker, Drechslera rostrata (Drechs.) Richardson & Fraser, Exserohilum halodes (Drechs.) Leonard & Suggs.) Perfect state is Setosphaeria rostrata Leonard. Similar to northern and southern leaf blights. Temperature and relative humidity are important factors in disease development. H. rostratum occurs also on the leaves of lovegrasses, sorghums, Sudangrass, pearl millet, and several other grass hosts. Resistant hybrids and varieties.

HIGH TEMPERATURES

‘‘‘High Temperatures: ‘‘‘ High temperatures (38-52 °C) cause scalding of maize leaf tissues, usually associated with low soil moisture and hot, dry winds.

HOLCUS SPOT

‘‘‘Holcus Spot: ‘‘‘ Round to elliptical spots, ranging from 2 to 10 mm in diameter, appear toward the tips of lower leaves. The pathogen is a short rod with rounded ends, 0.6-1.2 x 1.5-3 or more µm (av. 0.73 x 2.13 µm), Gram-negative, and motile with one to several polar flagella, sometimes in long chains. Bacteria overwinter in crop residue and invade the host through stomata. Resistant hybrids and varieties. Crop rotation.

HYALOTHYRIDIUM LEAF SPOT (HYALOTHYRIDIUM SPP)

‘‘‘Hyalothyridium Leaf Spot (Hyalothyridium spp.) ‘‘‘ This disease is prevalent in several Latin American countries. Leaf spots range from small, tan, elliptical spots with brown borders to large, almost circular, light brown blotches that enlarge up to 1.5 cm in diameter, sometimes so numerous that most of the leaf surface is covered. Hyalothyridium sp. Pycnidia are immersed, globose, thin-walled, and subhyaline to pale brown.

IDENTIFYING RUSTS

‘‘‘Identifying Rusts: ‘‘‘ Puccinia sorghi can be differentiated from P. polysora by the larger, more sparsely distributed and elongated uredia; darker uredospores; erumpent telia; and teliospores with thicker walls, longer pedicels, and a bullet-shaped terminal cell.

IRON

‘‘‘Iron: ‘‘‘ Iron chlorosis, the name given to iron deficiency symptoms, may occur in corn grown on high pH soils containing free calcium carbonate. Iron availability decreases rapidly as soil pH increases. Water-logging of the soil worsens the symptoms, but the effect may be temporary and disappear as the soil dries. Iron chlorosis typically occurs on the new leaves as a distinctive yellow striping of the tissue between veins while the veins remain green. Severely chlorotic leaves may be almost white and have necrotic areas. The best method of prevention is variety selection. So-called iron-efficient cultivars are less susceptible to chlorosis and should be planted if chlorosis has been observed previously and soil conditions are conducive to development of deficiency. Soil applications of iron fertilizers are generally ineffective in correcting iron deficiency, but banding of high rates (75-100 lbs/acre) of ferrous sulfate in the seed row have been shown to be effective. The most effective corrective treatment is foliar sprays of soluble iron materials, such as iron chelates or ferrous sulfate. A 0.5 to 1.0% solution of ferrous sulfate with a surfactant sprayed in enough water to wet the leaves (about 20 gals/acre) is effective. Multiple applications at 7-14 day intervals may be necessary.

JAVA DOWNY MILDEW (PERONOSCLEROSPORA MAYDIS)

‘‘‘Java Downy Mildew (Peronosclerospora maydis) ‘‘‘ This systemic downy mildew is of great importance because reductions in yield of 40% may occur. White-to-yellow streaking, followed by necrosis and browning of the streaks, are characteristic leaf symptoms. The pathogen produces two kinds of hyphae-one is straight and sparsely branched; the other lobed, irregularly branched, and clustered. Infected maize plants grown during the dry season are the primary source of inoculum in Indonesia. Conidial formation in P. maydis requires free moisture, darkness, and temperatures below 24 °C. Resistant hybrids and varieties. Systemic fungicide seed treatments.

KERNAL RED STREAK

‘‘‘Kernal Red Streak: ‘‘‘ Red streaking of the kernel pericarp is a symptom of kernel red streak (KRS).

LANCE NEMATODES (HOPLOLAIMUS SPP)

‘‘‘Lance Nematodes (Hoplolaimus spp.) ‘‘‘ At least five species of the large, robust lance nematodes (Hoplolaimus spp.) have been associated with maize.

LATE WILT (CEPHALOSPORIUM MAYDIS)

‘‘‘Late Wilt (Cephalosporium maydis) ‘‘‘ The first symptom of late wilt, which has only been reported from Egypt and India, is moderately rapid wilting of the leaves beginning at tasseling time. Cephalosporium maydis Samra, Sabet & Hingorani The conidiophores are 30-250 µm long, mostly branched, with conidia forming successively and exogenously in the heads. C. maydis is primarily soilborne and may infect maize through the roots or mesocotyl. C. maydis infects maize seedlings more easily than older plants. Resistant hybrids and varieties. Rotation with rice or sesame. Balanced soil fertility based on a soil test.

LEPTOSPHAERIA LEAF SPOT (LEPTOSPHAERIA SPP)

‘‘‘Leptosphaeria Leaf Spot (Leptosphaeria spp.) ‘‘‘ Leptosphaeria sp. produces small tan-to-cream lesions on the leaves that become large and concentric or streaked, covering large areas of the leaves.

LIGHTNING

‘‘‘Lightning: ‘‘‘ Lightning injury is rare on corn.

LOW-TEMPERATURE SUNSCALD AND COLD BANDING

‘‘‘Low Temperature Sunscald and Cold Banding: ‘‘‘ Injury occurs when chilly(5-15 °C), clear, dewy nights are followed by clear, sunny mornings.

MAGNESIUM DEFICIENCY

‘‘‘Magnesium Deficiency ‘‘‘ Magnesium deficiency normally occurs on acid soils of low magnesium limed with calcitic limestone. High soil K or high rates of fertilizer K can induce magnesium deficiency, especially in low magnesium soils. Symptoms normally occur as interveinal chlorosis on the lower leaves. Small necrotic spots may also develop giving the appearance of spotted striping. Symptoms are similar to manganese deficiency except, because magnesium is mobile in the plant, the symptoms appear first on older leaves. As severity increases leaf tips, edges and underside may become reddish-purple and eventually die. Magnesium deficiency is prevented by liming with dolomitic limestone. On high pH soils, or where dolomite is not available, soluble salts of magnesium are effective. Magnesium sulfate or potassium-magnesium sulfate is commonly used.

MAIZE BUSHY STUNT

‘‘‘Maize Bushy Stunt: ‘‘‘ The disease is thought by some researchers to be the same as or closely related to Mesa Central corn (maize) stunt. The first symptom is chlorosis of the margins of the whorl leaves followed by intense reddening of the tips of older leaves, although some maize varieties do not redden. A mycoplasmalike organism (MLO) has been found associated with maize bushy stunt (and Mesa Central stunt) disease. The MLO is found only in the phloem of infected plants. Several leafhopper species transmit the MLO but only D. maidis and D. elimatus are efficient vectors. No control measures have been developed.

MAIZE CHLOROTIC DWARF (MCDV)

‘‘‘Maize Chlorotic Dwarf (MCDV) ‘‘‘ The earliest symptom caused by maize chlorotic dwarf virus (MCDV) is a chlorosis of the young leaves in the whorl. Double infections of MCDV and MDMV do occur in some areas. MCDV is transmitted by the leafhopper Graminella nigrifrons (Forbes) and G. (Deltocephalus) sonora (Ball) in a semipersistent manner. The isometric (31 nm in diameter) maize chlorotic dwarf virion contains RNA.

MAIZE CHLOROTIC MOTTLE (MCMV)

‘‘‘Maize Chlorotic Mottle (MCMV) ‘‘‘ Maize chlorotic mottle virus (MCMV) produces fine chlorotic streaks in the youngest leaves 7-10 days after inoculation. The virus can be transmitted mechanically and by six economically important leaf beetles (Chrysomelids): Oulema melanopa L. (cereal leaf beetle), Chaetocnema pulicaria Melsheimer (corn flea beetle), Systema frontalis (Fab.) (flea beetle), Diabrotica undecimpunctata Mannerheim (southern corn rootworm), D. longicornis (Say.) (northern corn rootworm), and D. uirgifera LeConte (western corn rootworm). Infections by MCMV cause a mild disease in dent and flint maize types (floury and sweet are more susceptible) but mixed infections with maize dwarf mosaic or wheat streak mosaic result in a severe disease described as corn lethal necrosis.

MAIZE DWARF MOSAIC (MDMV)

‘‘‘Maize Dwarf Mosaic (MDMV) ‘‘‘ Symptoms are highly variable and the following description depicts frequently seen effects. MDMV is commonly confused with maize stunt and maize bushy stunt. Maize dwarf mosaic virus (MDMV) is a long, flexuous, rod-shaped virion 12-15 x 750 nm that is transmitted mechanically.

MAIZE MOSAIC (MMV)

‘‘‘Maize Mosaic (MMV) ‘‘‘ Symptoms of this disease (also called enanismo rayado, corn stripe, and sweet corn mosaic) appear as chlorotic spots, short lines, and broken to nearly continuous fine-to-broad chlorotic stripes centered on the secondary or tertiary veins. Maize mosaic virus (MMV) is transmitted in a persistent manner by the corn planthopper, Peregrinus maidis (Ashmead). When maize mosaic and maize streak occur in the same locality, the two diseases can be confused because of similarity of symptoms.

MAIZE RAYADO FINO OR FINE STRIPING DISEASE (MRFV)

‘‘‘Maize Rayado Fino/Fine Striping Disease: ‘‘‘ Early symptoms consist of chlorotic dots, spots, or' short lines distributed more or less uniformly over the youngest leaf and centered on the secondary and tertiary veins. Maize rayado fino virus (MRFV) is transmitted in a persistent manner by the leafhopper Dalbulus maidis (Delong & Wolcott), D. elimatus (Ball), Graminella nigrifrons (Forbes), Baldulus tripsaci Kramer and Whitcomb, and Stirellus bicolor (Van Duzee), following a latent period of 7 to 22 days. Maize Colombian stripe (del rayado Columbiana del maiz) and Brazilian corn streak are diseases of maize caused by strains of MRFV.

MAIZE ROUGH DWARF (MRDV)

‘‘‘Maize Rough Dwarf (MRDV) ‘‘‘ Maize rough dwarf virus (MRDV) dwarfs all parts of infected plants, inducing small swellings (galls) on the veins on the lower surfaces of young leaves.

MAIZE STREAK (MSV)

‘‘‘Maize Streak (MSV) ‘‘‘ Initial symptoms consist of minute, round, cream-to-white scattered spots on the youngest leaves. Maize streak virus (MSV) is transmitted by five leafhopper species of the genus Cicadulina of which C. mbila (Naude) is the most important.

MAIZE STRIPE (MStpV)

‘‘‘Maize Stripe (MStpV) ‘‘‘ The first symptoms of maize stripe, caused by the maize stripe virus (MStpV), are numerous chlorotic spots and narrow streaks that appear at the base and extend outward on the youngest leaves. MStpV is transmitted in a persistent manner by the corn planthopper, Peregrinus maidis.

MAIZE WALLABY EAR (MWEV)

‘‘‘Maize Wallaby Ear (MWEV) ‘‘‘ Symptoms of maize wallaby ear disease occur in two forms: mild consisting of ephemeral galls and enations followed by plant recovery, and severe with numerous galls on most or all secondary veins, severe stunting, and no grain yield. Severely affected plants have upright dark green leaves with edges rolled upward and inward that stand out stiffly at right angles to the stalk.

MANGANESE DEFICIENCY

‘‘‘Manganese Deficiency ‘‘‘ Corn has a relatively low manganese requirement and deficiencies are rare. Manganese deficiency is favored by high soil pH, low soil manganese levels, and sandy soils high in organic matter, such as peat or muck soils. Symptoms are vague but are generally characterized by interveinal chlorosis of the young leaves similar to iron deficiency. Manganese deficiency is best prevented by band applications of manganese sulfate. Banding manganese with an acid fertilizer such as ammonium sulfate improves manganese availability. Broadcast applications of manganese on high pH soils are not recommended. Manganese EDTA applications on high pH soils are not consistently effective and in some cases have actually worsened the deficiency. Insoluble manganese fertilizers such as manganese oxide are not effective. Manganese deficiency detected early in the season may be corrected with foliar sprays of manganese sulfate or manganese EDTA. Multiple applications may be necessary.

MINIIMIZING MYCOTOXINS PRODUCTION AFTER HARVEST

‘‘‘Minimizing Mycotoxin Production after Harvest: ‘‘‘ Harvest as soon as the moisture content allows minimum grain damage (usually 24 to 26% moisture). Adjust the harvesting equipment for minimum kernel damage and maximum cleaning. Dry the shelled grain to at least 15% moisture as rapidly as possible but not longer than 24 to 48 hr after harvest. Cool the grain after drying and maintain dry storage conditions. Clean the grain and bins thoroughly before storage to remove dirt, dust and other foreign matter,crop debris, chaff, and cracked or broken kernels. Store in watertight structures that are free from insects and rodents. Continue periodic aeration and probing for hot spots at intervals of one to four weeks throughout the storage period. Several organic acids-propionic, isobutyric, acetic, and mixtures of these with ammonium isobutyrate are registered for use on high- moisture grain in storage and sold under various trade names.

MISCELLANEOUS NEMATODES

‘‘‘Miscellaneous Nematodes: ‘‘‘ Cyst nematodes (Heterodera and Punctodera spp.), cystoid nematodes (Meloidodera spp.), pin nematodes (Paratylenchus spp.), and reniform nematodes (Rotylenchus spp.) have occasionally been found parasitizing maize or are frequently associated with its roots.

MOSAIC VIRUS

‘‘‘Mosaic virus: ‘‘‘ Mottled leaves, poor or slow growth. Some death of leaf tissue along margin. No control. All varieties can become infected but certain ones are more tolerant and produce well even when infected.

MOYLBDENUM DEFICIENCY

‘‘‘Molybdenum Deficiency ‘‘‘ Molybdenum is rarely, if ever, deficient in maize. If deficiency occurs, it is likely to be seen only in extremely acid soils. Leaves may die at the tip, along the margins, and between veins. In some cases symptoms may resemble nitrogen deficiency. Molybdenum deficiency is best prevented by liming acid soils. Because of the extremely small amounts of molybdenum needed by plants, seed treatments of molybdenum fertilizer are the best method of assuring adequate distribution.

MYCOPLASMA DISEASES

‘‘‘Mycoplasma Diseases: ‘‘‘ Mycoplasmas and spiroplasmas are minute, filterable bacteria in the class Mollicutes. Evidence from electron microscopy and antibiotic treatments indicates that organisms closely resembling classical mycoplasmas are involved in many plant diseases previously thought to be caused by viruses. Cell wall-less procaryotes (mycoplasmas and spiroplasmas) have thus far been found associated with only three (two?) diseases of maize, Rio Grande maize stunt, Mesa Central maize stunt, and maize bushy stunt.

MYCOTOXINS AND MYCOTOXICOSES

‘‘‘Mycotoxins/Mycotoxicoses: ‘‘‘ Mycotoxins are fungal metabolites that are toxic when consumed by animals or man. Aflatoxin B1, present in corn, is among the most potent of known carcinogens. Aflatoxin has been implicated in primary liver cancer in humans. Three genera of fungi-Aspergillus, Penicillium, and Fusarium (Gibberella)-are most frequently involved in cases of mycotoxin contamination in corn.

NEEDLE NEMATODES (LONGIDORUS SPP)

‘‘‘Needle Nematodes (Longidorus spp.) ‘‘‘ These nematodes (Longidorus spp.) can be among the most devastating pathogens of maize.

NEMATODE DISEASES

‘‘‘Nematode Diseases: ‘‘‘ Nematodes, also called nemas or eelworms, are unsegmented roundworms that inhabit fresh and salt water, decaying organic matter, soil, plants, and animals throughout the world. Plant-parasitic nematodes are typically microscopic, transparent, mobile, and vermiform. Passive movement of nematodes occurs in water, soil, and infected plant parts. Nematodes are extracted from soil by various flotation, sieving, and centrifugation procedures and from roots by incubation techniques. Most plant-parasitic nematodes become inactive at temperatures below 10 °C and over 35 °C. More than 100 species of plant-parasitic nematodes have been reported to feed on or are associated with the roots of maize. Virtually all fields will contain more than one species of plant-parasitic nematode. The nematode genera Longidorus, Xiphinema, Trichodorus, and Paratrichodorus transmit viruses to various crop plants.

NIGROSPORA EAR OR COB ROT (NIGROSPORA ORYZAE)

‘‘‘Nigrospora Ear/Cob Rot (Nigrospora oryzae) ‘‘‘ Symptoms are not conspicuous until the ears are harvested. Nigrospora oryzae (Berk. & Br.) Petch. (Syn. Basisporium gallarum Moll.). Perfect state is Khuskia oryzae. Spores are black, spherical to ovoid, 10-16 µm (mostly 12-14 µm) in diameter, and borne singly on short, branched conidiophores, 3-7 µm thick. The fungus overseasons on plant debris in the field, particularly in undeveloped secondary ears. Maize grown in infertile soil appears to be more susceptible than that grown in fertile soil, possibly because poor nutrition causes premature dying. Full-season, adapted hybrids and varieties resistant to stalk rots and leaf blights. Balanced soil fertility. Control of root-feeding and stalk-feeding insects. Resistant hybrids and varieties. Early harvest. Proper storage (below 18% moisture initially for ears, 15% for shelled grain) will prevent further growth of the pathogen, but is not a control measure in the field.

NITROGEN DEFICIENCY

‘‘‘Nitrogen Deficiency ‘‘‘ Nitrogen deficiency in young plants is characterized by overall stunted, spindly appearance and pale, yellow-green foliage. In older plants lower leaves show a characteristic V-shaped yellowing from the tip down the leaf to the midrib. Yellowing progresses to firing of the leaves and premature death. The entire plant becomes progressively more yellow as severity increases. Ears are small and poorly filled at the tip. Low grain protein may also indicate nitrogen deficiency. Nitrogen deficiency may occur on all soils. Heavy rain or irrigation can leach available soil nitrogen and induce deficiency. Sandy soils of low organic matter are most prone to nitrogen deficiency. Nitrogen deficiency is prevented by applying nitrogen fertilizer at appropriate rates depending on soil, environment, and yield level.

NORTHERN OR TURCICUM LEAF BLIGHT (HELMINTHOSPORIUM TURCICUM)

‘‘‘Northern/Turcicum Leaf Blight (Helminthosporium turcicum) ‘‘‘ Long, elliptical, grayish-green or tan lesions ranging from 2.5 to 15 cm in length develop first on the lower leaves. T. turcica, the sexual stage, occurs rarely, if ever, in nature but produces black, globose pseudothecia in the laboratory. Two biotypes of H. turcicum from maize isolates have been described. H. turcicum overwinters as mycelia and conidia in infected leaves, husks, and other plant parts, except in the northernmost areas where maize can be grown. Northern corn leaf blight occurs sporadically in most humid areas of the world where maize is grown. H. turcicum attacks sorghum, Sudangrass, Johnsongrass, gamagrass, and teosinte. At least two types of resistance are known: small lesion size and few lesions (polygenic), and chlorotic lesions with little or no sporulation and yellowish halos (monogenic). Resistant hybrids and varieties. Where practical in seed-production fields or on sweet corn for fresh market, fungicides

NUTRIENT DEFICIENCIES

‘‘‘Nutrient Deficiencies: ‘‘‘ Nutrient deficiencies are difficult to diagnose from visual symptoms alone since adverse weather, disease, insect, and various other injuries or other factors may cause similar manifestations.

NUTRIENT EXCESSES

‘‘‘Nutrient Excesses: ‘‘‘ In highly acid, aluminum and iron oxides may accumulate and cause internal necrosis near shoot apices or in vascular bundles in the nodes. Manganese toxicity may occur in very acid soils of high manganese content. Boron toxicity may occur if corn follows crops fertilized with high boron rates.

OCHRATOXINS(NEPHROTOXINS)

‘‘‘Ochratoxins (Nephrotoxins): ‘‘‘ Ochratoxins, produced primarily by Aspergillus ochraceus and Penicillium viridicatum, cause listless-ness, huddling, diarrhea, tremors, and other neural abnormalities in broiler poultry.

OTHER BACTERIAL LEAF SPOTS OR BLIGHTS

‘‘‘Other Bacterial Leaf Spots/Blights: ‘‘‘ Several bacteria occur on maize leaves causing water-soaked, elliptical, or irregular lesions that may coalesce to form large necrotic areas.

OTHER EAR ROTS

‘‘‘Other Ear Rots: ‘‘‘ Several species of Aspergillus and Rhizopus on maize ears have been described. Helminthosporium maydis, H. rostratum, H. carbonum and other species may infect maize ears.

OTHER HERBICIDE INJURIES

‘‘‘Other Herbicide Injuries: ‘‘‘ Other preemergence or post-emergence herbicides used at excessive rates, or chemicals to which maize has a low tolerance, may cause abnormal color, leaf spots, twisted or malformed shoots or roots, plants that never emerge, and stunting.

OTHER STALK INFECTIONS

‘‘‘Other Stalk Infections: ‘‘‘ Numerous other fungi cause maize stalk infections, but are normally of little economic importance. Physoderma maydis Miy., the causal agent of Physoderma brown spot, also produces water-soaked lesions on maize stalks beneath the leaf sheath. Other fungi are generally considered secondary invaders although some can cause stalk rots following artificial inoculations.

OZONE

‘‘‘Ozone: ‘‘‘ Symptoms on sweet maize are dull, gray-green, water-soaked lesions that develop into tan to white necrotic areas on both leaf surfaces. Ozone injury may be confused with insect or mite damage, chlorine or fluoride injury, or natural senescence.

PENICILLIUM ROT (PENICILLIUM SPP)

‘‘‘Penicillium Rot (Penicillium spp.) ‘‘‘ Penicillium rot occurs primarily on ears injured mechanically or by insects. Penicillium oxalicum Currie & Thom is the most common, although other species, including P. chrysogenum, P. glaucum, and P. funiculosum, may commonly occur. Penicillium species are soil-inhabiting fungi with long conidiophores that branch in a broomlike fashion.

PERONOSCLEROSPORA MISCANTHI

‘‘‘Peronosclerospora miscanthi: ‘‘‘ Peronosclerospora miscanthi (T. Miyake) C.G. Shaw (Syn. Sclerospora miscanthi T. Miyake) has been used to inoculate maize but has not been found occurring naturally on this host.

PEROXYACETYL NITRATE

‘‘‘Peroxyacetyl Nitrate: ‘‘‘ Maize is generally considered resistant to peroxyacetyl nitrate (PAN) injury.

PHAEOSPHAERIA LEAF SPOT (PHAEOSPHAERIA MAYDIS)

‘‘‘Phaeosphaeria Leaf Spot (Phaeosphaeria maydis) ‘‘‘ Lesions are initially small, pale green or chlorotic, becoming bleached or dried with dark brown margins. The perithecia are spherical to subglobose with papillate ostioles. The fungus persists in diseased plant parts in the field. The disease is favored by high rainfall and relatively low night temperatures.

PHILIPPINE DOWNY MILDEW (PERONOSCLEROSPORA PHILIPPINENSIS)

‘‘‘Philippine Downy Mildew (Peronosclerospora philippinensis) ‘‘‘ This is the most serious downy mildew disease in The Philippines where yield losses range from 15 to 40% or more. Systemic symptoms may appear in the first true leaf as a complete chlorosis or chlorotic stripes nine days after planting while local symptoms may appear when plants are in the two-leaf to three-leaf stage until tassels and silks are formed. The mycelium is branched, slender (8 µm in diameter), irregularly constricted, and inflated. P.philippinensis penetrates maize leaves through stomata by germ tubes from germinating conidia. Night temperatures of 21-26 °C and free moisture are essential for conidial production, germination, and infection. Resistant hybrids and varieties. Sanitation; rogue and destroy infected plants. Systemic fungicide seed treatment and fungicide-oil sprays are applied in some areas.

PHOSPHORUS DEFICIENCY

‘‘‘Phosphorus Deficiency ‘‘‘ Phosphorus deficiency is most commonly observed on soils with insufficient phosphorus. Very acid and alkaline soils are most conducive to deficiency. Phosphorus deficiency may also be induced by cold soil temperatures and soils which are too wet or too dry. Root growth restrictions caused by compaction, insect damage, root pruning by cultivation, herbicide injury, and positional unavailability may cause phosphorus deficiency. Mild deficiencies are characterized by stunted growth and dark green leaves. As severity increases, some varieties may show purpling or reddening of the stalk and leaves beginning early in the growing season. Phosphorus deficiency causes reduced root growth, small ears with undeveloped kernels, and delayed maturity. Where low soil test indicates phosphorus deficiency, band application of phosphorus fertilizer is more effective. Soil test phosphorus levels should be maintained at sufficient levels for optimum yield levels.

PHYSALOSPORA (BOTRYOSPHAERIA) EAR ROT (BOTRYOSPHAERIA FESTUCAE)

‘‘‘Physalospora (Botryosphaeria) Ear Rot (Botryosphaeria festucae) ‘‘‘ Physalospora ear rot develops as a dark brown to black, felty mold growth on all parts of the ear. The black, submerged perithecia are gregarious with conical necks that emerge through the cuticle.

PHYSODERMA BROWN SPOT (PHYSODERMA MAYDIS)

‘‘‘Physoderma Brown Spot (Physoderma maydis) ‘‘‘ Brown spot or Physoderma disease is generally of minor importance although serious localized outbreaks have been reported. Lesions first appear as very small, oblong to round, yellowish spots on the leaf blade, leaf sheath, stalk, and sometimes on the outer ear husks and tassels of maize. Physoderma maydis Miyake (Syn. P. zeae-maydis Shaw) Sporangia produced in pustules in infected tissue are smooth, brown, flattened on one side with a circular cap or lid, and 18-24 x 20-30 µm. The thick-walled sporangia overseason in infected tissue or in soil and germinate in the presence of moisture to produce numerous zoospores. Brown spot infection occurs generally in meristematic tissues in the whorl and is closely associated with the host's morphological development. Resistant hybrids and varieties. Clean plowdown of crop debris.

POOR POLLINATION

‘‘‘Poor pollination: ‘‘‘ Poor pollination caused by failure to plant enough corn at one time; insufficient soil moisture- especially from silking to harvest; hot weather or high winds 2 to 3 weeks before harvest, not fertilized as directed. Incomplete kernel development or shriveled kernels. Plant at least 3 to 4 rows at least 8 feet long each time planting is made. Water and fertilize as directed. Grow varieties adapted to area.

POPPED KERNEL

‘‘‘Popped Kernel: ‘‘‘ Popped kernel appears as an irregular break in the seedcoat over the kernel crown.

POTASSIUM DEFICIENCY

‘‘‘Potassium Deficiency ‘‘‘ Potassium deficiency is most common in soils of low potassium content but may be induced by very wet or dry soils, compaction, and nutrient stratification by tillage. Soils most commonly low in potassium are sandy soils, organic soils, and highly weathered soils. Removal of forage crops containing large amounts of potassium may produce deficiencies in the following crop. Early deficiency symptoms are yellowing or firing of tips and margins of older leaves. Symptoms progress up the plant as severity increases. Potassium deficient plants have weak spindly stalks, have fewer brace roots, are prone to lodging, and are more susceptible to disease. High nitrogen rates accentuate the deficiency. Deficiencies are best prevented by application of potassium chloride or potassium sulfate according to soil test.

PURPLE LEAF SHEATH

‘‘‘Purple Leaf Sheath: ‘‘‘ Irregular purplish-brown blotches and spots of varying size form on the leaf sheaths, usually after silking.

PYTHIUM STALK ROT (PYTHIUM APHANIDERMATUM)

‘‘‘Pythium Stalk Rot (Pythium aphanidermatum) ‘‘‘ This rapidly developing rot is usually confined to a single internode just above the soil line. Pythium aphanidermatum (Eds.) Fitzp. (Syn. P. butleri Subr.) and other Pythium species P. aphanidermatum can be cultured on water or cornmeal agar. See seed rots and seedling blights. The disease develops primarily during extended periods of hot (32 °C), wet, or very humid weather. Resistant hybrids and varieties.

REFUSAL FACTOR OR VOMITOXIN(DEOXYNIVALENOL)

‘‘‘Refusal Factor/Vomitoxin (Deoxynivalenol): ‘‘‘ Fusarium roseum f. sp. cerealis (sexual stage Gibberella zeae), growing in maize ears before harvest or in storage, may produce several toxins that make the grain unpalatable to swine.

RHIZOCTONIA EAR ROT (RHIZOCTONIA ZEAE)

‘‘‘Rhizoctonia Ear Rot (Rhizoctonia zeae) ‘‘‘ Rhizoctonia ear rot is recognized in its early stages by a salmon-pink mold growth on the ear. Rhizoctonia zeae Voorhees R. zeae overseasons as dormant mycelium and sclerotia in kernels, soil, or on and in plant debris.

RING NEMATODES (MACROPOSTHONIA AND CRICONEMOIDES SPP)

‘‘‘Ring Nematodes (Macroposthonia and Criconemoides spp.) ‘‘‘ Macroposthonia and Criconemoides spp. represent one of the largest groups of plant-parasitic nematodes.

ROOT ROTS

‘‘‘Root Rots: ‘‘‘ No sharp demarcation between seedling blight and root rot exists, and many of the same causal organisms attack both stem and root tissues. The disease cycle of Pythium root rot and seedling disease. The most important organisms are species of Pythium, including P. irregulars Buis., P. debaryanum Hesse, P. graminicola Subram., P. splendens Braun, P. uexans dBy., P. rostratum Butl., P. paroecandrum Drechs., and P. ultimum Trow. Injury-free seed of high germination. Good cultural practices, e.g., planting seed in warm, fairly moist soil (above 12.8 °C ); proper seedbed preparation; correct placement of fertilizer, herbicide, and other pesticides. Seed-protectant fungicides, e.g., captan or thiram.

ROOT-KNOT NEMATODES (MELOIDOGYNE SPP)

‘‘‘Root Knot Nematodes (Meloidogyne spp.) ‘‘‘ Several species of Meloidogyne attack maize and can cause severe damage, especially in warmer regions of the world.

ROOT-LESION NEMATODES (PRATYLENCHUS SPP)

‘‘‘Root Lesion Nematodes (Pratylenchus spp.) ‘‘‘ These nematodes (Pratylenchus spp.) are among the most common parasites of maize.

RUST

‘‘‘Rust: ‘‘‘ A disease caused by a fungus. Brown spots on leaves with powdery rust-colored spores. Leaves may discolor. Favored by cool temperatures and high humidity or overhead sprinklers. Resistant varieties available. Fungicides available.

SAMPLING FOR MYCOTOXINS AND SAMPLE PREPARATION

‘‘‘Sampling for Mycotoxins/Sample Preparation: ‘‘‘ An adequate sample of suspect grain must be obtained for any assay method selected.

SEED ROTS AND SEEDLING BLIGHTS

‘‘‘Seed Rots/Seedling Blights: ‘‘‘ Germinating maize kernels may be attacked by a number of soilborne or seedborne fungi that cause seed rots and seedling blights.

SEPTORIA LEAF BLOTCH (SEPTORIA SPP)

‘‘‘Septoria Leaf Blotch (Septoria spp.) ‘‘‘ In rather cool, humid environments Septoria sp. may produce small, light green to yellow-to-brown lesions on the leaves.

SILK-CUT

‘‘‘Silk-Cut: ‘‘‘ Despite its name, silk-cut has nothing to do with the silk of the ear.

SOIL CRUST

‘‘‘Soil Crust: ‘‘‘ Beating rains on fine-textured soil may produce such a hard crust that seedlings have difficulty penetrating it.

SORGHUM DOWNY MILDEW (PERONOSCLEROSPORA SORGHI)

‘‘‘Sorghum Downy Mildew (Peronosclerospora sorghi) ‘‘‘ This disease is present around the world. Systemically infected plants are chlorotic, sometimes stunted, and occasionally have white-striped leaves and abnormal seed set. The conidiophores are erect, spreading, fragile, hyaline, 180-300 µm long, usually dichotomously branched, and emerge singly or in groups from stomata on the lower and sometimes the upper sides of leaves. The oospores of P. sorghi, which survive several seasons in soil, and conidia probably constitute the primary inoculum. Conidia are produced between 17 and 29 °C (optimum often 24-26 °C depending on the geographic isolate). Resistant hybrids and varieties offer the best means of control. Destroy infected crop debris and Sorghum weeds. Rogue and destroy infected plants as they appear in the field. Protective and systemic fungicide seed treatment. Plant at the onset of the rainy season in cool soil. Avoid maize-sorghum rotations in fields where the disease has occurred, and do not sow maize adjacent to a field of maize or sorghum that can provide conidia for infection.

SOUTHERN(POLYSORA)RUST (PUCCINIA POLYSORA)

‘‘‘Southern (Polysora) Rust (Puccinia polysora) ‘‘‘ This disease resembles common rust but with very subtle differences. The uredospores are yellowish to golden, mostly ellipsoid orobovoid, and measure 20-29 X 29-40 µm. Teliospores of P. polysora are rare and are not known to germinate. Southern rust is favored by high temperatures (27 °C) and high relative humidity.

SOUTHERN OR MAYDIS LEAF BLIGHT (HELMINTHOSPORIUM MAYDIS)

‘‘‘Southern/Maydis Leaf Blight (Helminthosporium maydis) ‘‘‘ Lesions on the leaves caused by Race 0 are elongated between the veins, tan, 2-6 x 3-22 mm long, with limited parallel margins and buff-to-brown borders. The two races are morphologically similar but Race T is specifically pathogenic to maize containing Texas male-sterile cytoplasm (Tm-sc) while Race 0 is not. Cultural isolates of Race 0, but not Race T, sporulate well on V-8 juice agar. Pseudothecia of C. heterostrophus, which occur rarely in nature, appear as tiny specks on or in maize tissue. The fungus overwinters as mycelium and spores in maize debris in the field and Race T on kernels in cribs, bins, and elevators. Southern corn leaf blight occurs worldwide, being important in regions of warm (20-32 °C), damp climate. Hybrids with normal cytoplasm are resistant to Race T. Genetic sources resistant to both Races 0 and T are available. Clean plowdown of crop debris may reduce early infections. Where practical in seed-production fields, fungicides may be applied, starting when one or two lesions occur per leaf and before the disease becomes epidemic.

SPIRAL AND STUNT NEMATODES

‘‘‘Spiral/Stunt Nematodes: ‘‘‘ Numerous species of spiral and stunt nematodes (Helicotylenchus, Tylenchorhynchus, Merlinius, and Quinisulcius spp.) are commonly associated with maize.

SPONTANEUM DOWNY MILDEW (PERONOSCLEROSPORA SPORTANEA)

‘‘‘Spontaneum Downy Mildew (Peronosclerospora sportanea) ‘‘‘ Spontaneum Downy Mildew has been reported in few countries around the world. The conidiophores are erect, hyaline, 315.6-441 µm long, emerging singly or in groups from stomata on the lower leaf surface.

STALK ROTS

‘‘‘Stalk Rots: ‘‘‘ Stalk rots are universally important and among the most destructive diseases of maize throughout the world. In most cases, stalk rots are caused by a complex of several species of fungi and bacteria that attack plants approaching maturity. Potassium deficiency enhances susceptibility to stalk rot (see potassium).

STEWART'S BACTERIAL WILT (ERWINIA STEWARTII)

‘‘‘Stewart's Bacterial Wilt (Erwinia stewartii) ‘‘‘ Stewart's bacterial wilt, sometimes called Stewart's leaf blight or maize bacteriosis, is common around the world. Infected sweet maize hybrids wilt rapidly, resembling plants suffering from drought, nutritional deficiency, or insect injury. Although most field or dent maize is not as susceptible as sweet maize, there are some very susceptible inbred lines and hybrids. The corn flea beetle (Chaetocnema pulicaria Melsh.), both adult and larvae of the twelve-spotted cucumber beetle (Diabrotica undecimpunctata howarti Barb.), toothed flea beetle (C. denticulata Ill.), as well as larvae of the seed corn maggot (Hylemya cilicrura Rond.), wheat wireworm (Agriotes mancus Say), and May beetle (Phyllophaga sp.) are known vectors of E. stewartii. Mineral nutrition influences the susceptibility of maize hybrids to infection by E. stewartii. Disease forecasting is based on the sum of the mean temperatures (C) for December, January, and February. Resistant hybrids and varieties. Early applications of insecticides (e.g., carbaryl) to kill corn flea beetles.

STING NEMATODES (BELONOLAIMUS SPP)

‘‘‘Sting Nematodes (Belonolaimus spp.) ‘‘‘ These large, browsing ectoparasitic nematodes are found only in sandy soils.

STORAGE ROTS (ASPERGILLUS AND PENICILLIUM SPP)

‘‘‘Storage Rots (Aspergillus and Penicillium spp.) ‘‘‘ Storage rots may develop on ear or shelled maize in storage, causing a reduction in feed and market value. The first stages of kernel invasion by fungi are not detectable by the unaided eye. Storage rots are caused by many fungi, principally species of Aspergillus and Penicillium. Storage fungi thrive in shelled maize with moisture contents of 15-20% and at 21-32 °C. At moisture contents below 15% and temperatures below 10 °C, the risk of damage by storage fungi is very low. Dry shelled maize to 13-15% moisture and, where possible, aerate to maintain a uniform temperature of 4-10 °C throughout the bulk. Chemical storage additives are being used to a limited extent. Central American maize hybrids and varieties show evidence of resistance.

STUBBY-ROOT NEMATODES (TRICHODORUS AND PARATRICHODORUS SPP)

‘‘‘Stubby-Root Nematodes (Trichodorus and Paratrichodorus spp.) ‘‘‘ The relatively small, ectoparasitic, stubby-root nematodes are found mainly in sandy soils.

SUGARCANE DOWNY MILDEW (PERONOSCLEROSPORA SACCHARI)

‘‘‘Sugarcane Downy Mildew (Peronosclerospora sacchari) ‘‘‘ Severe outbreaks of this disease on corn have limited maize production in Taiwan. Sugarcane downy mildew is characterized by local lesions leading to systemic infection. Conidia and conidiophores are superficial. The fungus overseasons as mycelium in sugarcane. Moisture, temperature, and age of plants are the important factors in disease development. Resistant hybrids and varieties. Cultural practices: Prevention eliminate the fungus in sugarcane by using healthy sugarcane sets; isolation-grow maize in disease-free areas where sugarcane is not grown extensively, and do not interplant with sugarcane; eradication-rogue and destroy diseased plants; fungicides-systemic seed treatment and sprays are a general practice in some areas.

SUGARCANE MOSAIC (SCMV)

‘‘‘Sugarcane Mosaic (SCMV) ‘‘‘ Several closely related viral diseases distributed worldwide are caused by strains and substrains of sugarcane mosaic virus (SCMV). SCMV infection of maize has occurred in most tropical and subtropical regions where maize was grown next to infected sugarcane.

SULFUR DEFICIENCY

‘‘‘Sulfur Deficiency ‘‘‘ Sulfur deficiency is most commonly associated with low organic matter soils, acid sandy soils, and cold temperatures. Symptoms are stunting, delayed maturity, and a general yellowing of the foliage as in nitrogen deficiency. In contrast to nitrogen deficiency, symptoms of sulfur deficiency appear first on young leaves. Interveinal chlorosis may also appear. Sulfur deficiency delays maturity and reduces grain protein. Many sulfur-containing fertilizers are effective in preventing or correcting sulfur deficiency. Sulfate forms have greatest immediate availability. Elemental sulfur forms have delayed availability because biological oxidation to sulfate is necessary for plant availability. Cold temperatures and large particle size delay this oxidation.

SULFUR DIOXIDE

‘‘‘Sulfur Dioxide: ‘‘‘ Sulfur dioxide can injure maize plants of all ages.

SYMTOMS OF NEMATODE INJURY

‘‘‘Symptoms of Nematode Injury: ‘‘‘ Feeding of nematodes on maize roots reduces plant vigor and root growth, lowers the natural resistance of plants to other pathogens, and induces root lesions or galls and other deformations.

T-2 TOXINS AND OTHER TRICHOTHECENES

‘‘‘Toxins/Other Trichothecenes: ‘‘‘ Fusarium tricinctum and some strains of F. roseum f. sp. cerealis, F. equiseti, and F. lateritium produce T-2 and other toxic trichothecenes.

TAR SPOT (PHYLLACHORA MAYDIS)

‘‘‘Tar Spot (Phyllachora maydis) ‘‘‘ Black, sunken, glossy spots on the leaves are conspicuous when plants are heavily infected.

TASSEL EARS

‘‘‘Tassel Ears: ‘‘‘ A few scattered plants in a field may have combination tassels and ears in the same terminal structure.

TROPICAL RUST (PHYSOPELLA ZEAE)

‘‘‘Tropical Rust (Physopella zeae) ‘‘‘ The pustules are yellow, occur in small groups mostly 0.3-1.0 mm long on the upper leaf surfaces, and are covered by the epidermis except for a small pore or slit. The uredospores are sessile, ellipsoid or obovoid, 15-22 x 22-33 µm, colorless to pale yellow, with nearly hyaline, moderately echinulate walls, 1.5-2 µm thick. Teliospores of P. polysora are rare and are not known to germinate. The disease is favored by warm to hot, humid weather and low altitudes. Tropical rust appears to be limited to warm, humid areas. Resistant hybrids and varieties.

VIRAL DISEASES

‘‘‘Viral Diseases: ‘‘‘ The more than 600 plant-infecting viruses are macromolecules composed of either ribonucleic acid (RNA) or deoxyribonucleic acid (DNA) surrounded by a protective protein or lipoprotein coat. Viruses are transmitted to plants through pollination and through wounds created by animal (mainly arthropod and nematode) or fungal vectors, parasitic plants, mechanical inoculation, and man's deliberate or accidental activities (e.g., propagation of plants by cuttings, grafts, or budding and by planting infected seed). Viral infections range from latent to premature plant death. More than 40 viruses or virus strains have been reported to cause diseases of maize throughout the world.

WHEAT STREAK MOSAIC (WSMV)

‘‘‘Wheat Streak Mosaic (WSMV) ‘‘‘ Early symptoms appear as small chlorotic spots or broken streaks at the tips of young leaves.

WIND

‘‘‘Wind: ‘‘‘ Strong winds can cause several types of damage to maize plants 10-25 cm tall.

WITCHWEED (STRIGA ASIATICA AND HERMONTHICA)

‘‘‘Witchweed (Striga asiatica and hermonthica) ‘‘‘ This parasitic flowering plant is widely distributed throughout the world. Infected maize plants wilt and turn yellow producing symptoms resembling those caused by acute drought. S. asiatica is an obligately parasitic flowering plant with square, aerial stems and small, bright green, elongated leaves. S. hermonthica is generally larger than S. asiatica with bigger leaves and flowers with five calyx ribs. Witchweed overseasons as seeds most of which require 90 days (for S. asiatica) to 15 or 18 months' dormancy before germination. The seeds are disseminated by contaminated soil on machinery, equipment, tools, water, or wind. Prevent movement of Striga seed (in soil, plants, equipment) from infested to noninfested areas. Keep all crops free of weedy grasses. Apply appropriate herbicides to the base of maize stalks in hills or rows before Striga plants emerge or produce seed. Plant nonhost trap crops (e.g., legumes) to stimulate germination of witchweed seeds.

YELLOW LEAF BLIGHT OR PHYLLOSTICTA LEAF SPOT (PHYLLOSTICTA MAYDIS)

‘‘‘Yellow Leaf Blight/Phyllosticta Leaf Spot (Phyllosticta maydis) ‘‘‘ Yellow leaf blight or Phyllosticta leaf spot has appeared around the world. The fungus can be isolated and cultured on potato-dextrose agar (PDA) (22-25 °C ) after conventional surface sterilization of leaf tissue, and produces pycnidia under fluorescent P. maydis overwinters in maize or grass debris. Disease development is favored by cool, wet weather. Resistant hybrids and varieties offer the best means of control. Tm-sc cytoplasm is usually more susceptible than other cytoplasms. Fungicide applications may be feasible in certain seed-production fields. Clean plowing or rotation, or both, greatly reduces early-season infections and primary inoculum.

ZEARALENONE, ZEARALENOL AND ESTROGENIC SYNDROME

‘‘‘Zearalenone Zearalenol/Estrogenic Syndrome: ‘‘‘ Zearalenone and related zearalenol are almost exclusively produced by Fusarium roseum f. sp. cerealis (F. graminearum) growing in ears of corn standing in the field or in stored ear maize. Estrogenism in swine is usually most prevalent in winter and early spring because the fungus requires a period of relatively low temperatures to produce biologically significant amounts of zearalenone or zearalenol.

ZEARALENONE AND TRICHOTHECENE DETECTION

‘‘‘Zearalenone/Trichothecene Detection: ‘‘‘ The estrogenic syndrome, feed refusal, or vomiting may be one of the first signs of ingestion of a Fusarium toxin.

ZINC DEFIENCY

‘‘‘Zinc Deficiency ‘‘‘ Corn is very sensitive to zinc deficiency. Symptoms are most often observed in corn grown in high pH soils of low zinc content. Low soil organic matter, loss of topsoil by erosion or leveling, cold soil temperatures, high phosphorus levels, poor root growth, and compaction can all contribute to deficiencies. Symptoms usually occur early in the season and include shortening of the internodes, lack of vigor, and light streaking of leaves followed by a broad stripe of bleached tissue on each side of the midrib. Mild deficiency may cause yield losses and delayed maturity without visible symptoms. Zinc deficiency is best prevented by soil application of water soluble zinc fertilizers such as zinc sulfate. Zinc EDTA is also effective, but costs significantly more. Insoluble zinc sources such as zinc oxide in granular form are ineffective. Band applications of 1-5 lbs Zn/acre, such as in a starter application, are most effective. Banding of nitrogen and zinc together improves zinc uptake. Broadcast applications can also be used, but higher rates of 5-10 lbs Zn/acre are normally required for high pH soils. These rates may have residual effects for several years. Foliar application of soluble zinc fertilizers is also effective. A 0.5 to 1% zinc sulfate solution applied with a surfactant in enough water to wet foliage is an effective treatment. Repeated applications at 7-14 day intervals may be necessary.

ZONATE LEAF SPOT (GLOEOCERCOSPORA SORGHI)

‘‘‘Zonate Leaf Spot (Gloeocercospora sorghi) ‘‘‘ This disease has been reported around the world. The conidia are borne in a pinkish to salmon-colored slimy matrix and are hyaline, needle-shaped, straight or slightly curved, and of variable length, 20-195 X 1.4-3.2 µm (av. 82.5 X 2.4 µm). The fungus also attacks sorghum, Sudangrass, Johnsongrass, sugarcane, and bentgrass.