Sorghum:Diagnostics

SORGHUM PESTS AND NUTRITIONAL DISORDERS

ACREMONIUM WILT (ACREMONIUM STRICTUM)

‘‘‘Acremonium Wilt (Acremonium strictum) ‘‘‘ Acremonium wilt has been recognized in most sorghum-growing regions of the world and includes symptoms of foliar desiccation and vascular discoloration of the lateral leaf veins. Infection probably begins in the leaf blade or leaf sheath and spreads through the vascular system. Most sorghum varieties appear to be resistant to the disease, or else the pathogen develops so slowly that it does not cause serious losses.

ALUMINUM

‘‘‘Aluminum: ‘‘‘ Aluminum is not required for plant growth, but it has been reported to have beneficial effects at low levels. Aluminum toxicity predominantly affects roots, which turn dark or purple, remain short, and become brittle and thick, with little branching.

ANTHRACNOSE (COLLETOTRICHUM GRAMINICOLA)

‘‘‘Anthracnose (Colletotrichum graminicola) ‘‘‘ The fungus causing anthracnose, survives in debris and in wild species of sorghum and may be seedborne. The best form of control is to use resistant cultivars.

ANTHRACNOSE STALK ROT (COLLETOTRICHUM GRAMINICOLA)

‘‘‘Anthracnose Stalk Rot (Colletotrichum graminicola) ‘‘‘ The disease causes foliar rot, head rot, stalk rot and can be recognized by the irregularly mottled or marbled patterns of colonization within internodal tissue, particularly in the peduncle. Controls of anthracnose stalk rot depend on the same fundamental principles as those applied in controls of leaf anthracnose and panicle anthracnose.

BACTERIAL LEAF SPOT (PSEUDOMONAS SYRINGAE)

‘‘‘Bacterial Leaf Spot (Pseudomonas syringae) ‘‘‘ Bacterial leaf spot occurs on sorghum, sudangrass, johnsongrass, pearl millet, maize, and foxtail millet. Initial symptoms consist of water-soaked spots on the lower leaves and the disease is disseminated by wind-driven rain. Control measures consist of crop rotation, destruction of infested debris, and selection of seed from disease-free plants.

BACTERIAL LEAF STREAK (XANTHOMONAS CAMPESTRIS PV HOLCICOLA)

‘‘‘Bacterial Leaf Streak (Xanthomonas campestris pv holcicola) ‘‘‘ Bacterial leaf streak has been reported in all areas of the world. Lesions are first observed as small, interveinal, water-soaked streaks, which broaden into irregularly shaped areas with dry centers and red margins. The best forms of control include crop rotation, destruction of crop residue, and planting resistant cultivars or hybrids.

BACTERIAL LEAF STRIPE (PSEUDOMONAS ANDROPOGONIS)

‘‘‘Bacterial Leaf Stripe (Pseudomonas andropogonis) ‘‘‘ Bacterial leaf stripe has been reported in sorghum-growing areas of the entire world. The initial symptoms are small (1 cm), linear interveinal lesions. Bacterial leaf stripe is an aerobic, non-spore forming rod measuring 0.5-0.7 x 1-2 µm, with one sheathed polar flagellum. Field dissemination occurs principally by means of wind and rain. The best forms of control include crop rotation, destruction of crop residue, and planting resistant cultivars or hybrids.

BANDED LEAF AND SHEATH BLIGHT (R. SOLANI)

‘‘‘Banded Leaf and Sheath Blight (R. solani) ‘‘‘ Banded leaf and sheath blight is a sorghum disease of little economic importance and is generally limited to tropical areas of the world. Sorghum leaf blades and sheaths initially develop water-soaked, gray-green lesions, which become tan or reddish brown with white centers, are dry, and irregularly shaped (2-8 mm wide). Banded leaf and sheath blight are adapted to parasitic attack of aerial plant organs. Plants become infected from free sycelia, residue containing mycelia, and sclerotia that are transmitted by wind and rain from the soil surface to foliar sites. There are no control measures available.

BENZOIC ACIDS

‘‘‘Benzoic Acids: ‘‘‘ Since the mode of action of benzoic acid herbicides is similar to that of the phenoxy acids,injury symptoms are difficult to distinguish from those caused by 2,4-D or dicamba.

BORON

‘‘‘Boron: ‘‘‘ Boron deficiency has not been reported in sorghum. Excess boron can greatly reduce plant growth.

BROME MOSAIC

‘‘‘Brome Mosaic: ‘‘‘ Brome mosaic virus has been found naturally occurring on sorghum in Texas. Leaves show very narrow, interveinal yellow streaks and may appear spotted from a distance. The virus is an icosahedral particle approximately 25 nm in diameter. Sorghum infection occurs principally through sap transmission.

CALCIUM

‘‘‘Calcium: ‘‘‘ Calcium deficiency occurs first in the meristematic tissues and on newly emerging leaves. Calcium deficiency often occurs in sorghum plants greenhouses and growth chambers.

CEREAL ROOT KNOT NEMATODE (MELOIDOGYNE NAASI)

‘‘‘Cereal Root-Knot Nematode (Meloidogyne naasi) ‘‘‘ The cereal root-knot nematode has been reported as a parasite of cereals, grasses, and sugar beets in Wales, Belgium, England, Yugoslavia, Iran, and the United States. Stunted, yellow plants in irregular areas of a field are the most prominent symptoms. Females nematodes are pyriform, have a small neck, and are 0.5-0.7 mm long and 0.3-0.4 wide and the disease overwinters as eggs.

CHARCOAL ROT (MACROPHOMINA PHASEOLINA)

‘‘‘Charcoal Rot (Macrophomina phaseolina) ‘‘‘ Charcoal rot is a major disease in the drier sorghum-growing regions of Africa, Asia, Australia, and the Americas. Charcoal rot is expressed in early growth stages as seedling blight, damping-off, and dry rot. The disease survives in the soil as sclerotia, which provide the initial inoculum. High soil temperature and low soil moisture are the most important factors predisposing sorghum plants to infection and development of charcoal rot. Incidence of charcoal rot of sorghum grown by irrigation can be minimized by maintaining soil moisture during the postflowering period.

CHLORINE

‘‘‘Chlorine: ‘‘‘ Symptoms of excess chlorine are difficult to distinguish from those of excess sodium.

CHLOROACTEMIDES

‘‘‘Chloroactemides: ‘‘‘ Chloroactemide herbicides (e.g., alachlor, metolachloy, propachlor) are generally used only with safened seed (treated with Concep 11, Screen, or other herbicid safeners to protect the seedlings).

COPPER

‘‘‘Copper: ‘‘‘ Copper deficiency is rare in sorghum.

COVERED KERNEL SMUT (SPORISORIUM SORGHI)

‘‘‘Covered Kernel Smut (Sporisorium sorghi) ‘‘‘ Covered kernel smut is generally distributed in every sorghum-growing region of the world. Individual ovules are replaced by a smut sorus, which is covered by a persistent peridium. The disease produces small, echinulate teliospores, which are generally 4-7 µm in diameter, germinate with the seed and are seedborne. A number of fungicides are currently used as seed treatments to provide complete protection from the disease.

CRAZY TOP (SCLEROPHTHORA MACROSPORA)

‘‘‘Crazy Top (Sclerophthora macrospora) ‘‘‘ Crazy top has a wide geographic distribution. The earliest symptom of young plants is a yellow spotting of the leaves, similar to the mosaic symptoms of viral infection. Oospores germinate in free water in saturated soil and produce zoospores, which penetrate host tissue. The disease develops in young plants growing in waterlogged soil. Land subject to overflow or waterlogging should not be used for planting unless adequate drainage is provided.

DISEASES VIRUSES AND VIRUSLIKE ORGANISMS

‘‘‘Diseases viruses and viruslike organisms: ‘‘‘ Plant viruses are simple macromolecules with no active means of entering plant cells. Viruses are transmitted from diseased to healthy plants through wounds caused by contact or friction between plants and through injuries caused by people, machinery, or animals, including insects, other arthropods, and nematodes. Plant viruses are identified by their host range, mode of transmission, and physical and chemical properties. Plant viruses are named in the vernacular, usually with a compound of the names of the first host on which the virus was observed and the type of symptom noted. Viruses are among the most significant etiologic agents of sorghum diseases. Sorghum viruses invade nearly all plant cells and normally are transmissible by mechanical inoculation.

DOWNY MILDEW

‘‘‘Downey mildew: ‘‘‘ Sorghum is a host of at least four species of downy mildew pathogens including Sclerophthora macrospora, Peronosclerospora sorghi, P. philippinensis, and Sclerospora graminicola.

DROUGHT

‘‘‘Drought: ‘‘‘ Water deficiency is the most severe environmental limitation on grain yield of sorghum throughout the crop's area of adaptation. Additionally, some soils in sorghum production areas are prone to crusting, which usually occurs when high-intensity rainfall is followed by rapid drying. In slowly developing, mild or moderate water deficits, plants often do not have heat or water-stress symptoms.

ERGOT (SPHACELIA SORGHI)

‘‘‘Ergot (Sphacelia sorghi) ‘‘‘ Ergot, also known as honeydew or sugary disease, occurs only in Africa and Asia. In its early stage, the honeydew or sugary phase, the disease is recognized by the presence of sticky, pinkish to brownish liquid drops, which pass through the pores from infected ovaries. Sclerotia, which develop in infected ovaries, are cylindric and slightly curved, may be either hard or soft and either short or long, measuring about 0.4-0.6 X 1-2.5 cm, and are creamy gray. Primary inoculum is provided either by ascospores from germinated sclerotia or by conidia from collateral hosts. Secondary spread of the disease in the field occurs by means of conidia in the honeydew, which are disseminated from flower to flower by insects, wind, and rain. In dry weather little development of the pathogen occurs in the ovary. Sclerotia have not been reported to be toxic to humans or animals. The best form of control is to plant pathogen-free seed.

FOLIAGE FEEDERS

‘‘‘Foliage Feeders: ‘‘‘ Infestations of the greenbug are detectable by the appearance of reddish spots on the leaves, caused by toxins injected into the plant. The corn leaf aphid is most commonly found deep in the whorl of the middle leaf. In contrast to many other aphid species, the sugarcane aphid feeds primarily on older leaves. The yellow sugarcane aphid attacks sorghum at the seedling stage and also older plants. Nymphs and adults of the shoot bug suck sap and cause sorghum plants to appear yellow. The spittlebug sometimes does considerable damage by feeding on sorghum leaves and within leaf whorls, causing yellow spots and blotches on the leaves. The chinch bug destroys plants principally by withdrawing enormous quantities of sap from stems or underground plant parts. In the whorl stage of sorghum growth, young larvae of the fall armyworm feed on the tender parts of whorl leaves. Spider mites suck sap from the underside of leaves, beginning along the midrib of the lower functional leaves. As a result of larval feeding by the shoot fly the central leaf wilts and then dries up.

FOLIAR DISEASES

‘‘‘Foliar diseases: ‘‘‘ Sorghum is plagued by a broad range of foliar pathogens, some of which cause disease in a wide variety of environmental conditions. Damage from many foliar pathogens can be avoided through cultivation of taller genotypes of sorghum, because many of these pathogens are disseminated from primary infections on the lower leaves. Moderate levels of resistance are available for most of the foliar pathogens of sorghum. Some airborne foliar pathogens are much less affected by cultural practices, and consequently host resistance becomes a major consideration. Losses caused by foliar diseases can at times be reduced by arranging planting dates so that the crop can be grown in seasons when these diseases are less likely to appear.

FUSARIUM ROOT AND STALK ROT (FUSARIUM MONILIFORME)

‘‘‘Fusarium Root and Stalk Rot (Fusarium moniliforme) ‘‘‘ Fusarium moniliforme is a pathogen in tropical and temperate regions of the world. Lesions caused by Fusarium vary in size from small, circular spots to elongated streaks and are light red to dark purple. Plants infected with the disease contain large areas of reddish pith on the lower two to three internodes, and the upper internode or internodes have brown to red vascular bundles. Primary inoculum of Fusarium species consists of conidia and mycelia that have remained in crop debris. The best form of control is good management practices which include destroying infected tissues and only using clean seed and materials.

GRAIN MOLD

‘‘‘Grain Mold: ‘‘‘ Grain mold is one of the major constraints on high yields in early-maturing sorghum genotypes in regions where they flower and fill grain during periods of high rainfall and high relative humidity. Fusarium produces pinkish white mycelium, which is powdery in appearance in the early stages and later becomes fluffy or appears on grain as a salmon orange, cakelike mass of mycelia and conidia. Curvularia lunata appears as a shiny, velvety black, fluffy growth on the grain surface. Olpitrichum produces long conidiophores with a tuft of conidia at the tip, which can be seen as small, snowy white, bristly projections on glumes and grain held against the light. Alternaria and Helminthosporium appear as dull, grayish black mycelium, and sparse, bristly growth can be observed when early symptoms are expressed. Phoma sorghina produces small, round, black pycnidia embedded in the grain and resembling pinheads. Colletotrichumgraminicola can be observed as small, black, elongated acervuli studded with clusters of setae and usually forming concentric rings on the grain. The first visible growth of F. moniliforme, F. semitectum, and C. lunata is usually on the grain at the point where the glume ends. F. moniliforme infects lodicules, filaments, or pedicels within five days after anthesis. C. lunata invades the ovary wall directly from the colonized lemma, palea, lodicules, and filaments and even from the decaying style and from dead pollen trapped in the spikelet. Both fungi can hasten physiological maturity by 10-18 days which results in the production of small seeds. Almost all the mold-causing fungi are weak facultative parasites disseminated by seedborne, soilborne, and airborne spores. Grain mold damage can largely be avoided by adjusting planting dates so that plants enter the grain-filling stage and reach maturity during a period without frequent rains.

GRAY LEAF SPOT (CERCOSPORA SORGHI)

‘‘‘Gray Leaf Spot (Cercospora sorghi) ‘‘‘ Gray leaf spot is generally found in sorghum-growing regions where warm, wet weather predominates during the growing season. The initial symptoms are small, red spots on leaves and the pathogen persists on infected host residue, surviving host crop plants, infected wild sorghum, and seed. The best forms of control are to use resistant, crop rotation, and sanitation to reduce surface residue and kill surviving crop plants.

HEAD BLIGHT (FUSARIUM MONILIFORME)

‘‘‘Head Blight (Fusarium moniliforme) ‘‘‘ Head blight is characterized by partial or complete colonization of the panicle or its rachis branches causing their premature death. Head blight is the fungus most frequently isolated from head blight lesions. Prolonged periods of wet weather preceding anthesis apparently favor head blight.

HEAD FEEDERS

‘‘‘Head Feeders: ‘‘‘ Larvae of the sorghum feed on ovaries, preventing normal grain development and causing blasted panicles. Larvae of the corn earworm or bollworm, hatch from eggs laid on sorghum leaves, migrate to the whorl, and feed on the tender, folded leaves. Young larvae of the sorghum webworm feed on developing floral parts. A group of panicle-feeding true bugs damage sorghum by sucking sap from developing grain and rachis branches.

HEAD SMUT (SPORISORIUM REILIANUM)

‘‘‘Head Smut (Sporisorium reilianum) ‘‘‘ Head smut is a common disease that affects the inflorescence and occasionally the foliage of plants. Sterility of the panicle, frequent tillering, and soilborne spores germinate and penetrate directly in the nodal region of the shoot apex are all characteristic symptoms of the disease. Disease incidence is related to the abundance of soilborne teliospores, which are most dense near the soil surface. The best form of control is the use of resistant cultivars.

IRON

‘‘‘Iron: ‘‘‘ Iron deficiency can decrease plant growth and delay maturity. Disorders from excess iron often occur in plants grown in tropical soils and in many acidic soils.

LEAF ANTHRACNOSE (COLLETOTRICHUM GRAMINICOLA)

‘‘‘Leaf Anthracnose (Colletotrichum graminicola) ‘‘‘ Anthracnose is a disease that has been reported around the world, especially in areas where hot, humid conditions occur. The foliar phase of anthracnose, which may occur at any stage of plant development, usually appears on the leaves producing small, elliptic to circular spots, usually 5 mm or less in diameter. Midrib infection may occur on cultivars that have little leaf damage and whose response may be independent of leaf infection. Infections may also occur below and just above the ground. Mycelium of lead anthracnose is gray to olivaceous, septate, and sparingly branched. The fungus can survive as mycelium in host residue, wild sorghum species, and some weeds and as conidia or mycelium on seed. The best control for anthracnose is the use of resistant cultivars.

LEAF BLIGHT (EXSEROHILUM TURCICUM)

‘‘‘Leaf Blight (Exserohilum turcicum) ‘‘‘ Leaf blight is widespread in many humid areas of the world and appears as infected seedlings with small, reddish or tan spots that develop on their leaves. Conidia (10-20 X 28-153 µm), borne singly at the tips of the conidiophores, are light gray, straight or spindleshaped or slightly curved, and rounded at the ends. The fungus persists as mycelia and conidia in infected crop residues on or in the soil and on the glumes of sudangrass seed. Only a few resistant cultivars are available and should be used. In addition, rotation with nonhost crops may be the best forms of control.

LEAF SHEATH DISEASE

‘‘‘Leaf Sheath Disease: ‘‘‘ Four pathogens cause primary damage to sorghum leaf sheaths: Rhizoctonia spp., Sclerotium rolfsii, Ophiobolus spp., and Gloeocercospora sorghi. Some stalk rot pathogens (Colletotrichum graminicola and Macrophomina phaseolina) attack sorghum leaf sheaths, but their occurrence on leaf sheaths is usually dependant on the development of the more serious stalk rots.

LONG SMUT (TOLYPOSPORIUM EHRENBERGII)

‘‘‘Long Smut (Tolyposporium ehrenbergii) ‘‘‘ Long smut symptoms are easy to distinguish by the presence of elongated, cylindric, slightly curved sacs, 2-4 cm long, with a persistent membrane and may appear on any part of the panicle. Teliospores are strongly cemented together in masses known as spore balls, which are dark brown and of various shapes and sizes (30-240 µm in diameter). Spore balls, deposited on the soil surface, are the initial source of infection from season to season. The use of resistant varieties offers the best means of controlling long smut.

LOOSE KERNAL SMUT (SPHACELOTHECA CRUENTA)

‘‘‘Loose Kernel Smut (Sphacelotheca cruenta) ‘‘‘ Loose kernel smut occurs in most sorghum-growing regions except Australia, Oceania, and the Malaysian-Indonesian archipelago. Affected plants are sometimes detected before panicles emerge from the boot. Smut sori develop in the pistil and stamens and sometimes on the rachis and its branches and on glumes. Teliospores are globose to subglobose (about 6-10 µm in diameter), light yellowish brown or dark brown, and very minutely echinulate. Spore germination occurs readily with the production of a thick, usually four-celled promycelium, from which sporidia or long hyphal branches may develop. The disease is primarily seedborne. Panicle infection can also occur directly from airborne spores where crop flowering occurs continuously or is prolonged, as it is in successive sowings. Fungicide seed dressings are currently used for control.

MAGNESIUM

‘‘‘Magnesium: ‘‘‘ In sorghum with magnesium deficiency, plant growth is usually retarded, and the reproductive phase is delayed.

MAIZE CHLOROTIC DWARF (MCDV)

‘‘‘Maize Chlorotic Dwarf (MCDV) ‘‘‘ Maize chlorotic dwarf virus has been found in sorghum only in the United States, particularly in Mississippi, Louisiana, and Texas. The symptoms are slight stunting and tertiary veinal yellowing. Leafhoppers acquire the disease in probing infected plants and can transmit it to healthy plants as soon as 2 hours later, for a period of up to 48 hours. Herbicides to eliminate or control johnsongrass and systemic sprays to reduce leafhopper populations are effective in reducing the populations.

MAIZE DWARF MOSAIC (MDM)

‘‘‘Maize Dwarf Mosaic (MDM) ‘‘‘ Maize Dwarf mosaic is the most important widespread viral disease of sorghum in the world, occurring wherever sorghum and johnsongrass are found. Symptoms are spotting followed by a red discoloration that may appear on leaves, sheaths, and peduncles, depending on the strain of virus, the cultivar, and the temperature. Infected plants may be dwarfed. The disease persists in maize and sorghum as well as in many annual and perennial grasses. Johnsongrass, which generally is widespread where grain sorghum is grown, serves almost exclusively as the host of remaining inoculum, because its fleshy rhizomes survive underground. It is controlled by cultural practices that minimize the virus source. The best control is the use of the resistant or tolerant cultivars and planting in noninfected fields.

MAIZE STRIPE (MStpV)

‘‘‘Maize Stripe (MStpV) ‘‘‘ Maize stripe virus causes yellowing, early flowering, and excessive tillering in sorghum.

MANGANESE

‘‘‘Manganese: ‘‘‘ Manganese deficiency can reduce plant growth and development, but it is seldom a problem.

METHODS OF CONTROL

‘‘‘Methods of Control: ‘‘‘ Six commonly recognized methods of control are avoidance, exclusion of the pathogen, protection of the plant, eradication of the pathogen, breeding for host resistance to disease, and therapy. Avoidance is the practice of cultivating a crop in such a manner as to escape disease or the damage caused by disease. Exclusion of a pathogen from a geographic area can be accomplished through regulatory programs that involve inspection and seed certification. Protection of sorghum can be accomplished by the use of fungicides, indirectly by control of insect vectors, or by modification of host nutrition. Complete eradication of an established pathogen is biologically an unlikely event. Breeding for host resistance to disease is one of the most common and currently productive methods of actively controlling sorghum diseases. Therapy is the curing of diseased plants by chemotherapy, physical treatment, or surgery.

MILO DISEASE (PERICONIA CIRCINATA)

‘‘‘Milo Disease (Periconia circinata) ‘‘‘ Milo disease, a root and crown rot is a soilborne fungus that is pathogenic only on selected introduced milos and cultivars developed from them. Plants are susceptible to the pathogen at all stages of development. Although symptoms include effects on foliage and grain, the disease is very rarely isolated from tissues above the crown. The fungus can survive in or on residues from previous crops. The best form of control is the use of resistant cultivars.

MOLYBDENUM

‘‘‘Molybdenum: ‘‘‘ Molybdenum deficiency has not been reported in sorghum but it has often been reported in maize. Plants can usually tolerate high levels of molybdenum without detrimental effects.

NEMATODES

‘‘‘Nematodes: ‘‘‘ Nematodes, or eelworms, are small, multicellular animals that exist in almost every soil and water habitat in the world. Adult plant-parasitic nematodes are comparatively small, 0.4-4 mm long, with the average being about 1 mm, and 0.01-0.05 mm in diameter. Nematode damage to field crops is often difficult to ascertain and may closely mimic responses to drought stress, nutrient deficiencies, or other diseases and insect problems. Controlling nematodes is often complex and may not be sucessful unless the population exceeds the level at which economic damage occurs to a crop plant.

NITROGEN

‘‘‘Nitrogen: ‘‘‘ Plants with symptoms of nitrogen deficiency are stunted, spindly, and pale yellow. Excess nitrogen may promote lush green foliage and may delay maturity.

NUTRITIONAL DISORDERS

‘‘‘Nutritional Disorders: ‘‘‘ Yellowing from unavailability of iron or zinc causes significant losses and is a major deterrent to sorghum production in some areas. Differences in host genotype are important in controlling nutritional disorders. The best form of control is integrated pest management of diseases, insects, and weeds which can provide a changing pattern of disease and research approaches.

OVAL LEAF SPOT (RAMULISPORA SORGHICOLA)

‘‘‘Oval Leaf Spot (Ramulispora sorghicola) ‘‘‘ Oval leaf spot appears as small, water-soaked spots with tan, brick red, or purple borders. The disease produces intercellular mycelium, which forms knots in the substomatal cavities of the leaf mesophyll. Late in the season, sclerotia are formed on some spots, mostly on the undersides of leaves. There are no control measures available for this disease.

PANICLE AND GRAIN ANTHRACNOSE (COLLETOTRICHUM GRAMINICOLA)

‘‘‘Panicle and Grain Anthracnose (Colletotrichum graminicola) ‘‘‘ Anthracnose is a common disease especially in cloudy, warm, humid growing areas. The panicle and grain blight phase of anthracnose affects mature sorghum plants. Young lesions on the panicle appear as elliptic pockets or bars immediately beneath the epidermis. Sporulation occurs on the central rachis, on primary, secondary, and tertiary branches of the panicle, on glumes, and on seeds in late stages of development. The destruction of primary inoculum is important for control of panicle and seedborne infection. The most effective means of controlling panicle and grain anthracnose and seedborne infection is to plant resistant cultivars.

PEANUT CLUMP VIRUS (PCV)

‘‘‘Peanut Clump (PCV) </b></S></CF> PCV, a member of the Tobamovirus group, contains single-stranded RNA. The virus is mechanically transmissible and is soilborne.

PHOSPHORUS

‘‘‘Phosphorus: ‘‘‘ Phosphorus deficiency is characterized by stunted, spindly, dark green plants with overtones of dark red on the leaves.

PHYSICOLOGICAL LEAF SPOTS AND LEAF DISCOLORATION

‘‘‘Physiological Leaf Spots/Leaf Discoloration: ‘‘‘ Sorghum has numerous physiological disorders. All physiological discolorations take on the color of the plant anthocyanin which is purple, dark purple, reddish, or tan pigmented. Most physiological leaf disorders are simply inherited and can easily be eliminated in a breeding program.

POKKAH BOENG OR TWISTED TOP (GIBBERELLA FUJIKUROI PV SUBGLUTINANS)

‘‘‘Twisted top (Gibberella fujikuroi pv subglutinans) ‘‘‘ Twisted top was originally described on sugarcane in Java. Twisted top is characterized by deformed, folded, or discolored leaves near the top of the plant. An abnormality sometimes associated with twisted top is "knifecut," consisting of narrow, uniform transverse cuts in the rind, which give the impression that tissue has been removed with a sharp knife. The inflorescence may be attacked before emerging from the surrounding leaf sheath and consequently may be rotted or barren. Infection occurs during prolonged wet weather when the pathogen grows upward on the outside surface of sorghum stalks, where it temporarily becomes established in leaf sheaths or whorls. No control measures presently exist for this disease.

POTASSIUM

‘‘‘Potassium: ‘‘‘ Symptoms of potassium deficiency may not be visible before reductions in plant growth occur. Disorders from excess potassium seldom occur unless plants are grown under unusual conditions.

PYTHIUM ROOT ROT (PYTHIUM SPP)

‘‘‘Pythium Root Rot (Pythium spp.) ‘‘‘ Symptoms occurring on large adventitious, or buttress, roots are darkening, blackening, and the formation of sunken, red-brown to black lesions. Sometimes, at root death, the lesions or the entire root is tan. On the Texas High Plains, root infections of sorghum probably occur throughout the season but increase at the boot stage or later, when large numbers adventitious roots are being produced in irrigated fields with high soil temperature and high soil moisture. There are no control measures available for the disease.

ROOT AND STALK ROTS

‘‘‘Root and Stalk Rots: ‘‘‘ In irrigated sorghum, charcoal rot is not a problem. Early harvesting sometimes prevents direct losses from stalk rots, but early-harvested grain is higher in moisture than grain harvested later and therefore must be artificially dried before it can safely be stored. Fusarium stalk rot develops under stresses somewhat similar to those that foster charcoal rot, and at times the two occur simultaneously. The pathogens that cause root and stalk rots are everywhere, and exclusion of their inoculum is unlikely. Resistant cultivars will soon become available as control measures.

ROOT KNOT NEMATODES (MELOIDOGYNE SPP)

‘‘‘Root-Knot Nematodes (Meloidogyne spp.) ‘‘‘ Several species have been reported as parasites on sorghum. Typical symptoms of infestations include irregular areas containing yellow and stunted plants, delayed blooming, and yield reductions of up to one third. Symptoms of infestations are subtle including inconspicuous galls that form on roots, and the nematode has limited or no visible effect on plant growth. Adult females are 0.8-1.0 mm long and 0.5- 0.8 mm wide.

ROOT LESION NEMATODES (PRATYLENCHUS SPP)

‘‘‘Pratylenchus spp (Pratylenchus spp.) ‘‘‘ Numerous root-lesion nematodes have been associated with sorghum, but only P. hexincisus and P. zeae have been shown to be pathogenic to it.

ROUGH LEAF SPOT (ASCOCHYTA SORGHINA)

‘‘‘Rough Leaf Spot (Ascochyta sorghina) ‘‘‘ Rough leaf spot has been reported in most countries where sorghum is grown and is more common in humid areas. Rough leaf spot is easy to identify in the later stages of development by the hard, black, raised pycnidia of the fungus, which give a sandpapery roughness. The fungus is spread mainly by airborne conidia during wet weather. Losses resulting from rough leaf spot are generally minor, and where the disease is a problem on sweet sorghum, cultivars with high levels of resistance are available.

RUST (PUCCINIA PURPUREA)

‘‘‘Rust (Puccinia purpurea) ‘‘‘ Rust caused by Puccinia purpurea is known to occur wherever sorghum is grown. In the pustules, urediniospores and teliospores are interspersed with abundant paraphyses, which are short, and usually bent inward. Urediniospores germinate within 1-2 hours by a single germ tube, which adheres closely to the leaf surface. Oxalis corniculata is the alternate host of the disease. Uredinia and telia are sometimes infected in areas where rainfall is high. In regions where rust becomes an economically important disease, the growing of less susceptible or resistant cultivars may provide the only practical control.

SEEDLING DISEASES

‘‘‘Seedling Diseases: ‘‘‘ Seedling diseases can be caused by both seedborne and soilborne fungi. Problems with stand establishment generally occur when sorghum is planted in cold and wet soils and the seeds fail to germinate. Resistant reduction often occurs when sorghum is planted in hot soils (50 °C). Many fungicides applied to sorghum seed are not effective.

SMUTS

‘‘‘Smuts: ‘‘‘ Covered kernel smut and loose kernel smut would cause major losses if seed treatment fungicides were not used. Head smut has not yet been controlled chemically, and so the emphasis is on the use of host resistance for control. In sorghum-producing areas with a high probability of head smut, variation in the pathogen population has caused concern.

SODIUM

‘‘‘Sodium: ‘‘‘ Sodium is not required for sorghum growth. Excess of sodium can occur when salinity is a problem.

SOIL INSECTS

‘‘‘Soil Insects: ‘‘‘ Several species of true and false wireworms attack the planted seed and roots of sorghum. The white grub does its most obvious and significant damage to sorghum soon after plants have emerged from the soil. Cutworm larvae are surface feeders that cut off sorghum plants at or slightly below the soil surface. Larvae of the southern corn rootworm bore into the crown of sorghum just above the roots and kill the growing point.

SOOTY STRIPE (RAMULISPORA SORGHI)

‘‘‘Sooty Stripe (Ramulispora sorghi) ‘‘‘ Sooty stripe has been found widely distributed in most areas of the world. The disease occurs on sorghum at all growth stages from seedling through maturity. The fungus grows slowly, with optimum growth at 28 °C and a pH of 4.0, forming a circular, compact, and crumpled colony with an entire margin. The pathogen survives as sclerotia in leaf residue on or below the soil surface. Crop rotation or destruction of infected leaf debris is recommended as a means of reducing initial inoculum.

SORGHUM DOWNY MILDEW (PERONOSCLEROSPORA SORGHI)

‘‘‘Sorghum Downy Mildew (Peronosclerospora sorghi) ‘‘‘ Sorghum downy mildew has a wide geographic and climatic range. Sorghum downy mildew occurs as either systemic or localized infection. The localized form of the disease results from conidial infection of leaves after emergence. Oospores in the soil germinate and invade the roots of sorghum seedlings. The minimum soil temperature for infection of sorghum by oospores is 10 °C. Sorghum downy mildew can be reduced or controlled by seed treatment with metalaxyl, a systemic fungicide, growing resistant cultivars, deep plowing, and crop rotation.

SORGHUM STUNT MOSAIC(SSMV)

‘‘‘Sorghum Stunt Mosaic (SSMV) </b></S></CF> This virus includes chlorotic and necrotic mottling, streaking of leaves, severe stunting, and reduced seed set. The sorghum stunt mosaic virus (SSMV) belongs to the Rhabdovirus group and is a bacilliform particle measuring approximately 68 X 220 nm. Hosts include maize and wheat in addition to sorghum. The virus is not mechanically transmissible.

SOUTHERN SCLEROTIAL ROT (SCLEROTIUM ROLFSII)

‘‘‘Southern Sclerotial Rot (Sclerotium rolfsii) ‘‘‘ Sclerotium rolfsii is a soilborne pathogen that attacks numerous plant hosts in warm regions of the world. The initial symptom is a water-soaked lesion, either bright red, purple, or brown, at the base of the lowest leaf sheath in contact with the soil. Thick, fan-shaped, superficial mycelial mats are often formed on the inner surface of leaf sheaths, in the outer ligule area of dry leaf sheaths, and sometimes in the area where a ligule contacts the subsequent sheath. The disease is a parasite capable of growing saprophytically in soil surface layers. There are no control measures available at this time for the disease.

STEM FEEDERS

‘‘‘Stem Feeders: ‘‘‘ The first indication that a plant is infested with the spotted stem borer is the appearance of small, elongated windows in young whorl leaves, where the larvae have eaten the upper surface but have left the transparent lower epidermis intact. Other borers, including the pink borer, the sugarcane borers, and the maize stalk borer feed on full leaves and especially leaf sheaths during early instars, and they tunnel into stems through internodes during later instars.

STORAGE MOLDS

‘‘‘Storage Molds: ‘‘‘ Storage molds of sorghum grain are caused by fungi that grow at relatively low moisture levels in the grain. Symptoms of fungal colonization in sorghum grain include darkening of the embryo or germ and molding, heating, and mustiness of the grain. Aspergillus Link species have upright, simple conidiophores, which arise from prominent foot cells and terminate in globose or vesicles. Penicillium Link species produce conidiophores without foot cells. Under certain conditions, species in both genera produce mycotoxins that are toxic to humans and animals. The rate of molding of stored grain depends on the inoculum, the environment, and the grain. The most common control method is prevention of fungal development by both drying or cooling the grain to a point at which fungi cannot grow (moisture content less than 13% and temperature below 5 °C). Numerous other methods can protect grain from storage fungi, but they may be of limited value with sorghum.

STORAGE MOLDS (ASPERGILLUS AND PENICILLIUM SPP)

‘‘‘Storage Molds (Aspergillus and Penicillium spp.)</b></S></CF> Storage molds cause darkening of the embryo, heating and mustiness of the grain. As the temperatures increase, the severity of the molding also increases. The best forms of control are to monitor the moisture content and temperature of the grain, air or ground, and relative humidity. Proper storage will increase the quality of grain over short or long periods of time.

STUNT MENATODES (TYLENCHORHYNCHUS, QUINISULCIUS, MERLINIUS SPP)

‘‘‘Stunt Nematodes (Tylenchorhynchus, Quinisulcium, Merlinius spp.) ‘‘‘ Species of Tylenchorhynchus, Quinisulcius, and Merlinius are medium sized ectoparasitic nematodes found in soil around the roots of numerous plant species, particularly cereal crops. In general, the root system of an infested plant may be poorly developed and have fewer feeder roots, and some of the root tips may be short and thickened.

SUGARCANE FIJI DISEASE (SCFD)

‘‘‘Sugarcane Fiji Disease (SCFD) </b></S></CF> Plants infected with the disease may have leaf galls, shortened stalks, stiff malformed leaves, and poorly formed heads. The leafhopper Perkinsiella saccharicida Kirkaldy transmits the virus to sorghum. The virus is an isometric particle measuring about 70 nm in diameter and containing double-stranded RNA.

SUGARCANE MOSAIC (SCMV)

‘‘‘Sugarcane Mosaic (SCMV) ‘‘‘ Sugarcane mosaic virus has a wide host range of perennial and annual grasses. Sugarcane mosaic is soilborne and aphids acquire the disease in probing infected plants for periods as short as 30 seconds to 1 minute. Vector control has not been economical or effective in controlling sugarcane mosaic.

SULFUR

‘‘‘Sulfur: ‘‘‘ Symptoms of sulfur deficiency appear on the upper, emerging leaves and are more pronounced at the base than at the tip. Disorders from excess sulfur may be due to sulfur dioxide, an air pollutant from industrial plants burning coal and fossil fuels.

TAR SPOT (PHYLLACHORA SACCHARI)

‘‘‘Tar Spot (Phyllachora sacchari) ‘‘‘ Tar spot occurs in Madagascar, Malaysia, Thailand, India, and the Philippines. The disease causes black spots on sorghum leaves and ascospores penetrate the leaves and produce small, round or sometimes elongated, raised spots. The use of resistant cultivars seems to be the most promising control measure. Tar spot does not become a problem on sorghum grown in the dry season.

TARGET LEAF SPOT (BIPOLARIS SORGHICOLA)

‘‘‘Target Leaf Spot (Bipolaris sorghicola) ‘‘‘ Target leaf spot occurs in the United States, Sudan, Israel, India, Cyprus, the Philippines, and Taiwan. The disease first appears as reddish or grayish dots, which later develop into definite spots, assuming an elliptic or oval to cylindric form. The fungus attacks plants at all stages development. The disease may be controlled with resistant cultivars.

TRIAZINES

‘‘‘Triazines: ‘‘‘ Injury from triazines (e.g., atrazine, cyanazine, propazine) is most likely to occur on sandy soils containing little organic matter or following heavy rains.

VESICULAR ARBUSCULAR MYCORRHIZAE

‘‘‘Vesicular-Arbuscular Mycorrhizae: ‘‘‘ Mycorrhizae are symbiotic associations formed between certain fungi and the roots of higher plants, which under normal conditions benefit both symbionts. In sorghum, primary root systems can be infected very rapidly after germination.

VIRAL DISEASES

‘‘‘Viral Diseases: ‘‘‘ Various strains of sugarcane mosaic virus cause mosaic symptoms that are typical of the problems caused by insect vectored viruses or viruslike pathogens. Control of viral diseases of sorghum depends in part on eradication of susceptible collateral hosts, avoidance of vectors, and the use of host resistance.

WATER AND TEMPERATURE STRESSES

‘‘‘Water and Temperature Stresses: ‘‘‘ Sorghum is recognized as one of the more heat-tolerant and drought-tolerant cereal grains, and this characteristic explains its popularity as a summer crop in the central and southern Great Plains of the United States and in much of the semiarid tropics.

WATER OVERSUPPLY

‘‘‘Water Oversupply: ‘‘‘ Even though sorghum is grown principally in drought-prone regions, problems can arise from an oversupply of soil water on the wetter fringes of production areas. Excessive rainfall during the harvest period can also cause reductions in grain quality.

WITCHWEED (STRIGA SPP)

‘‘‘Witchweed (Striga spp.) ‘‘‘ Most of the damage to sorghum caused by Striga occurs before the parasite emerges from the soil. Striga causes losses to the host plant by diverting the host's water and nutrients into its own system. Seeds of Striga are minute and are produced in enormous numbers, an average plant produces between 0.1 million and 0.4 million. After germination, the radicle tip grows toward the nearest host root with this movement possibly being a chemotropic response. Before its emergence, Striga is completely dependant on the host roots for water and nutrients. Plants belonging to the genus Striga are annual herbs, usually scabrid, with opposed lower leaves and alternating upper leaves. Control of Striga is a complicated task because of the extremely long viability period of its seed in the soil and the very specific environmental requirements for seed germination.

YELLOW SORGHUM STUNT

‘‘‘Yellow Sorghum Stunt: ‘‘‘ The only sorghum disease definitely shown to be incited by a mycoplasma is yellow sorghum stunt. It is characterized by a yellow-tinged cream color that makes affected plants conspicuous from a distance. Plants that are infected early in the season when they are less than 0.3 m high remain at the height they had attained at the time of infection, but their stalks continue to increase in diameter. Electron micrographs of diseased leaf sections reveal mycoplasmalike bodies in sieve elements of the phloem. Sorghum varieties with adequate resistance to the disease are available and can be grown if the disease becomes serious.

ZINC

‘‘‘Zinc: ‘‘‘ Zinc deficiency occurs in young leaves, which become uniformly pale green to yellow and the discoloration progresses from the base toward the tip.

ZONATE LEAF SPOT (GLOEOCERCOSPORA SORGHI)

‘‘‘Zonate Leaf Spot (Gloeocercospora sorghi) ‘‘‘ Zonate leaf spot is common on sorghum during wet periods and may also infect maize, millet, sugarcane, and numerous other grasses. The disease is conspicuous on sorghum leaves as circular, reddish purple bands alternating with straw colored or tan areas, which form a concentric, or zonate, pattern with irregular borders. The fungus remains as sclerotia formed within the dead tissue of old leaf lesions, where they appear as small, raised bodies in lines parallel with the veins. Losses from this disease can be reduced by crop rotation and clean cultivation to destroy residues of susceptible weed hosts.

ZONATE LEAF SPOT ON LEAF SHEATHES (GLOEOCERCOSPORA SORGHI)

‘‘‘Zonate Leaf Spot on Leaf Sheaths (Gloeocercospora sorghi) ‘‘‘ A leaf sheath blight is common in sorghum fields in southern Texas. Initial infection may occur anywhere on basal leaf sheaths. Zonate leaf spot on leaf sheaths is associated with a high incidence of zonate leaf spot on leaf blades and sheaths. Crop rotation and burial of crop residue help to reduce initial inoculum.