Small Grains:Diagnostics

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SMALL GRAIN PESTS AND NUTRITIONAL DISORDERS


ABIOTIC DISORDERS

‘‘‘Abiotic disorders: ‘‘‘ Environmental factors such as low temperature, inadequate nutrient levels, and inadequate or excessive soil moisture can cause abnormal growth of grain plants and reduced grain production. Under some circumstances, herbicides applied to the grain crop or herbicide residues remaining from a previous crop can injure grain.


ATRAZINE

‘‘‘Atrazine: ‘‘‘ Atrazine can cause the leaves of small grains to turn yellow or brown and injury occurs if residues left from a previous crop are not broken down sufficiently.


BACTERIAL BLIGHTS (XANTHOMONAS CAMPESTRIS AND PSEUDOMONAS SYRINGAE)

‘‘‘Bacterial blights (Xanthomonas campestris and Pseudomonas syringae) ‘‘‘ Bacterial Leaf Streak and Black Chaff. Bacterial leaf streak develops small, water-soaked spots on the leaves that enlarge and turn brown. Lesions develop into streaks between leaf veins on barley but streaking is not common on wheat leaves. If wet weather occurs during heading, the bacteria may infect glumes and cause symptoms known as black chaff which is dark, water-soaked streaks that form between the veins of glumes and usually appear at the base of the awns. Bacterial Leaf Blight of Wheat. Bacterial leaf blight of wheat requires wet or humid and cool weather in the spring. Leaf blight first appears after the boot stage as small, grayish-green, water-soaked spots on the upper foliage. Lesions enlarge and turn a light tan and may grow together to form large blotches or streaks. Halo Blight and Stripe Blight of Oats. Halo blight occasionally kills seedlings and leaves, leaf sheaths, and panicles of older plants can also be attacked. The bacteria that cause halo blight and stripe blight are seedborne. The first symptoms of both diseases are water-soaked spots on leaves that become tan or light brown in the middle with pale green margins (halos). They may grow together to form blotches. Eventually lesions of both kinds of blight turn brown, and heavily infected leaves die. Plants may recover and produce healthy growth during warm dry weather. The best forms of control include using clean seed, crop rotation, and practices that eliminate crop residue and control grass hosts.


BARLEY STRIPE (PYRENOPHORA GRAMINEA)

‘‘‘Barley stripe (Pyrenophora graminea) ‘‘‘ Barley stripe is a seed-transmitted disease that occurs wherever barley is grown but does not occur on grains other than barley. It is most severe where sprinkler irrigation is used or high humidity occurs during heading. Diseased tissue turns brown as the fungus produces spores, then dries up and splits lengthwise along leaf veins, giving leaves a frayed appearance. Symptoms usually first appear on the second or third leaf just as it is forming. Barley stripe is controlled by using certified seed or seed treatment.


BARLEY YELLOW DWARF (BYDV)

‘‘‘Barley yellow dwarf (BYDV) ‘‘‘ Barley yellow dwarf causes stunting, foliage discoloration, reduced root development, and reduction in yield. The oldest leaves turn bright yellow or have a red discoloration, which begins on leaf margins and tips and spreads toward the base of leaves. Barley yellow dwarf is managed by planting resistant cultivars and avoiding early fall or late winter plantings that expose young plants to peak aphid populations.


BLACK POINT

‘‘‘Black point: ‘‘‘ Black point, also known as kernel smudge, is a darkening of grain kernels caused by fungi that infect the seed coat when high humidity or rain occur during grain filling. The embryo end of affected kernels may become shriveled. Both barley and wheat may be affected, but the disease is primarily a problem of durum wheat. Lodging increases the incidence of black point. The best forms of control include avoiding excessive nitrogen fertilization and late season irrigations.


BROMOXYNIL

‘‘‘Bromoxynil: ‘‘‘ Leaf tips may turn brown if bromoxynil is applied following a frost or if high rates are used on oats. Leaf tips of barley may turn white following a bromoxynil application and all plants generally recover from the injury.


CHLORSULFURON

‘‘‘Chlorsulfuron: ‘‘‘ Application of chlorsulfuron during cold weather or to small grains may cause yellowing and stunted growth. Plants usually recover after a few weeks.


COVERED SMUT (TILLETIA CARIES AND TILLETIA FOETIDA)

‘‘‘Covered smut (Tilletia caries and Tilletia foetida) ‘‘‘ Plants infected with covered smut are stunted and heads emerge later than normal. Glumes of infected wheat and barley spikes are spread apart and spikes appear darkened and are often distorted. The spore masses on wheat are often called bunt balls and have a distinctive odor of decaying fish. Florets of oat plants with covered smut appear gray because the black spore masses are contained within a layer of remaining hull and lemma tissue.


CROWN RUST (P. CORONATA)

‘‘‘Crown rust (P. coronata) ‘‘‘ Crown rust pustules are reddish orange, oblong, and appear mainly on leaves, but also on stems. Smaller size, orange color, and lack of ragged edges distinguishes them from the pustules of stem rust. Shiny black, nonerupting pustules containing teliospores are formed on the undersides of leaves when oats begin to ripen.


CULTURAL PRACTICES

‘‘‘Cultural Practices: ‘‘‘ Following the recommended cultural practices for your area helps minimize disease losses. Many of the pathogens that cause small grain diseases survive between crops on crop residue or as spores in the soil. Removing, burning, plowing under, or allowing the decomposition of crop residue between grain crops reduces the primary inoculum of several important diseases. Other control measures include avoiding both excesses and deficiencies of nutrients and choosing a correct planting date to avoid some diseases.


DICAMBA

‘‘‘Dicamba: ‘‘‘ Dicamba may cause weak growth of tillers, lodging, delayed flowering, failure of kernels to develop, and deformed spikes. Injury can result when application is improperly timed or when grain is stressed by hot and dry or cold weather. Grain can also be injured if planted too soon after high rates of dicamba have been used.


DICLOFOP

‘‘‘Diclofop: ‘‘‘ Diclofop may kill roots or parts of the stem and crown. Yellow or brown patches may develop along the edges of leaves, and some leaves may die. Injury can occur if plants are stressed by waterlogged soil, cold and foggy weather, drought, or phosphorus deficiency.


DIFENZOQUAT

‘‘‘Difenzoquat: ‘‘‘ Difenzoquat may cause yellowing and tip-burn of leaves if applied when grain plants are stressed by cold and wet or hot and dry weather or by nutrient deficiency.


ERGOT (CLAVICEPS PURPUREA)

‘‘‘Ergot (Claviceps purpurea) ‘‘‘ Claviceps purpurea The most prominent symptom of ergot is the presence of bluish black, elongated sclerotia replacing one or more kernels of a grain spike. Sclerotia are solid and grayish inside, distinguishing them from the dark spore masses produced by smut fungi. The fungus that causes ergot survives winter as sclerotia in or on the soil. The best form of control is to use clean seed, rotate the crop , and practice deep tillage.


FLORAL DISEASES

‘‘‘Floral diseases: ‘‘‘ Floral, or flower-infecting, diseases of small grains are caused by fungi that destroy developing grain kernels, replacing plant tissue with spores or spore-producing structures. The most important flower-infecting diseases of small grains are the smuts. Seed treatments and seed certification are used to eliminate the seedborne inoculum of these diseases.


FOLIAR DISEASES

‘‘‘Foliar diseases: ‘‘‘ Foliar diseases inhibit grain filling by reducing photosynthesis of the host and some foliar diseases reduce the number of kernels per spike if they develop during early growth stages. Important foliar diseases are caused by fungi that survive on crop residue, volunteer grain plants, or weed hosts and are spread by wind or splashing water. The best forms of control include timely applications of foliar fungicides, using resistant cultivars, using certified seed and seed treatment fungicides, and careful monitoring of wet conditions.


HERBICIDE INJURY

‘‘‘Herbicide injury: ‘‘‘ Small grains can be injured by herbicides that are applied at the wrong growth stage or applied when plants are stressed by adverse environmental conditions. The best form of control is to follow the label directions regarding proper application procedures.


KERNEL BUNT (TILLETIA INDICA)

‘‘‘Kernel bunt (Tilletia indica) ‘‘‘ The kernal bunt fungus produces dark spore masses on infected kernels that have a fishy odor. However, usually only a few kernels in each head are infected and only part of each infected kernel is replaced by smut spores. In the presence of free moisture, spores germinate to produce a secondary type of spore, which is spread by wind and infects individual florets during flowering. The fungus then replaces some or all of the kernel. Smuts are controlled effectively by using certified seed and seed treatments.


LEAF BLOTCH (PYRENOPHORA AVENAE)

‘‘‘Leaf blotch (Pyrenophora avenae) ‘‘‘ Leaf blotch survives on crop residue and inside the hull of infected seed. As the disease progresses, lesions develop on the leaves as small, reddish purple spots that develop into oblong blotches. Leaf blotches turn reddish brown as they enlarge, and leaves eventually turn brown and die. Practices used to control other diseases, such as use of certified seed, seed treatment fungicides, crop rotation, and elimination of crop residue, help control leaf blotch as well.


LEAF RUST (PUCCINIA RECONDITA)

‘‘‘Leaf rust (Puccinia recondita) ‘‘‘ Leaf rust pustules are reddish orange and oval, and they form on leaf blades and sheaths but not stems. They lack the ragged edges present on stem rust pustules. Individual pustules remain small (2 to 3 mm in diameter), but entire leaves may appear covered with rust when severely affected. As plants dry up at the end of the season, pustules may turn dark and shiny with the formation of dark brown, thick-walled teliospores. The best form of control is to apply foliar fungicide treatments.


LEAF SCALD (RHYNCHOSPORIUM SECALIS)

‘‘‘Leaf scald (Rhynchosporium secalis) ‘‘‘ Leaf scald of barley appears as dark, bluish gray spots. These spots expand into oval or elliptical lesions that have light, bluish gray centers surrounded by a dark brown ring. The light-colored center and dark brown margin are characteristic of barley leaf scald. As lesions age, their centers turn dry and light brown. Lesions may grow together as the disease progresses, and entire leaves may be covered and killed if the disease is severe. Leaf scald is controlled by planting the most resistant cultivars available, eliminating crop residue, controlling volunteer barley and grass weeds, and using crop rotation.


LOOSE SMUT (USTILAGO TRITICI)

‘‘‘Loose smut (Ustilago tritici) ‘‘‘ Loose smut symptoms usually are not apparent until heading. Heads of infected plants usually emerge earlier than healthy heads, and consist of olive black masses of teliospores in the place of kernels. The spore masses are contained within thin gray membranes that rupture soon after heads emerge. The spores are blown away by air currents, leaving behind the rachis of the head and some spores clinging to fibers where spikelets normally would be.


LOW TEMPERATURE INJURY

‘‘‘Low temperature injury: ‘‘‘ Small grains are most sensitive to low temperatures during early seedling development and flowering. Severe frosts can injure the grain spike anytime after it has begun to form. If frost occurs several days in a row, seedling leaves emerge with bands of yellow or white where cells were killed. Light frost during flowering inhibits pollination. Those florets that are flowering at the time low temperatures occur remain sterile. Try to avoid low temperature injury during flowering by matching cultivar maturity with planting date so that heading will occur after the greatest danger of frost is past.


NAPROPAMIDE

‘‘‘Napropamide: ‘‘‘ If small grains are planted too soon after napropamide or pronamide has been used in a broadleaf rotation crop, injury can occur. These herbicides inhibit germination and the formation of secondary roots. Seedlings may fail to emerge or will emerge but develop only two or three leaves before turning yellow and dying.


NET BLOTCH (PYRENOPHORA TERES)

‘‘‘Net blotch (Pyrenophora teres) ‘‘‘ Net blotch first appears as tiny spots or streaks on leaves. The spots may initially be dark green and water-soaked, but turn light brown as they develop. Diseased areas expand and a dark brown, netlike pattern develops within the light brown lesions. Surrounding tissue becomes yellow. The disease reduces yield and grain quality. Net blotch is controlled by planting the most resistant cultivars available for the area, avoiding early planting, eliminating crop residues and volunteer barley, using crop rotation, and planting clean seed.


NUTRIENT DEFICIENCIES

‘‘‘Nutrient deficiencies: ‘‘‘ Symptoms of nutrient deficiency can be caused by low levels of nutrients in the soil or conditions that interfere with the plant's ability to obtain nutrients from the soil. Excessive soil moisture may reduce soil levels of nitrogen and sulfur by increasing leaching and can increase nitrogen loss to denitrification. Cold soil or high soil pH make it difficult for plant roots to obtain phosphorus from the soil. Use raised-bed plantings or careful leveling of fields during bed preparation to improve drainage and avoid ponding of water in low spots. This decreases the likelihood of nitrogen loss to denitrification.


PARAQUAT

‘‘‘Paraquat: ‘‘‘ If paraquat spray drifts onto small grain foliage, white spots or patches of bleached tissue develop. Affected areas may turn brown and dry up. Scald lesions can be distinguished by their oval, lens shape and the development of a dark brown margin.


PESTICIDES

‘‘‘Pesticides: ‘‘‘ Seed treatment and foliar-applied fungicides may he used for disease control small grains. The fungi that cause smut diseases and barley stripe survive and are transmitted on or in seed. Use of certified seed greatly reduces the incidence of these diseases, but they can also be controlled with seed treatments. Foliar-applied fungicides usually are not economical for small grains and are more effective for rusts and powdery mildew.


PHENOXY

‘‘‘Phenoxy: ‘‘‘ The phenoxy herbicides 2,4-D and MCPA can cause stunting, distortion of leaves, distortion of spikes, and failure of kernels to develop. Injury occurs when the herbicides are applied at the wrong growth stage or when weather conditions following application favor unusually rapid growth of grain plants.


POWDERY MILDEW (ERYSIPHE GRAMINIS)

‘‘‘Powdery mildew (Erysiphe graminis)’‘‘ Powdery mildew develops most often on the upper surface of lower leaves. The disease first appears as white or light gray spots, usually on the upper side of the leaf, with yellow or pale green spots on the bottom leaf surface. The spots enlarge and the grayish white patches, which are made up of fungus tissue, take on a fluffy, powdery appearance as they begin to produce spores. Resistant cultivars of barley and wheat are available. Crop rotation, elimination of crop residue, and control of volunteer grains and weed hosts greatly reduce inoculum survival from one season to the next.


RESISTANT CULTIVARS

‘‘‘Resistant Cultivars: ‘‘‘ Several diseases of small grains can be controlled effectively by using resistant cultivars. Because new strains or races of pathogens can develop, consult your local farm advisor or pest control adviser for a list of cultivars that currently are resistant to the important diseases in your area.


ROOT/CROWN DISEASES

‘‘‘Root/crown diseases: ‘‘‘ Two important diseases of small grains, common root rot and take-all, are caused by fungi that invade roots and lower stems. Both these diseases darken the roots and crown area of affected plants and cause the development of white, sterile spikes. The pathogens survive on crop residue, volunteer grains, or grass weeds, and are dispersed by movement of contaminated crop residue and infested seed. These diseases are managed by using crop rotation, eliminating crop residue, managing soil fertility, and conserving soil moisture.


ROOT ROT/SCAB (BIPOLARIS SOROKINIANA AND FUSARIUM SPP)

‘‘‘Root rot/scab (Bipolaris sorokiniana, Fusarium spp.) ‘‘‘ This disease is characterized by stunting, development of few tillers, heads that lack grain or have a few shriveled kernels, and spikes of diseased plants turn white. The crown and lower stems of affected plants have a light to dark brown discoloration. Plants have a poorly developed, darkened root system. Symptoms are most pronounced and losses most severe when plants are stressed for water. Some of the fungi that cause common root rot may also cause a blight of grain heads, called scab, when wet conditions occur during heading. The best forms of control include conservation of soil moisture in dryland situations, provision of good soil drainage where needed, crop rotation, and clean cultivation between crops.


RUSTS (PUCCINIA SPP)

‘‘‘Rusts (Puccinia spp.) ‘‘‘ Yellow-orange to brownish red, powdery rust spores appear on leaves, stems, and sometimes flower parts of infected plants. As plants begin to dry up at the end of the season, rust pustules turn dark and shiny as spores called teliospores are formed. Rust infections increase water loss and decrease the amount of photosynthate available for grain filling causing fewer kernels that are smaller and shriveled. The effects of rust on grain yields are more pronounced if the disease becomes severe before or during flowering. Use of resistant cultivars is the primary means of controlling rust diseases.


SEED CERTIFICATION

‘‘‘Seed Certification: ‘‘‘ The use of certified seed helps reduce the introduction of pathogens that may contaminate grain seed. These include the fungi that cause scald and net blotch of barley and leaf blotch of oats.


SEPTORIA LEAF BLOTCH (SEPTORIA SPP)

‘‘‘Septoria leaf blotch (Septoria spp.) ‘‘‘ Septoria leaf blotch reduces grain filling, causing poorly filled heads and shriveled grain. The disease is more pronounced during periods of damp weather and water-soaked or yellowish spots or flecks appear on the lower leaves. These spots develop into irregular shaped, yellow areas that turn reddish brown with grayish brown, dry centers. The disease can be controlled by planting a resistant cultivar or with properly timed fungicide sprays and destroying infected crop residue.


SMUT DISEASES

‘‘‘Smut diseases: ‘‘‘ This disease is characterized by fungi that develops just behind the growing point. As heads form, the contents of grain kernels are replaced with masses of dark smut spores called teliospores. Blackish masses of spores in the place of healthy spikes or dark spore masses replacing grain kernels are characteristic. Teliospores are spread by wind to flowering spikes of healthy plants, where they infect florets.


STEM RUST (PUCCINIA GRAMINIS SUBSP TRITICI)

‘‘‘Stem rust (Puccinia graminis subsp tritici) ‘‘‘ Stem rust develops elongated pustules on stems but also on leaves and leaf sheaths, and are distinguished by their brownish or brick red color and conspicuously tattered edges. A film of moisture is necessary for infection. The rust fungus grows between host cells just under the outer plant surface or epidermis. Fungus tissue grows just below the epidermis and as urediospores are formed, the epidermis bursts and the rust pustules appear. The best form of control is the use of resistant cultivars and applying foliar fungicides.


STRIPE RUST (PUCCINIA STRIIFORMIS)

‘‘‘Stripe rust (Puccinia striiformis) ‘‘‘ Stripe rust pustules are yellow-orange, develop mainly on leaves and spikes, and tend to grow together into thin stripes that run lengthwise between the veins of blades or on the inner surface of glumes. Stripe rust symptoms usually appear earlier in the season, because the fungus develops at lower temperatures than the other rust fungi. The best form of control is the use of resistant cultivars.


SUMMARY

‘‘‘Summary: ‘‘‘ A number of diseases can be important factors limiting small grains production. Local weather conditions, relative resistance of cultivars to the diseases present, and use of disease-free seed are primary determinants of what diseases are important in a given area. Cropping history and soil fertility are also important. Many diseases of small grains are favored by moderate temperatures and high humidity or frequent rains, conditions that usually prevail from late fall through early spring in some of the major grain-growing areas. The best form of control is to use resistant cultivars, certified seed and use good cultural practices.


TAKE ALL (GAEUMANNOMYCES GRAMINIS SUBSP TRITICI)

‘‘‘Take all (Gaeumannomyces graminis subsp tritici) ‘‘‘ Take-all is a disease that occurs primarily on fall-planted wheat and barley. Yields are reduced because grain filling is inhibited or prevented. Plants affected by take-all may begin to show symptoms of stunting and reduced tillering early in the growing season. Plants also develop white heads that lack grain or contain shriveled kernels, are pale with few tillers, and are more prone to lodging than healthy ones. The bases of affected stems are darkened and have a shiny black appearance when the leaf sheath is pulled away. Roots become blackened and brittle and a layer of dark brown or black fungus mycelium develops underneath the lowest leaf sheaths. Take-all is managed by using crop rotation, using raised-bed planting in poorly drained soils, and avoiding certain forms of nitrogen fertilizer.


TRIFLURALIN

‘‘‘Trifluralin: ‘‘‘ Trifluralin residues may stunt the growth of small grains may appear bluish green, but germination and emergence usually are not affected.


WATER STRESS

‘‘‘Water stress: ‘‘‘ Small grains can be stressed by inadequate or excess water and develop a bluish-green color and leaves curl longitudinally. Severe water stress during jointing results in stunted plants and reduced yields regardless of subsequent irrigations or rainfall. Waterlogged soil may cause leaves to turn yellow or purple. Severe waterlogging kills plants.