Crop Diseases in Corn, Soybean, and Wheat Gail Ruhl Department of Botany and Plant Pathology Purdue University West Lafayette, IN 47907 Email: ruhlg@purdue.edu

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LEAF AND STEM DISEASES

SOYBEAN Disease Name: Bacterial Blight Pathogen: Bacterium. Pseudomonas syringae pv. glycinea Symptoms: Initial symptoms are small angular watersoaked spots on leaves. Lesion centers dry out and turn brown to black with watersoaked margins and yellow halos. Lesions may coalesce resulting in large blighted areas. Affected tissue often drops out, giving a tattered appearance to the leaves. Rarely a serious disease in Indiana soybeans. Conditions: Cool, rainy weather. Outbreaks often follow thunderstorms. Inoculum Survival: Seed, infected crop residue. Inoculum Dispersal: Airborne bacteria from rainsplash, mechanical (cultivators). Management: Resistant cultivars, plant pathogen-free seed, crop rotation, avoid cultivation when foliage is wet.

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SOYBEAN Disease Name: Brown Spot Pathogen: Fungus. Septoria glycines Symptoms: Irregular light-brown lesions, ranging in size from small specks to a few mm in diameter. Lesions eventually darken to brownish black. Lesions are primarily found on leaves, but can also occur on stems, petioles and pods. Early season infection is restricted to unifoliate and first trifoliate leaves. Conditions: Extended warm, wet weather. Inoculum Survival: Seed, infected crop residue. Inoculum Dispersal: Airborne spores. Management: Plant pathogen-free seed, crop rotation, foliar fungicide at R-3 stage, plow under crop residue.

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SOYBEAN Disease Name: Pod and Stem Blight Pathogen: Fungi. Diaporthe phaseolorum var. sojae and Phomopsis longicola Symptoms: No definite leaf or stem lesions are produced under field conditions. Fungal spore-bearing structures (pycnidia) appear as black specks in linear rows on dead stems and poorly-developed pods. Pod blight phase results in poor quality seeds and seed decay. Seedling blight may occur from seed infections. Conditions: Warm wet weather at R7 to R8 stages favors seed infection and decay. Inoculum Survival: Seed, infected crop residue. Inoculum Dispersal: Airborne spores. Management: Crop rotation, seed-applied fungicides for seedling blight. Foliar fungicide at R6 stage in seed production fields if risk of seed infection is high. Timely harvest.

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SOYBEAN Disease Name: Downy Mildew Pathogen: Fungus. Peronospora manshurica Symptoms: Pale green to yellow spots on upper leaf surface. Lesions turn grayish brown to dark brown with a yellow or light green margin. Tufts of gray fungal growth may be visible on lower leaf surface during moist weather. Severely affected leaves turn brown and drop. Whitish fungal growth may be present on interior of pods and seed coats. Systemically infected seedlings from infected seeds are stunted with mottled foliage. A common disease, but rarely causes significant yield loss. Conditions: High humidity and cool temperatures favor disease. Inoculum Survival: Seed, infected crop residue. Inoculum Dispersal: Airborne spores. Management: Crop rotation, plow under crop residue, resistant cultivars, fungicide seed treatment.

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ROOT AND LOWER STEM DISEASES

SOYBEAN Disease Name: Phytophthora Rot Pathogen: Fungus. Phytophthora sojae Symptoms: Seed rot and pre-emergence damping-off. Root and stem rot of older seedlings. In more mature plants chlorosis and wilting of leaves, with a dark brown discoloration on lower stem progressing upward from the soil line. Root rot of older plants may also occur. Affected plants are clustered in field. Conditions: Heavy soils and soil saturation for more than 24 hours promote disease. Disease is more severe with reduced-till. Highest risk with no-till. Inoculum Survival: Soil, infected crop residue. Inoculum Dispersal: Soilborne spores. Management: Resistant cultivars, fungicide (metalaxyl) seed treatment or metalaxyl in seed furrow, improve soil drainage.

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SOYBEAN Disease Name: Rhizoctonia stem rot Pathogen: Fungus. Rhizoctonia solani Symptoms: Pre-emergence or post-emergence damping off of seedlings. Brown or reddish brown lesions on larger seedlings and young plant stems at the soil line and on the taproot. Infected stems may break in the lesioned area. Conditions: Stress conditions- soil compaction, planting too deep, chemical damage. Warm and dry soil prior to planting followed by wet conditions after planting favors disease. Inoculum Survival: Soil, infected crop residue. Inoculum Dispersal: Soilborne hyphal fragments and survival structures. Management: Systemic fungicide seed treatments. Promote good soil conditions that favor rapid seedling development. Avoid herbicide or other chemical damage. Avoid planting too deeply and compacting soil.

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SOYBEAN Disease Name: Sclerotinia Stem Rot Pathogen: Fungus. Sclerotinia sclerotiorum Symptoms: Wilting and death. Dead leaves remain attached to stems. Tan to white lesions at nodes can girdle stems. Cottony fungal growth may be present on diseased plant parts. Large black structures (sclerotia) form in pith and on infected stems. Conditions: Cool, wet weather favors disease. Inoculum Survival: Sclerotia in soil, infected crop residue, seed. Sclerotia may survive for up to 7 years. Inoculum Dispersal: Airborne spores from structures on sclerotia. Management: Deep plow to bury sclerotia, then work soil shallow for 5 + years to keep sclerotia below soil surface. No-till a nonhost crop (corn, sorghum, small grains) for 2 years. Avoid close spacing and tall, bushy cultivars in fields with disease history. Avoid excessive irrigation before flowering. Cultivars with cv. Williams in parentage may be more susceptible.

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SOYBEAN Disease Name: Brown Stem Rot Pathogen: Fungus. Phialophora gregata Symptoms: Symptoms are usually not evident until late in the growing season. Leaves may suddenly discolor, with brown tissue between veins and green tissue along veins. Leaves remain attached. Browning of pith inside stem of infected plants. Conditions: Disease progresses more quickly in older than younger plants, regardless of temperature. Inoculum Survival: Soil, infected crop residue. Can survive 3-5 years. Inoculum Dispersal: Soilborne hyphal fragments and spores. Management: Resistant cultivars. Avoid susceptible cultivars for 3 years.

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SOYBEAN Disease Name: Sudden Death Syndrome Pathogen: Fungus. Fusarium solani race A Symptoms: Yellow interveinal blotches, which become necrotic leaving green tissue along the leaf veins. Leaf blades drop, leaving petioles attached to the stem. Foliar symptoms similar to brown stem rot. Light brown or gray discoloration occurs in internal taproot tissue, while pith remains white. Conditions: Often occurs with soybean cyst nematode, but SCN not necessary for disease to occur. Cool wet soils in early spring favor infection. Inoculum Survival: Soil, infected crop residue. Inoculum Dispersal: Soilborne spores and hyphal fragments. Management: Crop rotation. Plant as late as possible in the spring, and use cultivars from more than one maturity group. Manage SCN. Timely harvest.

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SOYBEAN Disease Name: Charcoal Root Rot Pathogen: Fungus. Macrophomina phaseolina Symptoms: Loss of vigor in mature plants. Leaves turn yellow and wilt but remain attached. Light gray or silver discoloration in taproot and lower stem after flowering. Small black fungal structures (microsclerotia) present in taproot and stem tissues. Conditions: High soil temperatures, dry weather. Plants that are under stress from adverse environmental or cultural conditions are more susceptible. Inoculum Survival: Microsclerotia in soil or infected crop residue. Inoculum Dispersal: Soilborne microsclerotia. Management: Crop rotation, adequate fertilization, irrigate to keep soil moisture high, seeding density and planting date to encourage early canopy closure.

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SOYBEAN Disease Name: Soybean Cyst Nematode Pathogen: Nematode. Heterodera glycines Symptoms: Slight to severe stunting with slight to severe chlorosis. Gradual yield decline over several years. Decreased nodulation, and canopy slow to close. Symptoms are similar to nitrogen and/or potassium deficiency. Some root decay. Cysts on roots first appear as white, lemon-shaped females on root surface. Cysts turn dark brown with age. Conditions: Symptoms are more pronounced under dry soil or other stress conditions. Can occur in any soil type, but often more apparent in light or sandy soils. Inoculum Survival: Cysts in soil. Inoculum Dispersal: Soilborne cysts and juvenile nematodes. Management: Crop rotation for 2 or more years between susceptible crops, good weed control, resistant cultivars. Avoid moving infested soil with equipment or seed.

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VIRUS DISEASES

SOYBEAN Disease Name: Bud Blight Pathogen: Virus. Tobacco Ringspot Virus Symptoms: Stunting occurs when young plants are infected. Minor disease in most fields in most years. Stem terminal will curve, forming a crook. Lateral buds become brown and drop off. Brown discoloration of stems, beginning at nodes. Leaves are dwarfed and curl upward. Pods often abort, or are underdeveloped. Infected plants remain green longer that healthy plants (delayed maturity). Symptoms often develop first on plants near the edges of a field. Conditions: Warm, dry conditions that favor vector development. Inoculum Survival: Seed, possibly weed hosts. Inoculum Dispersal: Vector. Thrips, dagger nematodes. Management: Virus-free soybean seed.

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SOYBEAN Disease Name: Soybean Mosaic Pathogen: Virus. Soybean Mosaic Virus Symptoms: Stunting, mottling and curling of leaves. Leaves may be puckered and misshapen, resembling 2,4-D injury. Diseased pods may be stunted and curved. Seeds from diseased pods may be discolored (hilum bleeding). A common disease, but rarely causes significant yield losses. Conditions: Cool, dry conditions which favor vector. Inoculum Survival: Seed. No overwintering hosts in northern US. Inoculum Dispersal: Vector. Aphids. Management: Resistant cultivars, virus-free soybean seed, remove symptomatic plants from seed production fields.

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