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Brown Spot of Soybean
Causal Agent:
The causal agent of brown spot is the ascomycete fungus, Septoria glycines. It produces brown, globose fruiting bodies called pycnidia. The pycnidia are embedded in leaf tissue where they overwinter in the field in plant debris. The pycnidia produce hyaline, septate conidia, or infective spores, that will germinate in water on the leaf surface.
Symptoms and Signs:
Symptoms of brown spot first appear as small brown lesions on the cotyledons or unifoliate leaves and then move up the plant as the season progresses but remain in the lower canopy. Reddish-brown to dark brown spots, which are angular in shape, vary in size from small specks to ¼ inch in diameter. Individual lesions can coalesce, surrounding tissue turns yellow or chlorotic and premature defoliation then occurs. Lesions can range in size anywhere from small specks to spanning several square centimeters of leaf surface. Defoliation typically starts at the bottom of the plant and proceeds upwards. Irregular brown lesions with indefinite borders may also develop on infected stems, pods, and petioles.
Symptomatic leaves are usually located in the lower canopy.
Characteristic small, brown, irregular lesions surrounded by leaf chlorosis.
Comparison of lesions of bacterial blight (left) and those caused by S. glycines (right).
Quantification of disease severity using percentage of leaf area affected. Percentage is based on lesion area, not chlorosis.
Disease Cycle:
The fungus, Septoria glycines, overwinters on infected soybean debris. Warm, moist weather conditions will favor spore production on crop residue. The spores are disseminated by wind and rain to nearby soybean plants and leaves, which can infect through stomatal openings. Infections usually begin in the lower canopy and spread upward. Disease severity will increase during prolonged periods of leaf wetness (6-36 hours). Fruiting bodies (pycnidia) develop in mature lesions which provide inoculum for secondary infections of leaves, stems, and pods.
Moderate temperatures (60⁰-85⁰F) and high moisture favor infection and disease development. Hot, dry weather tends to halt the spread of S. glycines.
Disease Management:
Yield losses due to brown spot are rarely severe as most cultivars today have high levels of resistance. Cultural practices targeted towards limiting residue build-up can further limit losses
Host Resistance: It is important to use a cultivar with resistance towards S. glycines to avoid significant yield loss. In productions with no-till or narrow rows, host resistance is increasingly important.
Crop rotation: Rotating with non-hosts allows time for soybean straw containing fungal fruiting bodies to degrade. This disease has been known to be more severe in continuously cropped soybean fields.
Tillage: Plowing under soybean straw can promote rapid decay of debris.
Scouting: Losses of 2 to 4 bushels per acre have been reported in Ohio in very carefully controlled field studies, rarely is it economically feasible to manage this disease. For seed companies, monitor levels of defoliation near R6 to be sure that varieties still have good levels of resistance. Discard varieties when defoliation of the lower canopy reaches the mid-level of the plant. Proper identification of this disease will allow for more effective management.