There are several foliar diseases of soybeans in Ohio. Yield reductions may occur if greater than 50% defoliation occurs early in the season. However, it is rare that foliar diseases have caused substantial losses in Ohio. This may change as more and more fields are in continuous soybean production.
Small chlorotic flecks will develop on the top surface of soybean leaves infected by this fungus. Under periods of high relative humidity, the fruiting structures, called sporangiophores, will appear as light purple fuzz on the underside of the leaves. Older leaves are resistant to this fungus but young leaves are highly susceptible. Seeds may also be affected and will have a thick white crust on the surface. This crust contains oospores, which are the overwintering structure. Severely infected leaves will drop off prematurely.
This leaf spot is caused by a bacterium. The lesions are small, angular, and surrounded by yellow. Lesions may coalesce or grow together, and older lesions may drop out of the leaves, giving the leaves a tattered appearance. The bacterium survives on the soybean seed and infects the cotyledons. Water splashing spreads the bacterium within the plant canopy.
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| Figure 32. Bacterial blight of soybean caused by Pseudomonas syringae. Small brown lesions are surrounded by bright yellow. |
This leaf spot is also caused by a bacterium and produces lesions, primarily on leaves, and first appears as small yellowish-green areas with reddish-brown centers. A small, raised pustule usually develops at the center of the lesion, typically on the lower leaf surface. This pustule or raised area distinguishes this bacterial disease from other leaf spotting pathogens.
This fungus overwinters on soybean straw left in the field. This fungus produces spores on the residue which are splashed onto the leaves. Severe infections can cause early defoliation in the spring and premature defoliation near maturation. The leaf spots are reddish-brown in color and irregular in shape and vary in size from minute flecks to approximately 1/4 inch in diameter. Infected leaves will quickly turn yellow and drop. This disease is worse in fields in continuous soybean production.
This foliar disease is caused by another residue-borne fungal pathogen. This leaf spot has gray or light tan centers with narrow reddish-brown borders. Premature defoliation occurs on heavily spotted leaves.
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| Figure 33. Frog-eye leaf spot, a fungal disease on soybean leaves caused by Cercospora sojina. |
Both of these diseases are stem and crown rots, but they both have very classic foliage symptoms. Sudden death syndrome is caused by the fungus, Fusarium solani f. sp. glycinea, and infects the roots and crowns. Brown stem rot is caused by the fungus Phialophora gregata and infects the stems, particularly the pith area. Foliar symptoms for both of these pathogens begin as yellow chlorotic areas on the leaves. These will eventually turn brown or necrotic. Look for the key diagnostic features for these two diseases on the stems and crowns of the affected plants.
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