Soybean Disease Atlas

2^nd Edition

Southern Soybean Disease Workers

VIRAL DISEASES

Fig. 24

Soybean Mosaic

Soybean mosaic is caused by soybean mosaic virus (SMV) and is the most widely distributed virus diseases of soybeans. it is spread by planting diseased seed and by at least 31 species of aphids.

Symptoms of SMV vary depending on the soybean cultivar, the age of the soybeans, the virus strain, and the temperature. Symptoms are most noticeable under cool temperatures of 18 to 24C. When temperatures rise above 30C., leaf symptoms may be masked. The youngest and most rapidly growing leaves show the most severe symptoms.

The leaves of SMV infected plants are distorted and narrower than normal, and develop dark green swellings along the veins (Fig. 24). Infected leaflets are puckered and curl down at the margin. Plants infected early in the season are stunted, with shortened petioles and internodes.

Diseased seed pods are often smaller, flattened, less pubescence, and curved more acutely than pods of healthy plants. Infected seed are mottled brown or black, usually smaller than seeds from healthy plants, and germination may be reduced.

Fig. 25

Bud Blight

Bud blight, caused by the tobacco ringspot virus (TRSV), can be a serious disease of soybeans. Yields may be reduced 25-100% depending on the time of infection. The virus is spread by planting infected seed, but the amount of infected seed produced is usually extremely low.

Infected plants are stunted and the pith of stems and branches show a brown discoloration, first near the nodes, then throughout the stem. Leaves on infected plants are smaller, wrinkled, and have a bronze discoloration. Buds become brown, necrotic, and brittle; hence the name bud blight. The most strirking symptom is the curving of the terminal bud to form a crook (Fig. 25). Diseased pods are often aborted or contain no seed. In the field, plants that are infected remain green after healthy plants have matured.

Fig. 26

Bean Pod Mottle

This disease, caused by the bean pod mottle virus (BPMV), often occurs in combination with soybean mosaic virus. The virus is transmitted mainly by the feeding of certain insects, particularly the bean leaf beetle. The virus is not seed transmitted.

In the field, diseased plants show a mild yellow mottling on young actively growing leaves (Fig. 26). As these leaves approach maturity the mottling becomes masked. Plants infected with both soybean mosaic and pod mottle are stunted, have distorted foliage, misshapen fruit, and necrotic tissue. Seed from plants infected with pod mottle are smaller than normal.

Fig. 27

Peanut Mottle

Peanut mottle is caused by the peanut mottle virus (PMV). It was first observed in 1971 and is the most common virus disease of soybeans in Georgia. The virus is seedborne in peanuts, but not in soybeans. Aphids can carry the virus from infected peanut fields or from other legumes in nearby fields. Volunteer peanut plants remaining in soybean fields are a source of inoculum for soybean infection.

A mosaic of dark green and yellow areas is produced in older leaves (Fig. 27). Ring patterns of yellow patches develop on the third and fourth leaves following infection. Infected leaves pucker and curl downward at the margins.