Soybean Disease Atlas

2^nd Edition

Southern Soybean Disease Workers

STEM AND POD ROT DISEASES

Fig. 10

Pod and Stem Blight

(Diaporthe phaseolorum var. sojae)

Pod and stem blight occurs in every soybean producing area of the United States. It is a major cause of seed damage, resulting in poor seed quality. This poor seed quality results in grade reductions and lower prices for the producer. The fungus overwinters on seed or on crop residue. Infection can occur early, but it is usually a late season disease, favored by warm, humid weather. Disease is more severe if harvesting is delayed.

The most characteristic symptom of this disease is the arrangement of black fruiting structures in linear rows on the stems (Fig. 10). Infected seed crack and shrivel and are often covered with white mold. These seed fail to germinate, or produce weak seedlings with brownish-red pinpoint lesions on the cotyledons. Reddish-brown streaks appear on the seedling stem near or below the soil line.

Pod and stem blight is managed by planting pathogen-free seed and foliar application of fungicides from mid-flowering to late pod. Harvesting soybeans promptly at maturity will reduce pod and seed infection.

Fig. 11

Stem Canker

(Diaporthe phaseolorum var. caulivora)

Stem canker has caused widespread damage in the southeastern United States in recent years. The disease can be very destructive because it kills plants from flowering to maturity. The fungus overwinters on diseased stems and infected seed. Initial symptoms are observed after flowering. The incidence of disease is greater in wet seasons.

Although symptoms vary on susceptible and resistant cultivars, the first symptoms are small reddish-brown lesions at the base of branches or petioles. These lesions elongate, become sunken and may girdle the stem. Leaves exhibit an interveinal yellowing which becomes necrotic (Fig. 11). Leaves on dead plants wither but remain attached.

The disease is controlled by using pathogen-free seed and planting resistant cultivars. Crop rotation will reduce overwintering inoculum.

Fig. 12

Anthracnose

(Glomerella glycines)

Anthracnose occurs in all major soybean growing areas of the world. The disease reduces stands, seed quality and yields. Losses are greatest in the southeastern United States. Although seedling infection can occur from overwintered fungus on infected seed or crop residue, anthracnose is generally a late-season disease. Late infections occur during bloom or early pod development when conditions are wet and humid for a prolonged period.

The fungus produces an abundance of spores which infect and kill lower branches, leaves, and young pods. Foliar symptoms include leaf rolling and veinal necrosis. Symptoms appear on stems, pods, and petioles as red or dark brown areas. Later these areas are covered with black fruiting bodies (acervuli) (Fig. 12) that resemble tiny pin cushions. infection of young pods results in empty pods at maturity. Pods infected later contain shriveled or moldy seed, and may have dark lesions on the seed coat.

Anthracnose is controlled by applying foliar fungicides between bloom and pod fill, and planting pathogen-free seed. Rotation will reduce disease incidence.