Introduction

While not a common occurrence for soybean producers in North Carolina, persistent warm, wet weather just preceding and at crop maturity may increase pod blight and seed decay damages.

Pathogen(s)

While many fungi can cause seed decay and pod blight, damages are most commonly caused by ascomycete fungi in the Diaporthe / Phomopsis complex. This disease complex can be seen in the eastern part of the continental United States. While many species can be attributed to this complex, seed decay is primarily caused by D. longicolla (syn. P. longicolla) and pod blight by D. sojae (syn. P. phaseoli).

Host Range

Common crop hosts include lima bean, cowpea, peanut, soybean, and other rotational vegetable crops as well as weeds like velvetleaf and pigweed.

Symptoms

Affected seeds appear wrinkled, white, and lightweight. Seedlings grown from infected seed may be slow to emerge. Reddish brown lesions appear on the cotyledons of infected seedlings near the soil line. Evidence of infection in mature plants can be seen on the stem and pods. The fruiting body of the fungi, pycnidia, appears as black specks on the pod and stems. Pycnidia will form in a linear fashion down the stem and appear more randomly scattered on pods.

Life Cycle and Favorable Conditions for Disease

The pathogen overwinters in seed, crop residue, and soil. Disease development is favored by warm, wet conditions and severity is increased by insect injury, virus infection, and potassium deficiency. Early in the growing season, the fungus is splash dispersed to lower portions of soybean stems causing stem canker symptoms. Seed damages are favored by warm, wet, and humid conditions during pod development (R3-R8). At the end of the growing season, the fungus survives in the remaining crop debris until suitable hosts are present.

Management

Avoid planting infected seed where possible. Since severity of infection is increased in early maturing cultivars, using a full season cultivar may reduce severity. Some cultivar resistance is available against Phomopsis seed decay; however, none is available for stem and pod blight. Efforts should be made to harvest early maturing cultivars first, as delayed harvest can also increase severity. Rotating to non-host crops such as corn and wheat will also help decrease the disease potential for subsequent years. Fungicide seed treatments help to protect seedlings against Diaporthe species. Foliar fungicides applied between R3 and R5 may help maintain seed quality if risk of infection is high.

Useful Resources

The Crop Protection Network Fact Sheet on pod and stem blight has more information on the pathogen and management considerations.

The NC State University Plant Disease and Insect Clinic provides diagnostics and control recommendations.

The NC State Extension Plant Pathology portal provides information on crop disease management.

The North Carolina Agricultural Chemicals Manual provides pesticide information for common diseases of North Carolina. The manual recommendations do not replace those described on the pesticide label, and the label must be followed.

Acknowledgements

This factsheet was prepared by the NC State University Field Crops and Tobacco Pathology Lab in 2020.

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