Causal Agent

Diaporthe phaselolorum, [anamorph Phomopsis sojae and Phomopsis longicolla (fungus)] 

 

Hosts

Soybeans. 

 

Symptoms

Most characteristic symptom is the development of black fruiting structures (pycnidia) of the fungus arranged in linear rows on the stems, petioles, and pods. Not all infected pods produce pycnidia, but mature pods with pycnidia always contain infected seed. 

 

Heavily infected seed are badly cracked, shriveled, and are frequently covered by a white moldy growth (fungal mycelium). Lightly infected seed may not produce symptoms of P. longicolla infection but produce weak seedlings that are infected with the pod and stem blight pathogen. Heavily infected seed often will not germinate. Delayed harvest, during rainy falls, increases the amount of P. longicolla seed infections.

 

Control

Pod and stem blight is controlled by rotating to non-host crops, sanitation (plowing down soybean residue), planting disease-free seed, timely harvest, and applying foliar fungicides. Foliar fungicides do not consistently increase yields of plants infected with pod and stem blight, but they can improve seed quality. This is especially true in years when warm weather and frequent rains occur during the reproductive phase of soybean development. Research has shown that one application of a labeled foliar fungicide at growth stage R6 can improve seed quality. Seed quality can also be maintained by timely harvest. Please contact your local county extension office for current information.