Phomopsis seed decay

The fungus Diaporthe tersa (with its anamorph Phomopsis longicolla) is a significant plant pathogen responsible for causing stem and pod blight, commonly known as Phomopsis seed decay. This disease is a major concern for soybean producers worldwide, as it can cause substantial yield reductions and severely degrade the quality of the harvested grain.

Soybeans are the primary host for Diaporthe tersa. The pathogen is seed-borne and soil-borne, meaning it can survive in crop debris and infect new plantings the following season. Beyond soybeans, the fungus can occasionally colonize other legumes, maintaining its presence in the field even in the absence of the primary host.

Visible symptoms often start as small, discolored lesions at the stem nodes, which later expand into large, necrotic patches. As the disease progresses, the stem tissue weakens and may crack open. Microscopic structures, such as pycnidia, appear on the surface of stems and pods. Infected seeds typically show significant decay, appearing chalky, shriveled, and often having a white or gray fungal growth.

The development and spread of Diaporthe tersa are highly dependent on warm and wet weather conditions. Extended periods of rainfall during the flowering and pod-filling stages are particularly conducive to the production and dispersal of fungal spores. High humidity acts as a catalyst for the colonization of plant tissue, leading to premature senescence and yield loss.

Effective management requires an integrated approach to minimize the impact of the disease. Key strategies include the use of disease-free certified seeds, applying effective fungicide seed treatments, and ensuring crop rotation with non-host plants. Proper management of crop residues and avoiding late harvests can also significantly reduce the survival rate of the pathogen in the field.