Phomopsis seed decay

Phomopsis seed decay, caused by the fungus Didymella longicolla (formerly known as Phomopsis longicolla), is a prevalent and destructive fungal disease of soybeans. It primarily affects the quality and viability of the seeds, but also causes stem lesions that can lead to premature plant death during the growing season.

The fungus is a necrotrophic pathogen that colonizes soybean tissues. It can persist in crop debris left on the soil surface, acting as a primary inoculum source for the following season. Additionally, the pathogen is frequently seed-borne, meaning that planting infected seeds can introduce the disease into new, clean fields.

Visible symptoms on stems typically appear as brown or dark-colored lesions located near nodes. On the seed, the infection is characterized by cracked, shriveled, and chalky-white appearances, which significantly reduce the commercial value of the grain. In severe cases, the presence of black, pimple-like pycnidia on the stems and pods confirms the presence of the pathogen.

The development and spread of Didymella longicolla are highly dependent on weather conditions. Warm and rainy weather, especially during the pod-filling and maturation stages, creates an ideal environment for spore production and dispersal. High humidity levels during late summer facilitate the rapid spread of the fungus across the canopy.

Effective management requires a multi-faceted approach. Growers should prioritize the use of high-quality, certified, and fungicide-treated seeds. Implementing a proper crop rotation, avoiding continuous soybean cropping, and choosing resistant or tolerant varieties are essential strategies to minimize economic losses caused by this pervasive pathogen.