Phomopsis seed decay

Phomopsis longicolla (taxonomically associated with the Diaporthe complex) is a major fungal pathogen that causes stem blight and pod and stem blight in soybeans. It is considered a significant threat to global soybean production, as it affects both plant health during the growing season and the quality of the harvested seeds.

The disease primarily manifests as elongated, light-colored necrotic lesions on the stems, often dotted with small, black fungal fruiting bodies known as pycnidia. When infection spreads to the pods, it can lead to stunted, shriveled, and discolored seeds, commonly referred to as "phomopsis seed decay," which significantly reduces marketability and germination rates.

The life cycle of Phomopsis longicolla involves overwintering on infested crop residues and infected seeds. During the growing season, particularly in warm and wet conditions, the fungus produces conidia that are splashed onto the lower stems and leaves by rainfall. The pathogen then colonizes the plant tissues, with symptoms typically appearing after the onset of reproductive stages.

High humidity and prolonged periods of rainfall are the primary drivers for disease outbreaks. The pathogen thrives when temperatures range from 20°C to 28°C. In dense canopy environments, lack of air circulation further facilitates the spread of spores, leading to widespread infection across the field and premature plant senescence.

Management strategies focus on a combination of cultural and chemical control measures. Essential practices include using high-quality, fungicide-treated seeds, implementing at least a two-year crop rotation with non-host crops, and managing tillage to encourage the decomposition of infected debris. Applying foliar fungicides at the R1 to R3 growth stages is recommended in high-risk areas to protect pods from colonization.