Soybean pod and stem blight

The fungus Diaporthe phaseolorum is the causal agent of soybean pod and stem blight, a serious agricultural disease affecting soybean crops worldwide. It belongs to a complex of species that cause significant damage to the structural integrity of the plant, primarily attacking the stems, petioles, and pods.

The disease primarily targets soybean crops but can also affect other legumes such as kidney beans and snap beans. The pathogen is seed-borne, meaning it can be introduced to new fields via contaminated seeds. Furthermore, the fungus survives in crop residues, making it a persistent challenge in no-till or reduced-tillage farming systems.

Symptoms appear as small, elongated light-brown lesions on the stems or nodes, which eventually turn grey. Under high humidity, the infected tissues develop numerous tiny black structures called pycnidia, often arranged in linear rows. Pods may become infected, leading to shriveled or moldy seeds that are covered in white mycelium growth.

Development is favored by warm, wet conditions during the reproductive stages of the crop. Moisture is crucial for the dispersal of fungal spores, which are spread by rain splash and wind currents. Temperatures ranging from 20°C to 28°C provide the optimum environment for the colonization of plant tissues by the fungus.

Economic damage is substantial, as the disease reduces both yield and seed quality. Severely infected seeds exhibit poor germination rates and reduced vigor, which can lead to stand establishment issues in subsequent seasons. Integrated management strategies are essential, including the use of certified disease-free seeds, balanced fertilization, and the timely application of systemic fungicides to protect the crop during vulnerable growth stages.