Soybean Phomopsis

The fungus Diaporthe aspalathi is the primary causal agent of soybean Phomopsis, a serious fungal disease that threatens soybean production worldwide. Formerly known as Phomopsis phaseoli, this pathogen belongs to the Diaporthaceae family. It is recognized as a significant constraint to yield, particularly in regions experiencing warm and humid growing seasons.

The host range is primarily limited to soybeans, though the pathogen can survive on various agricultural crops and wild legumes. The fungus is highly persistent and can survive as mycelium or as structures called pycnidia on infected crop debris left in the field. Additionally, it can be seed-borne, which allows for the long-distance spread of the pathogen through commerce.

Symptoms typically manifest as the plant approaches maturity. Brown, sunken lesions appear on the lower stems, often starting at the nodes. As the infection progresses, these lesions can expand and girdle the stem, leading to premature wilting and death of the plant. A characteristic sign of the disease is the emergence of small, black, pepper-like pycnidia on the stems and pods.

Environmental conditions play a critical role in disease development. Diaporthe aspalathi thrives in warm weather, generally between 20°C and 28°C, combined with prolonged periods of leaf wetness or high humidity. Spores are disseminated via rain splashes and air currents, infecting developing pods and seeds, which completes the infection cycle for the next season.

The impact of this disease includes significant yield loss, reduced seed quality, and poor germination rates of infected seeds. Effective management strategies for controlling the impact of Diaporthe aspalathi are essential for profitable production:

• Utilizing certified disease-free seed and applying appropriate fungicides as seed treatments.
• Implementing long-term crop rotation to reduce inoculum levels in the soil.
• Deep tillage to bury infected crop residues, accelerating their decomposition.
• Applying foliar fungicides during the R3–R5 growth stages if weather conditions favor the disease.