Purple seed stain

Purple seed stain is a destructive fungal disease caused by the pathogen Cercospora kikuchii. This fungus primarily affects soybean crops, leaving distinctive purple or pink discoloration on the seeds. The pathogen survives in crop residue and is frequently seed-borne, which creates a significant challenge for long-term disease management in large-scale agriculture.

The primary host for this fungus is soybean. The infection manifests on leaves, stems, pods, and seeds. Initial symptoms on leaves appear as reddish-purple spots which may expand and merge, leading to premature senescence of the foliage. These lesions reduce the photosynthetic capacity of the plants, ultimately stunting their growth and overall productivity during the critical grain-filling phase.

The most recognizable diagnostic feature is the staining of the seeds within the pods. The fungal infection causes a characteristic purple blotch on the seed coat, ranging from light pink to dark deep purple. While the seed interior may remain healthy, the infection lowers seed quality, marketability, and can significantly decrease germination rates if used for future planting.

Environmental conditions play a crucial role in the development and spread of the disease. The fungus thrives in warm, humid conditions with temperatures ranging from 20°C to 30°C. Frequent rainfall and extended dew periods during the reproductive stages promote rapid spore germination and infection, allowing the fungus to spread quickly throughout the entire field canopy.

• Apply effective fungicides at the R3 or R5 growth stages.
• Select resistant or tolerant soybean varieties available on the market.
• Plant certified, disease-free, and treated seeds to prevent spread.
• Practice crop rotation with non-host crops for at least two years.
• Incorporate crop residue into the soil to hasten decomposition.

The economic impact of purple seed stain is primarily measured in quality degradation and yield loss. Heavily infected seeds often have lower protein and oil content, making them less valuable to processors. Furthermore, the loss of leaf surface area reduces the plant's ability to fill pods, resulting in smaller, shriveled seeds that lower the overall harvested yield per hectare.