Pythium Seed and Root Rot of Alfalfa
Pythium seed and root rot is caused by multiple oomycete (not a true fungus) species in the Pythium genus. This disease is primarily a seedling disease, causing rotted seeds, roots, and damping off. Seeds become a mass of soft, brown rotted tissue. Pre- and postemergence damping off occurs early in the season. The tissue of infected seedlings becomes soft, brown, and water-soaked before collapsing. Some seedlings survive in the right conditions by developing roots above infected root tissue. On older plants, root systems are reduced and general plant growth and development is inhibited.
Pythium spp. survive as oospores in crop debris and soil. Pythium seed and root rot development is favored by wet soils and cool temperatures. Conditions that slow seedling development favor Pythium seed and root rot.
Pythium seed and root rot can be managed by implementing a variety of cultural practices and the use of fungicide seed treatments. Cultural practices include improving soil drainage, maintaining appropriate soil fertility, planting when conditions favor seedling development, and making up for stand loss through higher seeding rates. Mefenoxam and metalaxyl fungicide seed treatments are effective against many Pythium spp.
Prepared and reviewed by the Alfalfa Pest Management Working Group.
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