Aphanomyces Root Rot of Alfalfa
Aphanomyces root rot is caused by the oomycete (not a true fungus) Aphanomyces euteiches. Alfalfa productivity is reduced when seedlings die and chronic root infection occurs. Symptoms include stunted seedlings with chlorotic, purplish cotyledons and eventual leaf yellowing. Tan to gray discoloration of root and hypocotyl tissues becomes brown as disease progresses. Aphanomyces root rot can be confused with other seedling diseases, and can form a disease complex with Phytophthora root rot. Seedlings with Aphanomyces will remain standing for a time, unlike the quick plant collapse associated with Pythium and Phytophthora infection. Infection of established plants results in reduced root mass and decay of lateral roots. Foliar yellowing and stunting can appear similar to nitrogen deficiency.
Aphanomcyes root rot symptoms on seedlings.
Craig Grau
Aphanomcyes root rot symptoms on seedlings.
Craig Grau
Aphanomyces root rot is favored by poorly drained or wet soil. The pathogen survives on infested crop residue or in soil for several years.
The best way to manage Aphanomyces root rot is to plant Aphanomyces-resistant alfalfa varieties and improve soil drainage. Monitor at-risk fields for disease to inform management, which can also include planting non-host crops. Mefenoxam and metalaxyl seed treatments are not effective against A. euteiches. Seed treatments that contain the active ingredients tolclofos-methyl or pyraclostrobin may suppress Aphanomyces in new seedings, but the protective effect is short-term.
Aphanomyces root rot disease cycle on alfalfa.
Mimi Broeske, Nutrient and Pest Management Program, University of Madison-Wisconsin
Aphanomyces Root Rot in Alfalfa
Prepared and reviewed by the Alfalfa Pest Management Working Group.
Aphanomcyes root rot symptoms on seedlings.
Aphanomcyes root rot symptoms on seedlings.
Aphanomcyes root rot symptoms on seedlings.
Aphanomcyes root rot resistant alfalfa cultivars (left) compared to susceptible cultivars.
Gallery images courtesy Craig Grau and Deborah Samac.
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