Phytophthora Root Rot of Alfalfa
Phytophthora root rot is caused by the oomycete (not a true fungus) Phytophthora medicaginis. This disease can prune roots and kill plants at any stage of development. Pre- and postemergence damping off occurs early in the season. On older plants, the most characteristic symptom of Phytophthora root rot is a light to dark brown lesion that girdles the taproot. Lesions start at root tips or where the taproot is connected to the lateral roots. Older lesions become reddish brown and appear sunken. The entire crown and taproot may rot. New lateral roots may be initiated above lesions. Foliar symptoms include leaf yellowing, reddening of the lower leaf surface, wilted shoots, and stunting. Symptoms appear first in low spots in the field and spread outwards if conditions are favorable.
Phytophthora root rot can cause alfalfa seedling death early in the season.
Craig Grau
Phytophthora root rot alfalfa root lesions.
Craig Grau
P. medicaginis survives in the soil as oospores. Disease development requires prolonged wet soils with temperatures above 60°F.
Phytophthora root rot is best managed by planting resistant cultivars and improving soil drainage. Where Phytophthora root and stem rot is a problem, seed treatments targeting oomycetes can provide some protection when establishing alfalfa.
Prepared and reviewed by the Alfalfa Pest Management Working Group.
Phytophthora root rot can cause alfalfa seedling death early in the season.
Phytophthora root rot alfalfa root lesions.
Phytophthora root rot alfalfa root lesions.
Phytophthora root rot alfalfa root lesions.
Foliar symptoms include leaf yellowing, reddening of the lower leaf surface, wilted shoots, and stunting.
Phytophthora root rot resistant and susceptible (middle) alfalfa cultivars.
Gallery images courtesy Craig Grau.
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