Brown Root Rot of Alfalfa
Brown root rot is caused by the fungus Phoma sclerotioides and can contribute to winterkill. Circular brown lesions form on roots and cause girdling in taproots, lateral roots, and feeder roots. Lesions will occasionally have black borders, which is indicative of brown root rot. Plants death can occur if coalescing lesions sever the root. Dark-colored pycnidia (fungal reproductive structures) can form on or in root lesions. Infected plants appear to recover in the summer but become increasingly less productive each year. Degradation of root and crown tissue promotes winterkill. Stand decline from this disease is often blamed on a harsh winter.
Brown root rot symptoms.
Craig Grau
P. sclerotioides survives on indiscriminate organic matter in the soil. After plant infection, cool soils and snow cover for long periods promotes fungal growth. The pathogen develops slowly and symptoms may only appear after three or more winters. The pathogen can also infect winter wheat, red clover, and other perennial crops.
To minimize pathogen inoculum in a field, sow annual cereal crops in spring for two to three years prior to establishing alfalfa. Alfalfa cultivars designated as partially resistant and tolerant to P. sclerotioides exist. Maintain soil fertility and reduce plant stress.
Prepared and reviewed by the Alfalfa Pest Management Working Group.
Brown root rot symptoms.
Gallery images courtesy Craig Grau.
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