Rhizoctonia Root, Stem, and Crown Rot of Alfalfa
Rhizoctonia root, stem, and crown rot is caused by the fungus Rhizoctonia solani. Symptoms are diverse and can affect a variety of plant tissues. Pre- and postemergence damping off occurs early in the season. Sunken, tan root lesions occur where lateral roots connect to the taproot. Lesions are oval to circular and darken towards the outer edges. Girdling lesions can kill the plant. Lesions heal and darken due to pathogen inactivity during cool parts of the season. Infected buds and shoots will develop brown lesions at and below the soil line. “Flagging” occurs as buds and shoots die. Crown tissue starts to rot as disease progresses, and sunken lesions form at the stem base. Lower stems and roots shrivel with brownish discoloration. Light colored stem lesions with dark margins can form; concentric rings of growth may be evident in lesions. Wet and hot weather promotes the foliar phase of this disease, which causes tissue to collapse and appear water-soaked. Webs or mats of fungal growth can cover plant tissue (web-blight).
Rhizoctonia root, stem, and crown rot alfalfa stem lesions.
Craig Grau
Rhizoctonia root, stem, and crown rot alfalfa stem lesions (bottom) compared to lesions from anthracnose crown and stem rot.
Craig Grau
R. solani survives in plant debris or soil. Disease risk increases when wet soils or high humidity are present with warm to hot temperatures.
Management options for this Rhizoctonia root, stem, and crown rot are limited. Partial resistance to infection of seedlings is present in certain alfalfa cultivars. Consider early harvest to reduce web-blight losses.
Prepared and reviewed by the Alfalfa Pest Management Working Group.
Rhizoctonia root, stem, and crown rot alfalfa stem lesions.
Rhizoctonia root, stem, and crown rot alfalfa stem lesions (bottom) compared to lesions from anthracnose crown and stem rot.
Gallery images courtesy Craig Grau.
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