Yellow Soybeans! What to Make of It? How About Rhizoctonia!

In a previous Crop Pest Ontario issue #10 (June 29, 2006), Horst Bohner (OMAFRA Soybean Specialist) discussed yellowing of soybeans caused by "yellow flash" (typically seen after an application of glyphosate) or manganese deficiency. To further complicate matters, Rhizoctonia root rot infection can resemble the interveinal chlorosis (yellowing) associated with manganese deficiency or the pale yellowing due to poor nodulation (lack of nitrogen).

Basing your decision solely on leaf symptoms could led to a incorrect diagnosis. Dig the plants and examine the roots for a characteristic reddish Rhizoctonia lesion produced on the stem at or just below the soil line. These firm, dry, brick-red lesions can join, forming a sunken girdling of the stem that may move down the taproot, pruning roots along the way. Symptoms of the disease are very similar to plants infected with Phytophthora root rot. Severely infected plants may lose their leaves. Wilted and/or dead plants often occur in small patches. Stem lesions girdle the stem and weaken the plant, often causing infected plants to break at the soil line under stormy conditions. Stressful growing conditions favour this disease. Rhizoctonia root rot is most damaging when cool, wet conditions in the spring are followed by hot, dry conditions. These same conditions are favourable for Fusarium root rot.

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