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Phytophthora Rot on Soybean

July 14, 2000
Several reports of Phytophthora root rot on soybeans have been received from around the state. The Phytophthora organism can cause disease symptoms on soybeans at any stage of growth. Issue no. 2 of the Bulletin discussed damping-off, seed and seedling rot, and fungicide seed treatments for Phytophthora and other soilborne fungi. During the midsummer, the Phytophthora organism can cause a stem disease on soybeans. Midsummer symptoms resulting from infection by Phytophthora will vary with the susceptibility of the soybean cultivar. Plants can die throughout the growing season from infections that happened close to planting. Typical symptoms on older susceptible plants are yellowing between the veins and at leaf margins, along with chlorosis of upper leaves. The soybean plant often wilts soon after these symptoms occur. Leaves remain attached to the petiole after the plant dies. Lateral roots and taproots are often destroyed, and lesions girdling the stem can extend upward to the 10 node on the plant. Lesions on the stem are sometimes described as brown or chocolate brown. The important thing to remember is that infection from Phytophthora root rot starts in the soil, so stem lesions will begin at the soil line and move upward.

Phytophthora Root Rot on Soybeans
Photo courtesy of XB Yang, Iowa State

On more tolerant cultivars, this organism may rot roots and stunt plants, but plants are not killed. Stem lesions are long and narrow, brown in color, and sunken. Highly tolerant cultivars may appear as stunted and chlorotic plants showing rot of secondary roots and taproot discoloration. Symptoms can look like lack of nitrogen or damage from flooding. Often, damage from this pathogen appears after heavy rains, which have been prevalent in some areas of Illinois.--Loretta Ortiz-Ribbing

Author: Loretta Ortiz-Ribbing

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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