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Update on Root Rot and Top Dieback of Soybeans

July 31, 2003

About 2 weeks ago (week of July 14), we began receiving numerous reports of disease in soybean fields that was seriously damaging or killing plants at numerous locations in east-central Illinois. The main problems were reported within about a week after heavy rains had saturated soils in the region. This disease problem did not occur only where water was standing and plants were flooded; it also commonly occurred in fairly well-drained areas in fields. The problem seems to have sub-sided since that time, but plants continue to die in scattered patches in some fields. Root rot associated with this disease problem may continue to stress and kill plants, especially if the soils become dry over the next few weeks. This article describes the problem and gives an update on what we know about it at this time.

The most obvious symptom of disease was necrosis and wilting of the top of the plants. Additional symptoms were root rot, with few lateral roots present, sloughing-off of the root epidermis and cortex, red discoloration of the root where the sloughing-off had occurred, absent and rotted Rhizobia nodules, and, in some plants, brown discoloration in the pith that was similar in appearance to brown stem rot (BSR). In addition, some plants had necrotic and chlorotic spots on the leaves that were similar to BSR or sudden death syndrome (SDS), but no other clear symptoms were commonly seen above the soil line. Since that time, numerous reports have suggested a similar problem occurred in a number of other states in the upper Midwest.

Characteristic symptoms on top of soybean plant.

Infected plant with rotted root system.

Red discoloration on infected roots after epidermis and cortex have sloughed-off. (Photo courtesy Mike Hellmer.)

Brown discoloration in split stem of infected plant. (Photo courtesy Mike Hellmer.)

An article in issue no. 17 (July 18, 2003) of the Bulletin by Suzanne Bissonnette discussed Phytophthora root rot as one of the causes of this problem. It turns out that this disease problem appears to be a complex of different diseases, of which Phytophthora root rot is one of several diseases playing a part. A similar disease situation was reported in Ohio by Dr. Anne Dorrance. In Illinois and elsewhere, the diagnostic work is continuing, but this is what we presently know. Phytophthora has been confirmed to be present in many, but not all, of the infected plants by ELISA testing as well as by isolations in the laboratory. The presence of Pythium, a water mold­type root rot pathogen similar to Phytophthora, has been confirmed in some plants using the same methods. Fusarium fungi have also been frequently isolated from the infected roots. Several different types of Fusarium fungi can infect soybeans, and it is likely that at least some of the root rot that has occurred was caused by Fusarium infection. It is early for significant BSR infection, and we have not yet been able to determine whether BSR caused the internal stem discoloration observed.

In summary, it appears that heavy rains in early July enhanced infection of soybean plants by a complex of different pathogens in the warm soils. We will provide additional updates in future editions of the Bulletin when we get new information that will either clearly define the problem or be of value for disease management.--Dean Malvick

Author: Dean Malvick

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

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