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Midseason Soybean Problems

August 11, 2000

Frogeye leaf spot. Fungal leaf diseases are beginning to show up in the soybean crop. Omar Koester of the Monroe/Randolph Extension Unit reports very significant frogeye leaf spot in many area fields. He also reports that in their variety trials there is quite a bit of difference in susceptibility of different varieties. Very good observation. Frogeye leaf spot used to be common in the southern part of the state, but use of resistant varieties decreased the prevalence of the disease. However, in the past several years, frogeye leaf spot has been showing up more frequently.

Why? Well, it's a good question, but I can only give an indirect answer. First, we need to consider that some varieties are still showing resistance to the disease, so there are obviously still choices of cultivars with that resistance. That's the good news. Second, we need to consider that perhaps a different race of the fungus is increasing, and current resistance isn't effective. At least five races of the fungus have been reported in the United States. Third, we need to consider that cultivars are selected for various agronomic traits and for resistance to what will probably be the most likely diseases to affect them in the area in which they are grown. Selection for frogeye leaf spot may not have been a recent priority.

Frogeye leaf spot foliar symptoms. Courtesy of O. Koester.

Cercospora sojina is the fungus that causes the disease. Frogeye leaf spot is readily diagnosed in the field. Symptoms are small, round eyespots on the leaves. The center of the spot is light brown to grayish and surrounded by a fairly distinct purple to reddish brown border. Usually just leaves are affected. But stems and pods may be infected late in the season. Heavily infected leaves may die and drop off prematurely. Yield loss is rare; but when susceptible cultivars were grown, loss was reported up to 15% in heavily diseased fields. If pods become infected, the fungus can grow through the pod and infect the seed, resulting in poor seed quality and seed discoloration.

Sudden death syndrome. We have had several articles in recent issues of the Bulletin discussing sudden death syndrome (SDS) of soybean. Bill Tarter of Alvey Labs reports the disease is showing up frequently in all of the central-southwestern counties. I did make the mistake of asking which counties, and much to his credit Bill listed all of the counties in that part of the state in rapid succession. Environmental conditions have been very favorable for infection and development of SDS this season. If we enter a weather period that places the crop under some stress, expect symptoms to show up rapidly.--Suzanne Bissonnette

Author: Suzanne Bissonnette

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

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