Ohio Soybeans: Mid-Season Disease, Management Considerations

Phytophthora stem rot in soybeans. Photo: Ohio State University

The rains last week around Wayne County (5+ inches) provided for saturated soil conditions. Cultivars with low resistance to Phytophthora sojae are now developing Phytophthora stem rot. These fields were planted the first week of June and the stands were good but are now beginning to thin out. This has been the pattern, it takes 1 to 2 weeks for above ground symptoms to develop on cultivars with resistance packages that are no longer effective.

Phytophthora sojae has one host, soybean, and can adapt to some of the types of resistance that is bred into soybeans to manage this pathogen.  Scout fields of soybeans 1 to 2 weeks after a rain to look for symptoms. If you find a plant or two, probably don’t worry but if you easily find dozens of plants and the canopy is thinning due to loss in stand – look at the resistance package of the soybean. It is time for something new.

Another thing is that plants that were planted in this first week of June were in flower.  For those of you that have whitemold annually, there is an app developed by colleagues at University of Wisconsin. Sporecaster is the name (here). Just to be clear, this has not been validated under Ohio conditions. Last year, it was only 50% correct in predicting.

This year is going to be very strange as we don’t know what the late planting and all of the rain will impact whitemold, but if you try it – keep notes and go back and check to see if whitemold does develop. Most important, what is the resistance rating for the variety? The higher the resistance, the less likely that white mold above the yield loss levels will develop.

Frogeye leaf spot reports were low the last two weeks. For those fields now hitting the R2 growth stage, it is time to scout. For fields of soybean, known to be susceptible to this leaf spot, the fungicide timing is R3. There are lots of very effective materials available.

We have begun our surveys for leaves with frogeye leaf spot to determine if the fungicides are still effective.  If you would like to participate we would greatly appreciate the samples.


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