Damon Smith, Extension Field Crops Pathologist, Department of Plant Pathology, University of Wisconsin-Madison
What a difference a season can make! Last year at this time it was unseasonably hot and the drought was beginning to rear its ugly head. This season we find ourselves on the opposite end of the spectrum. Temperatures have been pretty cool and winter just doesn’t seem to want to give in to spring/summer. Last week we planted research soybean plots and then found ourselves with saturated soils and 2-inch soil temperatures sliding back into the upper 40s and low 50s at the Arlington Agricultural research station. I suspect many soybean growers found themselves in the same situation around the state.
So what kind of diseases do you run into in these cool wet soils? Species of Pythiums, Phytophthora sojae, and Phomopsis can all cause seed rots and/or seedling diseases when soils are cool and wet. To learn more about seedling disease visit the Soybean Plant Health website at http://fyi.uwex.edu/fieldcroppathology/soybean_pests_diseases/ and scroll down to “Seedling Diseases”.
Soybean fungicide seed treatments can help reduce seed rot. However, not all soybean seed treatments are the same. To determine if a fungicide was included in your seed treatment, consult the recently updated “What’s on your seed” chart at http://fyi.uwex.edu/fieldcroppathology/files/2013/01/Whats_on_your_seed_FINAL_4.pdf. Efficacy information for some of these active ingredients can be found on the chart “Specific Activity of Soybean Seed Fungicides”. The chart is located at http://fyi.uwex.edu/fieldcroppathology/files/2010/11/Specific_Activity_of_Soybean_Seed_Fungicides.pdf.
For Phytophthora, resistant varieties of soybean are also available. Consult your seed label to determine what resistance gene(s) your variety has in it. The table below shows which genes are resistant against which races. Remember that “field resistance” does exist in most varieties marketed in Wisconsin. That is to say that even though a certain gene (RPS 1a for example) might not be very effective for a specific race found in a field, varieties with that gene it will perform better than another variety with the same gene. However, “field resistance” can be overcome easily in situations where pathogen pressure is high.
For more information on Phytophtora root and stem rot which can occur later in the season you might also consult a new fact sheet recently posted at http://fyi.uwex.edu/fieldcroppathology/files/2013/04/Phytophthora-Root-and-Stem-Rot-of-Soybean.pdf and the Soybean Plant Health page on Phytophthora root and stem rot at http://fyi.uwex.edu/fieldcroppathology/soybean_pests_diseases/phytophthora_soybean/.