My dog, Lily, and I were dodging ice patches on our nightly walk this week and could finally sense a hint of spring. It is time to start getting ready for the field season. As I look back over my data and pictures of wipe outs in Ohio, the biggest losses have occurred when the super susceptible soybean variety or corn, or wheat, was planted into a field where inoculum (pathogen population) was high and environmental conditions often occur. The disease triangle is a great tool to remind us that limiting one of the factors (susceptible variety, inoculum, or environment) can greatly reduce losses due to diseases. Producers have the most influence over the variety that is planted in a given field. Now that the seed is ordered, or if you have field that has a risk of developing Sclerotinia white mold, Phytophthora seedling blight or stem rot or high populations of SCN, order that seed so you don’t get stuck later on down the road.
Phytophthora sojae- this is in all fields in Ohio that have high clay content and are poorly drained. We have a large and highly variable Phytophthora population here in this state, which most of the Rps genes are no longer effective. It is critical for these parts of the state to choose varieties with high levels of partial resistance (also called tolerance or field resistance). For the new producers in the state, an Rps gene is one that gives the plant resistance to some of the pathogen population but not all. Breeders love them as they are single genes and very easy to add to a new variety.
Sclerotinia sclerotiorum causes Sclerotinia stem rot or white mold and is most often in a problem in northeast Ohio, north central, and last year there were also reports of this disease in the southwestern part of the state. The sclerotia can survive for a long time, so it is important choose varieties with resistance. Again, each company has a different rating scale so it is important to read the information on the resistance. In the field, resistance is going to look like a fewer number of plants that infected and this resistance can be overwhelmed when conditions are very conducive to infection. Planting at lower populations, enhancing air drainage (don’t surround that field with corn).
Frogeye leaf spot. This fungus disease is caused by a Cercospora that is specific to soybean. There have been a few highly susceptible varieties that were planted in Ohio over the past decade. The trick to this one is to rotate, this fungus can survive on the residue that is left in the field. For the folks that insist on beans-back-to-beans, and for those that saw this in their fields last year – check the variety score for resistance to this pathogen and choose the best resistance.
Soybean cyst nematode. In addition to planting a resistant variety it is also good to know your numbers. Sounds like a cholesterol warning doesn't it? In the case of SCN, less than 500 eggs per cup of soil and keeping it under 1,000 is what we need to shoot for on some fields. Non-detectable levels are like gold. If you haven’t tested in a while, my first question is, were the yields on each field above? Equal? or below the county average? How many fields was the overall yield down to 30 bu/A. Target the lowest yielding fields first. If you have yield maps, especially for no-till farmers, target the low yielding pockets. SCN will stay in that one pocket, but the pocket will get larger over time with no-till. With cultivation, the pocket will get larger in the same direction that you till.
All of these diseases are readily managed with host resistance, but it is up to you to know which fields have the highest amount of disease pressure/inoculum. Read the seed company literature about their varieties, unfortunately some of the best information is in the teeny tiny print at the bottom of the tables. Honestly soybean varieties are not all resistant to everything. So take the time to know what you purchased, wish to purchase, and put it in the correct field. Don’t get sold a variety targeted for western Iowa – they do well there but not here.