Summer 2021 Soybean Issues 

Jul 19, 2021

This weather pattern during the summer has been tough on some of the soybean acres in our trade territory. The northern areas are looking better than the southern areas because as we move south we have received more rainfall causing yellowing and stunting in the fields. A few problems this yellowing could be are Phytophthora Root and Stem Rot, Potassium deficiency and Nitrogen deficiency. These problems are caused by excess moisture or areas that roots cannot grow properly. 

Potassium and Nitrogen deficiency 

Potassium and Nitrogen deficiency have been widespread this year. Yellowing on the outside of the soybean leaves is the sign of Potassium deficiency while overall plant yellowing (generally in poorly drained areas) are signs of nitrogen deficiency. Both issues are hard to fix in season and are best addressed out of the growing season. Our dry conditions earlier this year are the reason behind lack of potassium while wet conditions are the reason for nitrogen deficiency. Both issues are caused by the roots. When it is too dry potassium in the soil solution is not diffusing towards the plant roots and when conditions are too wet soybean nodules are not producing nitrogen. We generally do not see these symptoms at the same time, but we apply the same agronomics to fix them. We want to make sure that the root zone has an adequate water to air ratio and in our area that means we need better drainage in the fields. Our soils hold onto water very well, so the best management practice is to lower that water table with drainage tiles. Once those nitrogen deficient roots are no longer saturated the soybean can grow out of the yellowing stage and green back up. This makes us feel better, but we know that there was yield lost in that timeframe. The best way to maximize yield is to make sure our plants never have a bad day. 

Phytopthora Root Rot 

This is a plant disease that is caused by a fungal pathogen that feeds on soybean residue. It is uncharacteristic for Phytopthora to show up and kill soybeans that are a foot or more tall, but I think that is a big cause of the yellowing and stunting of soybeans this year. Phytopthora affects the roots and works its way up the stem about six inches. The characteristic stunting and dark stems in the low-lying wet areas of the field make me think Phytopthora is the culprit. I want to know 100% so we are sending samples to the Iowa State Plant diagnostic lab. At the lab ISU can confirm the disease is Phytopthora and tell us the specific strain of the disease. This is important information because soybean varieties have multiple Phytopthora genes we can choose from. We need to be able to determine which strain of disease and make sure we choose soybean varieties that protect against that specific strain. Phytopthora Root rot can stay in the soil for 10-15 years so if we see it once it is best to assume it can be there again when we have weather conditions that are conducive. Yield impacts range quite of bit but based on what I’ve seen I don’t feel it will be a total loss in those areas, but I could easily see a 5-10 bushel difference in them. We have just received too much rainfall on some of these fields.  

If any of your fields have issues going on in them reach out to your SFG agronomist. We like to spend time in the fields and help determine issues even if competitor products are used. Contact us as we would like to help make your 2021 crop year as profitable as possible.