The forecast for the coming week is for continued rain and in many cases this will fall on already saturated soils across the northern and west central part of the state. This is going to be tough on soybeans. Here is a guide to help differentiate among some key problems when these types of weather events occur.
1. Flooding: In fields that have had standing water for >48 hours, you’ll notice stunted soybeans and a smell as you approach the “drowned” out areas. When you dig the roots up, they may or may not be brown but the trick is that the outer layer of the root tissue, the cortical cells, can be easily pulled off leaving the white center of the root or root stele. The roots almost look like rat tails.
When plants are completely underwater for approximately 24-48 hours under high temperatures (>80°F), they will likely die. Plants respire more under high temperatures, oxygen is depleted, and carbon dioxide builds up suffocating the plant. Cool, cloudy days and cool, clear nights increase the survival of a flooded soybean crop. If the waters recede quickly and the plants receive some light rain, they can recover.
2. Poor nodulation: Yellow soybeans that can also be somewhat stunted – is often indicative of poor nodulation (see picture). Nodules are the small knots found on roots, often near the top of the root system. Nodules are the result of a symbiotic relationship between soybean and bacteria (Bradyrhizobium japonicum). These bacteria convert nitrogen into a form that is usable by the soybean plant.
Nodulation is reduced in wet soils. Soybeans at the V2 growth stage when grown in saturated soil for two weeks retain the ability to recuperate nodule function when normal (aerobic) conditions are restored. To determine if a nodule is actively fixing nitrogen (i.e., converting nitrogen to a usable form), split the nodule with your fingernail and examine the inside. If the inside of the nodule is pink or red, nitrogen is being fixed.
3. Disease: Flooded and saturated soil conditions will also provide the optimum conditions for the water molds that are common across the state. In these cases, the whole roots are brown, sometimes with dark brown lesions on the roots, and the tissue can be brown to tan. Both Phytophthora sojae and Pythium are contributing to this problem. Once the soybeans are at the V2 growth stage or greater, the protection from the seed treatment is gone and we are relying on the soybean plants’ defense system to mitigate the damage. For these areas in Ohio, the Rps genes for Phytophthora will only protect a few of the plants; we are relying on the partial resistance (field resistance, tolerance) part of the package.
What can you do for these, wait. When dryer weather returns the roots will re-establish. Anne has had some samples when dug up and mailed to Wooster, actually grew new roots during the transit. The roots just need some oxygen to get moving again. Check your drainage, these are excellent opportunities to see where some improvements can be made.
Henshaw, T.L., R.A. Gilbert, J.M.S. Scholberg, and T.R. Sinclair. 2007. Soya bean (Glycine max L. Merr.) Genotype response to early-season flooding: I. root and nodule development. J. Agronomy & Crop Science 193:177-188.
Sullivan, M., VanToai, T., Fausey, N., Beuerlein, J., Parkinson, R. and Soboyejo, A. 2001. Evaluating on-farm flooding impacts on soybean. Crop Sci. 41:93-100.