Over the past two weeks, Purdue researchers have received increasing reports of stressed or dying soybeans. In many cases, these symptoms have been most noticeable in areas that have received heavy rainfall. The warm, wet conditions have been favorable for Phytophthora root and stem rot, but also other root diseases, and it is important to accurately diagnose the cause of the symptoms observed in a field.
The classic symptoms of Phytophthora root and stem rot are soft, discolored roots and a dark brown lesion that is visible above and below the soil line and moves up the stem (Figure 1). Plants may be stunted and die prematurely. However, the dark brown stem lesion may not always be present, especially if the soybean variety has partial resistance to Phytophthora. In several instances, we have confirmed Phytophthora root and stem rot in soybean plants without seeing the typical brown lesion on the stem. We have also diagnosed Phytophthora root and stem rot in several fields where varieties with the Rps1k gene were planted. This Rps gene has typically performed well against Phytophthora stem rot in Indiana, but it is not infallible. Anne Dorrance of Ohio State University provided a nice summary of how Rps genes work and the level of disease control that can be expected from varieties with these genes in a recent newsletter article that can be accessed here: http://agcrops.osu.edu/newsletter/corn-newsletter/it-phytophthora-stem-rot-it-flooding-injury-or-it-both. In fields where the disease is confirmed, take note of the Rps genes (if any) in the variety, and the level of reported partial resistance in the variety. The next time these fields are planted to soybean, select varieties with Rps genes and a high level of partial resistance to Phytophthora stem rot.
Although Phytophthora root and stem rot is receiving the most attention in the state, we have also observed stunting, root discoloration and premature plant death associated with Fusarium and Rhizoctonia root rots in recent weeks. Therefore, if you are seeing patches of soybeans that are discolored, prematurely dying, or stunted, remember that to accurately determine the specific organism responsible for a suspected disease issue, it is necessary to submit samples to a diagnostic lab such as the Purdue Plant and Pest Diagnostic Lab, https://www.ppdl.purdue.edu/PPDL/index.html.