Source: Dr Anne Dorrance, OSU Extension
Soybeans across the state range from ready to harvest to still flowering. But in some fields, the yellowing was limited to pockets – some was sudden death syndrome or brown stem rot, charcoal rot, Phytophthora stem rot, and soybean cyst nematode. There are some other early yellowing situations that we are still working on an accurate diagnosis, but yellowing in these cases may be linked to fertility issues and/or related to late flooding injury. I think in 2018 we’ve observed just about everything, and it was all dependent on where in the state the soybeans were grown, how much rain occurred and when that rain fell, as well as the variety. It did seem that we had calls on the same variety from multiple regions.
The heat this past weekend is also going to move the crop fairly fast. So if you haven’t driven by the earliest planted fields – this is the week to do so. Sudden death syndrome is very widespread – but in most fields, it is limited to a scattering of plants throughout the wet areas. The plants were not severely affected as most of the fields I visited were holding their leaves and not defoliating as quickly as I have observed for the most susceptible varieties. Late season Phytophthora stem rot is also present – in this disease, the plant wilts, holds its leaves and develops a brown canker that extends from the base of the plant up the stem. Charcoal rot can also cause early yellowing or dying, and these symptoms were present last week in several areas of the state. To distinguish this from other diseases, cut open the tap root and look for the black dots embedded in the tissue and lower stem. When populations of soybean cyst nematode are high, plants will also mature earlier. For cyst, you can dig up the plants, shake the soil off and see the small white pearls (females) on the roots. Often we need a microscope as the cyst will turn tan to brown and becomes hard to see.
This round of late season scouting is important for variety selection, improving fertility applications for the fall, and prioritizing which fields to sample for soybean cyst nematode. Let’s just hope the weather cools so we can get out of the trucks and walk into the fields!