A few weeks ago, we looked at a field with suspected potash deficiency. Stemphyllium leaf spot was not very evident throughout the field. The leaves had more of an interveinal chlorosis (right). This could be micronutrient deficiency, nematodes, or even fusarium wilt. I sliced the stem where no fusarium was evident. I did see some galls on feeder roots, but not many. We pulled a nematode and soil sample to see what was going on.
The sample indicated Southern root-knot nematodes were well over 100 and the soil sample showed deficiency in potassium and manganese. The pH here was 7.4 which is above recommendations. This would limit our manganese. UGA Extension Fertility Specialist Dr. Glen Harris said although K and Mn are low, the leaf symptoms are characteristic of nematodes. Even though not much stemphyllium leaf spot and galls were observed, tests show that these problems existed. However, based on leaf symptoms, nematodes was our most limiting factor. September, October and November are recommended months to sample nematodes in cotton for root-knot if problems in the field are evident. Below is from the UGA publication Guide For Interpreting Nematode Assay Results: