Update:
Control methods for charcoal rot were compared in soybean cultivars from three maturity groups commonly grown in southeast Kansas. The control had no biological or chemical treatment. The biological treatment was a mustard plant, Mighty Mustard Pacific Gold (Johnny’s Select Seed), a mustard variety that produces high glucosinolate concentrations suggested to control soil-borne diseases. Chemical control was seed treatment of fungicide only; because there was very little charcoal rot pressure in 2015, no in-season fungicide applications were made. The fourth treatment included both biological and chemical treatments.

The mustard cover crop was planted in April. Canopy coverage of the mustard was measured with a light bar. The mustard cover crop was terminated in early June, and lightly incorporated prior to planting soybeans. Soil samples were taken prior to planting and after termination of the mustard seed. Soil microbial activity was measured in the soil samples with the Solvita Field Test.

Soybeans were harvested at maturity with a plot combine. No differences in yield were observed between treatments or between cultivars.

Plant samples were collected at R7-8 and analyzed for charcoal rot. Although there was no charcoal rot infection visible on the plants, the fungus was present in the plant samples. The biological treatment (mustard cover crop) had a significantly lower colony forming unit rating than the untreated check. The biological treatment was also lower than the chemical treatment and combined chemical + biological, but the difference was not significant.

Three of the cultivars (AG 4933, AG 4934, and AG 5332) had lower charcoal rot levels than the other cultivars. One cultivar, AG 4135, was particularly susceptible.

Note that environmental conditions likely played a role in the lack of disease pressure in 2015. Total rainfall in southeast Kansas during the 2015 growing season was higher than normal, and more importantly, was received nearly consistently throughout the growing season. The usual summer dry period that exacerbates disease pressure and inhibits yield was not experienced. More importantly, temperatures were exceptionally cool in southeast Kansas during 2015, with fewer than five days during the growing season exceeding 95°F, the temperature above which soybean yields begin to decline (Sassenrath et al., 2014).

The soybean cultivars selected for this study included two early maturity group 4’s, two late group 4’s and a mid- to early- group 5. One of the cultivars planted in 2015, AG4933 RR2Y, is not available. Instead, AG4835 will be substituted.

Reference:
Sassenrath, G.F., Liu, X., Shoup, D. 2014. Crop Yield Trends in Kansas. Agricultural Research 2014, Southeast Agricultural Research Center, Kansas State University, SRP 1105. Kansas State University Agricultural Experiment Station and Cooperative Extension Service. p. 66 – 70.