Two cultivars of soybean from each of three maturity groups (early IV, late IV and V) most commonly planted in Southeast Kansas will be planted in research plots at the Southeast Ag Research Center experiment fields outside of Columbus, KS. Cultivars from each maturity group will be selected as resistant (R) or susceptible (S) to charcoal rot, for a total of six cultivars tested. Fungal treatments will be:
1) no fungicide control
2) complete chemical control (seed treatment plus in-season fungicide application)
3) biological control
4) chemical and biological control

Biological control will be attained using a high-glucosinolate mustard with known biofumigant properties (Larkin et al., 2011). The mustard will be planted in the fall of the previous year, terminated in the spring with herbicide, and tilled into the soil prior to planting soybeans. All plots will be tilled to remove tillage as a variable. Weed and insect infestations in each treatment will be noted weekly during the growing season and controlled according to standard practices.

Production measurements will include in-season assessment of plant development and growth stage. Charcoal rot disease severity will be determined by randomly selecting ten plants per plot at the R7-R8 growth stage for root and stem severity rating. The plants will be scored by splitting the stem and taproot of each plant and rating the degree of gray discoloration and microsclerotia in the vascular and cortical tissues on a scale of 1-5. M. phaseolina root population levels will be estimated by grinding the split roots after the severity evaluation. The ground plant tissue will be plated on microbiological medium and incubated. Colony forming units (CFUs) of M. phaseolina will be counted and transformed to CFUs per gram of root tissue.

Sections of each plot will be hand-harvested at maturity to determine yield components of soybean (plants/acre; pods/plant; seed/pod; number of seed/acre; average seed weight and total seed weight/acre). Total plot yield will be taken with a plot combine and seed samples will be analyzed for seed size and quality. Economic analysis will be performed by developing partial budgets for each treatment from harvested yield and total input expenses.

Soil analysis will be performed to determine total biological activity of soils before planting cover crops and again after soybean harvest. Soil microbial activity will be measured with the Solvita respiration test system.

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.