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Potential Control Measure for Charcoal Rot in soybean

Over the last few years, I have penned a lot of words about using cover crops in Midsouth soybean production systems. And many of those words have referenced the need for a producer to define his/her goals when deciding to adopt a system that includes cover crops. But I can’t recall seeing or reading that one of the goals that should be considered is using a cover crop to control a soil-borne pathogen that is known to negatively impact soybean yield.

So when I came across the article “Pass the Mustard: Cover Crop Found to be Natural Barrier to Charcoal Rot”, my interest was piqued enough to delve into just what this was about. And to my surprise, the subject of the article was results from Kansas State University research where mustard (the same mustard plant that produces the seed from which the condiment is derived) was evaluated as a cover crop to suppress the charcoal rot pathogen which resides in soils in many soybean fields.

Charcoal rot (CR) is a disease of crops (including soybean) that is caused by the soil-borne pathogen Macrophomina phaseolina. The disease is most likely to limit soybean yield in field situations where hot and dry weather conditions occur. The fungus infects plant stems and ultimately will kill plant roots if conditions promoting its proliferation persist. Surveys over the last few years have estimated it to be a major cause of disease-related soybean yield losses in the Midsouth.

There are no known curative measures for CR–e.g., there are no efficacious seed treatment or foliar fungicides to control this disease. The only management options to limit damage from CR are using resistant soybean varieties (only moderately resistant varieties are available) and alleviating conditions such as drought that promote its damage potential (Luna et al., J. Int. Pest Mgmt., 2017). This means that producers are generally left to the mercy of this disease and they hope that it either does not rear its ugly head, or that conditions that promote its proliferation will soon be mitigated by improving weather conditions.

Now back to the subject of the above linked article from Kansas State University. It turns out that there is a compound called glucosinolate that is present in the mustard plant (Brassica juncea). Results from research indicate that this compound and its hydrolysis products have efficacy against soil-borne pathogens (Sotelo et al., Appl. Env. Microbiology, 2015). The premise for conducting the study covered here is that this compound and its secondary metabolites derived from mustard used as a cover crop will have a suppressive effect on the CR fungus.

The research that was summarized in the above article (click here for the report of results) covered a 3-year period. A mustard cultivar was planted in early spring at two sites (Columbus - ~37°N lat.; Manhattan - ~39°N lat.) in eastern Kansas. Mustard plants were allowed to grow until flowering, when they were terminated with a herbicide burndown.

Five treatments as follows were imposed on plots that were planted to a MG 4.1 soybean variety following mustard crop termination.

1. Control–no mustard cover crop;

2. Mustard cover crop with no incorporation–soybeans planted into standing mustard;

3. Mustard cover crop with no incorporation–cover crop rolled before soybean planting;

4. Mustard cover crop with no incorporation–cover crop mowed before soybean planting;

5. Mustard cover crop with disk-incorporation.

Soil and soybean plant samples were collected at R7-R8 and amount of fungal infection as indicated by the number of colony forming units (CFU) was determined in both the soil and plant samples. Results follow.

•   In a previous study, the number of CFU’s was reduced in soil by 8% and in plants by 50% in plots that had a mustard cover crop compared to an untreated control.

•   In this study, an unincorporated mustard crop (treatments 2-4) resulted in reduced CFU’s in the soil. The greatest reduction in CFU’s occurred where soybeans were planted into the rolled or mowed cover crop (treatments 3 and 4). Thus, the more intact the cover crop before soybean planting, the greater the control of the CR fungus in the soil.

•   At the site with the heaviest CR pressure, cover crop treatment did not affect soybean yield.

•   The authors concluded that these limited results indicate the potential for a mustard cover crop to help in controlling CR in a following soybean crop based on the significant reduction in disease pressure.

The positive effects from using any cover crop, regardless of the intended goal(s) for its use, will likely only be determined/realized after its long-term continued use. According to Dr. Gretchen Sassanrath, lead scientist of the project, the research study cited above will be continued. Hopefully, this continuation will be long enough to determine if an annual mustard cover crop will result in a cumulative repression of the charcoal rot fungus in an amount that will result in less reduction in soybean yield caused by the fungus. This could be a real breakthrough in the management of this problematic disease of soybeans. The preliminary results cited above certainly provide support for a continued research effort to determine its worthwhileness.

Composed by Larry G. Heatherly, Apr. 2019, larryheatherly@bellsouth.net