Charcoal rot in soybeans

Editor’s note: This article is from the archives of the MSU Crop Advisory Team Alerts. Check the label of any pesticide referenced to ensure your use is included.    

Although drought conditions aren’t especially conducive to the development of root diseases on soybean, there is one notable exception – Charcoal rot (Macrophomina phaseolina). (view photos) Although this disease organism survives in the soil and in crop residue from year to year, it develops when there is a high level of the pathogen in the soil, and plants are stressed by weather that is hot and dry. Recent rainfall and cooler temperatures should help to reduce plant stress, but plants damaged by this disease are likely to have already suffered some loss of yield. This pathogen can also cause a stalk rot in corn.

I have visited fields and looked at samples from Clinton County, Sanilac County and Ingham County in the last two weeks and confirmed charcoal rot in all of them. Soybean samples with charcoal rot have been confirmed by MSU Diagnostic Services, and charcoal rot has been identified by extension educators in other counties in the state. One field I visited had a large number of dead and dying plants, particularly in low areas. Many of the remaining plants were stunted, and the upper leaves were yellowing. There were a few pods at the upper nodes and many of those were flat. Symptoms in the other fields weren’t quite as severe, but stunted and yellowed plants appeared in patches in the field. The confirming field symptom was the numerous, tiny black fruiting structures (sclerotia) in the root and stem tissue. To the naked eye, the infected tissue has a grayish-black color, as if it had been sprinkled with powdered charcoal. If you use a hand lens or field microscope, you can clearly see the tiny, individual sclerotia. It is interesting to note that several of the fields I visited had two different soybean varieties planted side by side in the same field, and that there was a noticeable difference in how the varieties were affected by the disease. The less affected variety had considerably fewer dead and stunted plants, was much greener and appeared to have better pod set than the other variety.

Charcoal rot can infect soybean plants early in their development, but symptoms don’t usually show up until plants begin to flower, or even later as plants’ water needs increase during pod formation and filling. Early symptoms, such as yellowing of upper leaves and premature leaf drop, may be attributed to other causes, including drought. Symptoms develop in the driest areas of the field first. The fungus is most active when soil temperatures are in the 80-95°F range.

If you have charcoal rot in your field, it is advisable to rotate out of soybeans for two years. Small grains or corn can be planted during the rotation away from soybeans. According to the University of Wisconsin Soybean Plant Health website, even though corn is a host for the charcoal rot pathogen, it can’t support as high a population of the microsclerotia as soybean. Plowing or tillage is not an effective means of controlling the disease, as the sclerotia can survive in the soil for a long time. Cultural practices that limit plant stress, such as optimum fertility levels, moderate planting rates and good weed control practices can help to reduce disease severity. Choose soybean varieties that are less susceptible to charcoal rot. Also, plant later maturing varieties (to the extent that is practical) that will be in the less vulnerable vegetative growth stages during the hottest, driest weather.

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