About Charcoal Rot
Charcoal rot is caused by a soilborne fungus called Macrophomina phaseolina. The fungus survives in plant debris in the soil as microsclerotia (small, black structures). Microsclerotia can also inhabit seeds, lodged under cracks in the seed coat or on the seed surface. The fungi initially infect through the roots of the plant where they are able to germinate. After infecting the root, the pathogen will take over the root and stem tissue of the plant until soybeans reach their reproductive stages. As a result, water flow throughout the plant becomes limited, limiting plant growth and damaging vascular tissue. Symptoms of charcoal rot may not develop until later in the season when infected plants become stressed, even when infection occurs early in the season.
Charcoal rot Identification and Habitat
Disease symptoms typically appear during or after flowering. The most identifiable symptom of charcoal rot is the microsclerotia occupying the plant’s root and stem tissue. That being said, these symptoms may not be seen until the plant is already mature or dead. Often the pathogen will cause plants to lose vigor, brown, and wilt. Stunted growth is also a common occurrence in infected plants. Infected plants should appear in patches in the field, which is commonly one of the first signs of infection. Plants may die prematurely, but their leaves will remain attached. Affected plants, along with microsclerotia growth, may have grey discoloration in the woody parts of their stems. Typically affected areas include sandy areas, hillsides, and compacted headlands.
Characteristically, hot and dry weather are most favorable for disease development, especially during the plant’s reproductive stages. Severity of the disease will increase when infected plants have been grown under stressful conditions.
Charcoal Rot Management and Control Methods
Minimum or no tillage can help reduce charcoal rot because it cools soil and limits soil moisture. If a field has been affected by the disease, rotating the crop with a less susceptible host (e.g. wheat) for one to two years can be beneficial. Lower seeding rates should be considered as it may help minimize crop stress and yield loss. Always look for resistant cultivars; however, they are not common for charcoal rot.
No fungicide treatments have been found to provide consistent control of charcoal rot.