Charcoal Rot
Charcoal Rot (Macrophomina phaseolina)
Charcoal is caused by the widespread fungus Macrophomina phaseolina which can attack over 500 plants including soybeans, edible beans, corn and sorghum. The disease is worse in hot, dry conditions. Infected plants which reach maturity will have shrunken, lightweight kernels, or in severe conditions plants may die before maturity.

Charcoal rot can be considered to be a stress-related pathogen. The symptoms of charcoal rot, which most commonly occur after flowering and during dry weather, include rapid wilt and death of plants (Fig. 1), a silver-gray lesion at the base of the stem (Fig. 2) and minute, black, pepper-like structures (microsclerotes) scattered in the pith (Fig. 3). Rarely, charcoal rot occurs on the seed heads where it can infect developing seeds.
charcoal rot
Figure 1. Wilting and dead charcoal rot-infected sunflower
charcoal rot
Figure 2. Charcoal rot lesion at base of stem, cut open to reveal microsclerotes
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Figure 3. Silver to grey lesion at base of infected stems
The fungus survives as microsclerotes in infected residues or freely in the soil. Evidence from research on mungbean in Australia suggests that Macrophomina infects the roots of healthy seedlings, where it remains dormant until the plant reaches the flowering stage and suffers either heat or moisture stress, or both. Other work in that country has shown that the pathogen can survive in the roots of common weeds that do not display any signs or symptoms of infection, so it is likely that this mode of survival also occurs in the USA sunflower-growing areas.
charcoal rot
Figure 4. Abundant microsclerotes inside infected sunflower
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Figure 5. Close up of microsclerotes
Active management of charcoal rot is difficult. Fungicides are generally not considered effective, and although some sunflower cultivars have higher tolerance to Macrophomina infection (probably associated with better drought tolerance), none are immune. Methods to minimise stress on plants, such as using optimum nutrition, planting times and planting densities can sometimes be effective. Avoiding crop rotation with highly susceptible crops may lower risk. There is conflicting evidence on the role of cultivation in the survival of microsclerotes in infested soil.
Images
Figure 1. Wilting and dead charcoal rot-infected sunflower (Sue Thompson, USQ).
Figure 2. Charcoal rot lesion at base of stem, cut open to reveal microsclerotes (Tom Gulya, USDA)
Figure 3. Silver to grey lesion at base of infected stems (Sam Markell, NDSU).
Figure 4. Abundant microsclerotes inside infected sunflower (Sam Markell, NDSU).
Figure 5. Close up of microsclerotes (Sam Markell, NDSU).
Additional resource
Thompson S (2017). Charcoal rot Pp. 44-47 In Sunflower, Section 9 Diseases, GRDC Grownotes. Grains Research & Development Corporation, Barton ACT, Australia. https://grdc.com.au/__data/assets/pdf_file/0024/370617/GrowNote-Sunflower-North-09-Diseases.pdf
 
Other NSA resources
Disclaimer statements
Information based in part on and reproduced from Kandel, H., Endres, G. and Buetow, R. 2020. Sunflower Production Guide. North Dakota Extension Publication A1995. Informational updates made possible by the Sunflower Pathology Working Group, and is/was supported by the USDA National Institute of Food and Agriculture, Crop Protection and Pest Management Program through the North Central IPM Center (2018-70006-28883).
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