No. 20/August 8, 1997
Hot, dry conditions are mostly unfavorable for plant pathogens. Most prefer damp to wet conditions and moderate temperatures. Charcoal rot is a major exception. This fungal pathogen is active only in very hot and dry soils and when plants are under extreme moisture stress.
Charcoal rot, caused by the fungus Macrophomina phaseolina, derives its common name from the appearance of infected plant tissues. The fungus produces microsclerotia, very tiny black structures resembling charcoal dust, in the pith of corn plants or embedded in the taproot tissues of soybean. Microsclerotia are thick-walled survival structures, allowing the fungus to overseason until favorable weather the next year.
Another peculiar trait of this fungus is that it can attack both corn and soybeans. It causes a stalk rot in corn and a general root rot in soybeans. Most plant pathogens are limited to one major crop, which is why rotation works so well to reduce the survival of pathogens from one season to the next. This practice is not helpful with charcoal rot. The conditions that favor this pathogen, however, do not occur yearly; and losses from the stalk-rot phase are not as common as losses to other corn stalk rots. Soybeans are a much more common host plant in Illinois.
Symptoms on soybeans include a premature yellowing of the leaves, particularly in the top of the plants, and premature leaf drop. The pathogen normally attacks fairly late in the season and much of the yellowing and defoliation is usually attributed to dry spots in the field or to other factors that cause early ripening. However, examining the top of the plants shows that the uppermost pods have not filled; and, in some cases, the upper one-third of the plant has only flat pods without seed. Digging plants and using a knife to split the taproot or simply scraping the outer tissues layers from the taproot reveals the small charcoal-like structures.
Besides hot, dry conditions, charcoal rot is favored by low phosphorous levels in the field. Thus, fields that repeatedly show charcoal rot should be tested for soil-fertility levels. Although adding appropriate amounts of phosphorous does not control the disease, it can reduce severity of loss.
Other control measures include selection of adapted varieties suitable for areas where dry conditions may occur, proper seeding rates, and rotation with poor host crops like milo or corn. Although the pathogen can infest those crops, it does not cause the level of losses as with soybeans. In cases of severe disease levels, 2 years out of soybeans may be helpful. Plowing or tillage has not been shown to have a significant impact because the sclerotia are so long-lived, similar to what is seen with white mold of soybeans.
H. Walker Kirby,Extension Plant Pathology, (217)333-8414