Symptoms and Signs
In seedlings, symptoms appear after the pathogen infects the stems at the base of the developing cotyledons near the soil line. The fungus produces black, sunken cankers that have sharp margins and often contain concentric rings. The plant's growing tip may be killed or the stem broken where it is weakened by the canker. Infection may continue into the hypocotyl and root region or the primary leaf petioles. Root infection causes a brown to black necrosis. If plants are grown under dry conditions, young plants can be killed.
Infection of older seedlings and plants may cause stunting, leaf chlorosis, early defoliation, and plant death. On older plants, "charcoal dust" often appears on the surface of roots and stems, mainly near the soil line, and is diagnostic evidence for this disease. This charcoal dust effect is caused by the production of small, black microsclerotia just below the epidermis and in the vascular tissue. This symptom is also called ashy stem blight.
Comments on the Disease
Charcoal rot can affect common beans, blackeyes, and limas. The fungus is pathogenic on many crops including corn and sorghum. The pathogen can survive in both the seed and the soil.
The disease occurs mainly under high temperatures and drought stress conditions– especially if irrigation is delayed. It is capable of infecting plants at all stages of growth. Soils that are high in organic matter, such as those along the Sacramento River-San Joaquin Delta region, tend to have more problems with this disease. In addition, uneven, unleveled land (such as beans grown in newly planted orchards) and fields where the plants are stressed (such as from too much or too little irrigation water) tend to have increased risk for getting this disease.
Avoid drought stress, especially during periods of high temperature. A 3-year rotation with a cereal crop (except corn or sorghum) may help reduce soil inoculum.