- Charcoal rot is caused by the fungus Macrophomina phaseolina which overwinters in the soil and soybean residue as tiny, black structures known as microsclerotia.
- Microsclerotia is what gives this disease the charcoal-like appearance, as numerous microsclerotia can be found on lower stems of infected plants just under the epidermis.
- Premature death, while the leaves remain attached to the plant, is a symptom associated with charcoal rot.
- The development of symptoms is usually favored by heat and drought stress.
Pathogen Biology
Charcoal rot is caused by the fungus Macrophomina phaseolina which overwinters in the soil and soybean residue as tiny, black structures known as microsclerotia (Image 1). Microsclerotia infect nearby soybean roots, colonizing root tissue and stems and clogging vascular tissue, thus interfering with water uptake. Favorable conditions for root colonization of charcoal rot include high temperatures and low soil moisture. When these conditions are prevalent during the growing season, yield losses due to charcoal rot can be significant. Infection can happen throughout the season and even affect young soybean plants, but the speed of colonization by M. phaseolina increases after pod fill. However, even when infection happens early, symptoms may not even develop unless plants are under stress (mainly heat and drought stress). Microsclerotia is what gives this disease the charcoal-like appearance, as numerous microsclerotia can be found on lower stems of infected plants just under the epidermis. M. phaseolina can infect more than 500 crop and weed species including corn, grain sorghum, cotton, tobacco and sunflowers. Microsclerotia can survive in the soil for many years if soil conditions are dry, but can only survive for a few weeks in saturated soils.
Symptoms and Signs
Microsclerotia are a sign that can be used to distinguish charcoal rot from other infectious or non-infectious diseases, but they are usually not visible until plants are mature or dead. As infected plants mature, yellowing and wilting can be observed in patches in the field (Image 2). As the disease progresses premature death while the leaves remain attached to the plant can occur. Upon closer inspection, these plants may exhibit a gray discoloration on the lower part of the stem along with microsclerotia just under the epidermis. Symptoms may appear first on the driest parts of the field.
Management
Cultural: Planting soybeans in no-till fields where soil moisture is preserved can help reduce charcoal rot levels and severity. Irrigation, when possible, can help reduce water stress in dry years and reduce the severity of symptoms and yield impacts. Rotation to wheat can help reduce the pathogen load, but rotation to corn and sorghum will not reduce inoculum levels since these latter crops are also hosts of M. phaseolina.
Resistant varieties: Genetic resistance to charcoal rot can provide effective yield protection but complete resistance is not available. However, partial resistance has been identified in soybeans adapted to southern growing regions (maturity groups IV and V) but it is limited in northern maturity groups (III and earlier). Some seed companies will provide a charcoal rot rating for their products, but others will not, so selecting varieties may be difficult in these situations.
Image 1. Charcoal rot microsclerotia on soybean stem.
Image 2. Yellow and wilting soybean patches affected by charcoal rot.
