Charcoal Rot

Affected plants often wilt, ripen prematurely, and die in patches, especially during hot, dry spells late in the season. Splitting the lower stem and taproot reveals a greyish to silvery discoloration of the inner tissue that is finely peppered with tiny black microsclerotia, giving a dusty, charcoal-like appearance. The vascular tissue and pith may shred, and roots become discoloured and brittle.

Macrophomina phaseolina survives for long periods as microsclerotia in soil and crop residue. These germinate and infect roots, often early in the season, but the fungus tends to remain limited until the plant becomes stressed. Under hot, dry conditions during reproductive growth, the fungus colonises the vascular tissue extensively, blocking water movement and causing the rapid decline and premature death typical of the disease.

• Microsclerotia survive for long periods in soil and residue
• They germinate and infect roots, often early in the season
• Disease progresses when plants are stressed by heat and drought
• New microsclerotia form in infected tissue and return to the soil

The fungus has a very wide host range, and among field crops soybean, sorghum, maize, common bean, sunflower, and groundnut are commonly affected. Hot, dry weather, drought stress, high plant density, and low soil moisture during grain or pod fill all strongly favour severe charcoal rot, which is why it is often most damaging in dryland and heat-prone environments.

• Hot temperatures with dry soils and drought stress during reproductive growth
• High plant populations that increase competition for water
• Sandy or low-water-holding soils that dry quickly
• Short rotations among susceptible hosts that maintain microsclerotia

Because charcoal rot is driven by moisture and heat stress, management focuses on reducing that stress and lowering the soil population of microsclerotia. Avoiding conditions that push plants into drought during grain fill is central.

• Reduce moisture stress through irrigation where it is available
• Avoid excessively high plant populations that intensify competition for water
• Use rotation with less-susceptible crops to lower soil inoculum
• Maintain good soil structure and water-holding capacity
• Choose adapted varieties and manage other stresses that weaken plants