Charcoal Rot – Macrophomina phaseolina

Background Information
  • Charcoal rot occurs primarily in dry, hot conditions. This disease can cause minor to significant yield losses, and it is most common in the southern areas of the Midwest.
Disease Agent
  • Macrophomina phaseolina is a fungus that has a wide host range that includes corn, wheat, and some weeds. The fungus overwinters in dry soil or residue of host tissues, and it can be spread with contaminated seed.
Symptoms
  • Symptoms typically occur during or after flowering.
  • Brown lesions may form on the hypocotyls or emerging seedlings.
  • Leaflets may be small and ultimately wilt and turn brown.
  • The taproot and lower stem may become light gray.
  • Small black dots may form beneath the epidermis of the lower stem and in the taproot to give the stems and roots a charcoal-sprinkles appearance.
  • In addition, reddish-brown discoloration and black streaks can form in the pith and vascular tissues of the root and stem.
Time of Occurence
  • Seedlings may be affected in hot, dry soils in early summer, but the disease typically appears after mid-season.
Conditions Favoring Disease
  • Hot, dry weather
  • The disease is most severe where plants have been growing under conditions of stress or injury.
Disease Managment
  • Choose soybean varieties that express moderate resistance to this disease.
  • Manage fields to avoid drought stress.
  • Decreased disease has been reported with minimal tillage, perhaps due to cooler soils and less moisture stress.
  • Crop rotation with cereals or corn for 2 to 3 years may be beneficial.