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Charcoal Rot in Soybeans

If you notice yellow, wilted soybean plants this fall, charcoal rot might be to blame. Also known as dry weather wilt, charcoal rot affects soybeans throughout the U.S. UNL warns the disease can infect a number of plant species, including rotation crops such as corn.

Charcoal rot is caused by a fungal pathogen, Macrophomina phaseolina, and infection usually occurs early in the season. If you’ve faced dry weather, note that this disease thrives when the plant is under stress and in hot, dry conditions. Disease symptoms don’t typically appear until the reproductive stages and when favorable disease conditions develop.

  • Begin as patches of stunted plants, appearing after R1.
  • Leaves turn yellow and wilt, then they die, but remain attached to the plant.
  • The lower portion of the stem and tap root have a light gray or silver discoloration.
  • Tiny black specks, known as microsclerotia, are visible on the lower stem and root. Where the stem is split, it has the appearance of fine charcoal powder. According to Iowa State, microsclerotia can be found on the outside layer of the stem, just beneath the surface and inside lower stems of dead plants.

Black specks of microsclerotia on a soybean stem. Source – Syngenta

  • Microsclerotia are the survival structures of the fungus that overwinter on soil or plant residue. 
  • The fungal pathogen infects the soybean roots early in the season, but it remains suppressed until favorable conditions occur for disease development.
  • Charcoal rot development occurs and symptoms become visible during periods of high temperatures and low soil moisture during the reproductive stages.
  • Select soybean varieties that are less susceptible to charcoal rot. 
  • In fields with a history of charcoal rot, avoid high populations to help conserve soil moisture, and manage soil fertility levels to reduce plant stress. 
  • Crop rotation to small grains can help reduce inoculum, however corn is susceptible to charcoal rot and does not help reduce infected residue. 
Contact your Golden Harvest Seed Advisor with questions or for additional agronomic insights.

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