NPIPM:Charcoal rot
Compiled by: Buyung Hadi, from the materials by: Loren Giesler, Dean Malvick and Lawrence Osborne
Charcoal rot is a dry weather disease of soybean. The disease is common in the southern part of the U.S. Charcoal rot was first found on soybean in Minnesota in 1999 and in North Dakota in 2002. Its importance in the Northern Plains (?) soybean growing region is expanding, especially in areas where summers are hot and dry.
Causal Organism
Charcoal rot is caused by the soilborne fungus Macrophomina phaseolina. This pathogen has a large host range, including corn, sunflower and some weeds..
Symptoms and Signs
The pathogen causing charcoal rot may infect plants at any developmental stage. Infected plants may show no symptoms until drought stress and high air temperatures occur, usually during the R1 and R7 growth stages. In some cases, infected seedlings may develop brown lesions on the hypocotyls. At later stages, symptomatic plants show slightly smaller leaflets with reduced vigor. The leaves progressively wilt, turning yellow then brown. The wilting leaves typically remain attached to the leaf stalk. When split open, the lower stem and tap roots of infected soybean plants show a streak of light gray or silver. The outer stem tissue underneath the epidermis is typically peppered with small black specks (microsclerotia). Vascular and pith tissues of the stems and tap roots may also turn reddish brown. Due to similar symptom of pith tissue discoloration, charcoal rot may be confused with brown stem rot. However, pith tissue discoloration in plants with charcoal rot is limited to the lower part of the soybean stem, often no more than the fifth node, while the discoloration due to brown stem rot can reach higher portion of the stem. Additionally, the small black specks evident in the tissues of lower stems and tap roots with charcoal rot should be used to differentiate this disease from brown stem rot. Disease and symptom development is optimum under dry conditions, so the first symptomatic plants usually show up in parts of the field that are dry. In infected plants, upper pods may have poor fill. In severe cases, the upper one-third of the pods may have flat pods without seeds.
Life Cycle and Epidemiology
The small black specks peppering lower stem and tap roots tissues underneath the epidermis are fungual structures known as microsclerotia . In winter, the fungus survives as microsclerotia on crop debris. Microsclerotia can survive for a few years in dry soil, but only for a few weeks in saturated soil. Plant infection may occur at any time during the growing season. Many infections occur early in the season, but no obvious symptoms develop until mid-season, when environmental stresses occur. Hot and dry weather with soil temperatures of 80 to 95˚F for 2-3 weeks enhance symptom development. The pathogen causing charcoal rot may infect soybean seeds and lower the seeds’ germination rate.
Management Approaches
Cultural Practices
Fields with charcoal rot history should be rotated out of soybean for two years or more. Rotate soybean with plants that are less susceptible to charcoal rot, such as small grains. Fields should be managed to reduce and avoid drought stress to decrease the impact of charcoal rot infection. For example, reduced seeding rate may reduce drought stress and thus reduce the impact of charcoal rot. Optimized fertilizer rate, especially phosphorus, will not control charcoal rot, but it can reduce the disease impact on yield.
Host Plant Resistance
Soybean varieties vary in susceptibility to charcoal rot. Select more resistant varieties if the field has a history of charcoal rot infection. Selecting early maturing varieties may also help since the early maturing soybean will mature prior to drought stress.
Chemical Control
Seed treatment fungicides have not been shown to help manage charcoal rot.
