Plant Disease: Charcoal Rot

Taxonomy:

Distribution:

• Charcoal rot thrives in the hottest, driest part of the growing season.


• Most prevalent in southern states, but can develop anyplace where hot, dry conditions exist.


• The disease develops when there is a high level of the pathogen in the soil and when plants are under stress from hot dry weather.

Impact:

• Studies showed that M. phaseolina could reduce plant height, root volume, and root weight by more that 50%.


• Damage to the root system is most evident during the pod formation and filling stages, when demand for water and nutrient absorption is high.


• Because diseased plants have smaller root systems, resulting seeds tend to be fewer and smaller in size.


• Diseased plants will mature several weeks earlier, which further contributes to yield loss.


• The charcoal rot pathogen survives from year to year as microsclerotia in soil and infected crop residue

Symptoms:

• Root infection can occur very early in soybean plant development.


• M. phaseolina can infect up to 100% of soybean plants within 3 or 4 weeks after planting.


• The fungus can continue to infect soybean plants throughout growing season and charcoal rot symptoms can be evident in soybean plants of all ages.


• Plants infected after the seedling stage generally show no aboveground symptoms until the early reproductive phase.

Aboveground symptoms at R1

• Plants infected by the charcoal rot pathogen may have premature yellowing of the top leaves and premature leaf drop.


• This can be confused with normal maturity.


• Plants in the driest parts of the field will typically show symptoms first.


• Upper pods may have poor fill and general low plant vigor.


• In some cases, the upper one-third of the plant may have only flat pods without seed.


• Tap and secondary roots and lower stems will have light gray discoloration.


• Scrape the outer tissues and will have black, dusty microsclerotia when the surface is scrapped off.

Management:

• Plants grown in conditions of high temperatures, drought or poor fertility (too high or too low) are most susceptible.


• Any cultural practices that minimize plant stress will reduce the risk of charcoal rot.


• Reduce plant populations and optimize fertility levels, especially phosphorus.


• Good fertility won't control charcoal rot but it can reduce disease impact on yield.

• Select soybean varieties that are not highly susceptible to charcoal rot.


• Plant high quality, disease-free soybean seed.


• Plant the short season-season varieties that are practical.


• Plants are most susceptible to charcoal rot at the beginning at flowering.


• The shorter-season varieties to reduce the time for dry conditions during fill.

• In fields with a history of charcoal rot, rotate out of soybeans for two years or more.


• Small grains and corn can be planted during the two-year interval between soybeans.


• Corn is a host for the pathogen but generally support lower populations of microsclerotia in soil than does soybean.


• Microsclerotia can survive for a indefinite time in soil.


• The benefit of rotation will vary with the field.