In the 2003 growing season, charcoal rot caused by a fungus called Macrophomina phaseolina was prevalent in the soybean fields of Iowa, the first ever statewide occurrence. Damage by the disease was not identified by many producers since the disease was relatively new. Surveys covering areas from northern to southern Iowa showed that in the northern Iowa (north of Highway 3), the prevalence was 60 percent. In central Iowa (between Highway 3 and Interstate 80), 90 percent of fields sampled were positive with plants having M. phaseolina. In southern Iowa (south of Interstate 80), 20 percent fields had charcoal rot infested plants, which was considered an underestimation by Iowa State University agronomists.

Yield loss

When severe, the disease reduces yield by killing the plants at early soybean reproductive stages. In 2003 season, soybean yields were measured at Iowa State University Hinds Research Farm near Ames in a field severely infested by the disease using GPS. In this field, the disease was first noticed in early August. By late August, the disease affected most soybean plants in the field (photo 1). Yield level in charcoal rot infected area was much less than the uninfected areas (see yield map). In northwest area free of disease, the yield was over 50 bu/acre. Some portions were in 40-50 bu/a. In areas severely infested which was about half of the field, yield was under 30 bu/a and significant portion was less than 20 bu/a.

Above: yield map of soybean from a field infected by charcoal rot at the Hinds Farm.

Enlarge

Severe damage due to charcoal rot of soybean on Hinds Farm near Ames, Iowa.

Management

Charcoal rot is a disease whose epidemics are associated with droughts. Dry weather during the preceding few years should have built-up sufficient pathogen populations in Iowa fields high enough to cause further damage if drought returns. Future risk of disease depends on the length of the dry climate cycle or whether drought would return in next year or two.

One year corn-soybean rotation is ineffective to control charcoal rot since the fungus also causes corn stalk rot. However, planting corn for 3 years (at least 2 years) has been recommended if a field had severe soybean charcoal rot. The fungus it is less damaging to corn than to soybean. Three year in corn could also avoid the disease since Mother Nature may be out of her peak of dryness after 3 years. Rotation with small grain, such as wheat and barley, minimizes disease risk.

Avoid excessive seeding rates, as high planting density is associated with increased competition for moisture. Soybean with a high plant density uses more water from soil, which increases disease risk during a dry season.

There are differences in level of disease among varieties. As we observed this year, there are differences among soybean varieties with some varieties more susceptible than others. Selection of earlier mature varieties is a consideration to shorten the effect of dry period during a growing season.

Seed treatments and tillage have been shown ineffective to control the disease according to literature. No tillage is more likely to increase pathogen survival.