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Red root rot has been a problem in the Atlantic coast states since the late 1980s and has increased incidence in Colorado, Nebraska, Missouri and Oregon since the late 1990s. Yield losses as high as 15 to 20% have been reported in the Delmarva region (Delaware, Maryland, Virginia).1 Genetic resistance has been difficult to find and management options are limited.
Red root rot is caused by a complex of soil fungi that includes Phoma terrestris, Pythium species, and Fusarium species, of which Phoma terrestris is the primary pathogen.
The optimal temperature range for disease infection is 70 to 80 °F around the time corn begins to senesce. There are reports that infection may occur as early as mid-silking.1 High plant populations, high fertility, and irrigation can be common ingredients in infected fields. Recent research indicates that early infection by Pythium species (and others) can cause root damage that enables Phoma terrestris to invade corn roots more successfully.
Phoma terrestris produces dark, thick-walled microsclerotia resting structures in the roots of infected plants. The microsclerotia allow the fungus to overwinter, survive in the soil for many years, and are the primary source of inoculum.1
Red root rot symptoms typically appear just before senescence. Roots and basal stalk tissue (lower three internodes) infected with red root rot have a reddish pink discoloration (Figure 1).1 Roots become a deeper red color as the disease progresses (Figure 2). The reddish coloration can be confused with Fusarium or Gibberella stalk rots, but red root rot has a darker red color.
The root tips and roots may be shredded or frayed, similar to insect damage.2 The root mass can be small, making severe lodging more likely. Combining may be difficult because the entire root ball may be pulled up as plants are harvested. The small root mass, compromised root system, and lodging contribute to yield loss.
During the late stages of ear filling the disease can cause rapid, premature death of the plant. Foliar symptoms can occur over a four to five day interval and plant death of the most susceptible plants can occur within a week. Other above ground symptoms include a grayish green discoloration of leaves and stalks or a wilted appearance, which is also characteristic of other stalk rot diseases.
Management options are limited. Crop rotation with a non-host such as soybean can provide some control.1 Genetic resistance has been difficult to incorporate into corn products, although the rate of disease development varies greatly between corn products. Research on inheritance of disease resistance indicates that it is a polygenic trait with additive gene action, which has complicated breeding efforts.1 Environmental stress during the season may contribute to disease infection and severity.2