Cotton root rot often causes the rapid wilt and death of cotton in the late spring, summer and early fall. Dead and dying leaves remain attached to the plant, and the roots of dying or declining plants are rotted. Photo courtesy of AgStock Images/Harold Kaufman.
Cotton root rot is a soilborne disease caused by the fungi Phymatotrichum omnivorum (cotton/Texas root rot). The disease has afflicted the cotton industry for more than 100 years. Cotton root rot typically occurs in specific geographies in the southwestern United States, and is found in Arizona, California, Texas, New Mexico and Oklahoma. Cotton Root Rot is prevalent in calcareous, alkaline soils and in areas with high summer temperatures and moist conditions (Goldberg, 1999). It occurs only under elevations of 5,000 feet and in low organic matter, high-pH soils.
Although cotton root rot fungus is limited in its geographical distribution, it’s one of the most destructive and difficult-to-manage diseases for cotton growers. The fungal pathogens survive deep beneath the soil, as much as 12 feet or more. Cotton root rot possesses amazing survival capabilities, allowing it to persist indefinitely in the soil.
Identification and Lifecycle
Growers will typically see cotton root rot symptoms when soil temperatures climb to 82 degrees F or above. Initially, symptoms appear as a slight yellow or reddish-bronze color in cotton leaves. Within 24 to 48 hours after bronzing, the top cotton plant leaves will wilt, with the lower leaves wilting in 72 hours. By the third day, infected plants will permanently wilt and die. Although infected leaves remain attached, cotton plants can suddenly die, even after they appear to have achieved exceptional growth.
Another symptom to help identify the disease is severely decayed tap roots of wilted plants. The decayed roots may show a fuzzy mold and a brown stain along the tissue. An additional distinctive sign of cotton root rot is spore mats that form on the soil near dead plants. The spore patches initially look white in color, then turn a tan color before disappearing within a few days.
It’s generally believed that cotton root rot spreads to new areas within a field by gradual growth in the soil from plant to plant. The fungus spreads from plant to plant either through root contact or by slow growth of mycelial strands through the soil (Smith et al., 1962). According to Texas A&M Extension research, the disease appears as dead cotton plants in circular patterns in a field. These sections of infection slowly grow during the season or, in following crop years, may increase five to 30 feet per year in a cotton field. Plants infected earlier in the growing season will die before bearing fruit, whereas infection occurring at later growth stages will reduce cotton yield and lower lint quality (Ezekiel and Taubenhaus, 1934; Yang et al., 2005).
Cotton root rot fungus can survive indefinitely in the soil. The Phymatotrichum organism lives as sclerotia, which are tiny, resilient seed structures. According to Oklahoma State University Extension, this fungus seems to be native to certain fields and not much is known about how it moves into new fields.
Cotton root rot is one of the most destructive and difficult-to-control fungal diseases in cotton; it can also attack more than 2,000 other broadleaf plant species. Cotton growers face the threat of significant crop damage to seedlings and potential devastation near cotton maturity.
Because the disease invades plant roots, it shuts off water flow upward to cotton leaves, resulting in plant wilt. This additional stress tends to reduce cotton quality and lint yield. For example, Texas A&M University researchers estimate that cotton root rot in Texas can cause losses of $29 to $40 million annually.
Managing Cotton Root Rot
Although management efforts such as scouting and cultural practices can help reduce the incidence and impact of cotton root rot, none are highly effective, according to Oklahoma State University Extension. Thus, preventative management measures are key.
Field mapping of cotton root rot diseases can help growers gauge where infection may turn up in individual fields. Aerial monitoring and imaging can aid mapping of the disease’s location and progress in a field, which provides a sense of when or if a fungicide treatment may be economically beneficial. Ground scouting can also help in documenting problem areas in fields, such as locations where soil water is being prevented from moving upward from cotton roots toward leaves.
Crop rotation, such as three to four years of cotton into a grass crop like grain sorghum, may help deter cotton root rot, although it may not reduce sclerotia in the soil, according to Oklahoma State University Extension. In some studies, three to four years’ rotation to a grass crop has shown to reduce cotton root rot by up to 60 percent. Deep plowing is another option.
There is no varietal resistance to cotton root rot. A cotton grower’s best planting strategy against cotton root rot is to plant early and choose early-maturing varieties, which can help prevent the disease from taking over later in the growing season.
Some growers with a field history of cotton root rot may want to make an at-planting fungicide spray application with a labeled fungicide.
Bayer Solutions to Control Diseases in Cotton
Visit our cotton section to learn more about cotton disease management solutions from Bayer, or contact your local Bayer representative.