(Wines&Vines) - After more than 100 years of fighting a fungus that causes cotton root rot—and then four years of testing a new chemical product—researchers at Texas A&M AgriLife Research have found a way to stop the effects of that disease in vineyards in Texas and other parts of the Southwest.

Dr. David Appel, professor of plant pathology and Texas A&M AgriLife extension specialist, told Wines & Vines that cotton root rot has replaced Pierce’s disease as the major disease of concern in the Texas Hill Country and Gulf Coast viticultural regions. “Cotton root rot has been showing up as the grape industry has expanded,” Appel reported. “It’s a devastating problem for people who have it. It shows up after they’ve put in all the infrastructure—the posts, the trellis, the vines.”

Caused by a fungus, Phymatotrichopsis onmivora, cotton root rot attacks more than 2,000 species of plants including cotton, alfalfa, grapevines and other fruits, nut and shade trees, and ornamental plants. The fungus prefers calcareous clay loam soils with a pH between 7.0 and 8.5, in regions with high summer temperatures, according to the plant disease handbook from Texas A&M University. The disease is not present in the High Plains or East Texas regions where many vineyards are planted, but it has affected vineyards especially in the Hill Country west of Austin.  

Cotton root rot is also present in Oklahoma, Arizona and New Mexico, all of which have similar types of soil and environmental conditions. While it the malady has been found in limited areas of Southern California, the disease has not been a problem in any of the state’s major grape-growing regions.

 The fungus produces seed-like organisms called sclerotia that can survive in the soil for decades and then detect the roots of a susceptible crop growing near it. When the fungus finds grape roots in vineyards, it will attack, and healthy-looking vines wilt; the leaves turn brown but stay attached to the vine; and within two weeks, the vine dies. Fritz Westover, who owns Westover Viticulture and is based in Houston, noted that the only way until now to escape cotton root rot in vineyards was to avoid locations with a history of cotton root rot. “It wasn’t sustainable to plant grapes where cotton had been grown,” he stated.

One old trick farmers used to determine where to plant susceptible crops was to pre-plant the acreage with alfalfa, which was very susceptible to cotton root rot. If the fungus was present, then the land would have to be planted with monocotyledonous plants such as forage grasses, wheat or oats that have field resistance to the disease. After years of looking for a way to deal with cotton root rot, researchers had, in essence, given up.