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A case study of misapplication of the fungicide flutriafol to grapevines in Texas

David Appel: Department of Plant Pathology and Microbiology, Texas A&M University

<div>A major limiting factor in winegrape production in Texas is cotton root rot, caused <em>by Phymatotrichopsis omnivora</em>. Chemigation with Topguard Terra<sup>®</sup> (flutriafol) is used to control cotton root rot under a 24 (c) Special Local Need label. According to the label, the maximum annual rate of application is 15.3 fl. oz./ac., either in one spring application or in split spring and post-harvest fall applications. In September 2016, a grower using flutriafol for disease suppression reported unusual symptoms in the grape varieties muscat, mourverdre and chardonnay. The symptoms included severe interveinal chlorosis and necrosis, marginal necrosis, and leaf curl on the older leaves. The new growth that developed subsequent to the misapplication was asymptomatic. The grower reported applying the full rate of flutriafol in July through a subsoil irrigation system that delivered product directly to the root zone of the grapevine, rather than the standard drip irrigation. Plant and soil samples were collected for flutriafol analyses and sent to a commercial testing lab. Levels of flutriafol in the symptomatic foliage were 40-112 ppm. The new growth had levels of 10 ppm. Corresponding soil levels of flutriafol ranged from 0.05-0.46 ppm. The maximum levels were greater than 5 times those detected in soils treated using recommended methods in previously tested vineyards. This episode was an example of chemical damage due to unintended misapplication and serves as a warning to the dangers of deviating from label recommendations for control of cotton root rot with flutriafol.</div>