Economic Impact and Management of Verticillium Wilt on Irrigated Alfalfa Hay Production in Wyoming. M. S. Page, Research Assistant, Department of Plant, Soil and Insect Sciences, University of Wyoming, Laramie 82071-3354. F. A. Gray, D. E. Legg, and W. G. Kearl. Professor, Assistant Professor, Department of Plant, Soil and Insect Sciences, and Professor, Department of Agricultural Economics, University of Wyoming, Laramie 82071-3354. Plant Dis. 76:504-508. Accepted for publication 30 December 1991. Copyright 1992 The American Phytopathological Society. DOI: 10.1094/PD-76-0504.

Loss in irrigated alfalfa (Medicago sativa) attributable to Verticillium wilt (caused by Verticillium albo-atrum) was determined by comparing yields of resistant and susceptible cultivars grown in the presence of Verticillium wilt. This loss, in combination with the estimated number of hectares infested with V. albo-atrum and planted to susceptible cultivars, was used to calculate the economic impact of Verticillium wilt on alfalfa hay production. Test plots were established on 13 May 1982 near Dayton, WY. Verticillium wilt was first detected in the test plots before the third harvest in 1984. Resistant cultivars had less disease, higher yield, and higher plant stands, beginning in 1984, than susceptible cultivars. Average annual yield loss (average difference between the two cultivar groups from 1984 to 1987) attributed to Verticillium wilt was 0.88 Mg/ha. Verticillium wilt was found in nine of the 13 Wyoming counties surveyed. Goshen County had the highest incidence of infested fields (65%). The disease was not found in Fremont County, which has the most hectares of irrigated alfalfa in the state. Approximately 32,877 ha of alfalfa hay grown under irrigation in Wyoming were infested with V. albo-atrum. Of the total infested hectares, an estimated 85% (27,946 ha) was planted to susceptible cultivars, of which Ranger was the most frequently planted. With an average hay price of $70.46 per megagram, the annual loss in Wyoming hay production was estimated to be $1,732,786. Regression analyses were used to develop equations for predicting the year when alfalfa fields, grown in areas infested with Verticillium wilt, would fall below an acceptable yield (yield threshold) or plant stand (plant stand threshold) level. The use of these equations provides the basis for making economically sound decisions for managing alfalfa.