Irrigation Increases Verticillium Wilt Incidence in a Susceptible Alfalfa Cultivar

Verticillium wilt (VW) caused by Verticillium albo-atrum occurs in many irrigated alfalfa (Medicago sativa) fields in southwestern Saskatchewan, Canada, but is very rare in dryland stands. A trial was established in 1994 to determine if incidence of VW is affected by the amount of irrigation water applied. A line-source irrigation system was used to produce an irrigation water gradient on two adapted alfalfa cultivars, Beaver (VW-susceptible) and Barrier (VW-resistant). The irrigation gradient was subdivided into six irrigation level treatments within each cultivar plot. Irrigation plus precipitation from April to August averaged 265, 291, 309, 330, 351, and 353 mm for irrigation levels one to six, respectively, over 3 years. A 1-m-wide strip through the center of each subplot was inoculated with a spore suspension of V. alboatrum before the first harvest in 1995. Plant density (ground cover) and incidence of VW-diseased plants per subplot were recorded and forage yield was assessed twice per year. Beaver had more VW-infected plants in 1995 and 1996 than Barrier, and the number of diseased plants of Beaver increased with irrigation level. Ground cover of Beaver declined in 1997 and this response was correlated to VW-diseased plant counts from 1995 and 1996. There was no significant correlation between VW-diseased plant counts and ground cover for Barrier. Forage yield increased linearly with irrigation level for both cultivars in two harvests during 1995. However, yield of Beaver did not respond to irrigation during cut 2 in 1996. Beaver forage yield declined with irrigation level at cut 1 in 1997 because ground cover had declined by 40% at irrigation level six compared with 22% at irrigation level one. The ground cover of Beaver at irrigation level one was similar to that of Barrier at all irrigation levels. We conclude that VW will affect susceptible alfalfa cultivars on irrigated stands in proportion to the amount of irrigation water applied. Irrigated alfalfa producers should adopt VW-resistant cultivars even when irrigation is limited.