Susceptibility of Weedy Hosts from Pacific Northwest Potato Production Systems to Crop-Aggressive Isolates of Verticillium dahliae.

Verticillium wilt, caused by Verticillium dahliae, is a disease of dicotyledonous crops such as potato and has a wide host range and persistent, long-term survival structures called microsclerotia that can persist in soil for up to 14 years. Some V. dahliae isolates are particularly aggressive on a specific plant host while retaining the ability to infect a wide range of other hosts. Weeds can serve as hosts for V. dahliae but whether they serve as sources of inoculum for aggressive isolates of V. dahliae to crop hosts is unknown. The goal of this research was to quantify V. dahliae microsclerotia obtained from 16 weeds which were grown in the greenhouse. Potting medium was infested with one of eight V. dahliae isolates from potato, mint, sugar beet, sunflower, tomato, and watermelon. The isolates from mint and potato were aggressive on the host from which they were originally isolated. All 16 weeds were infected by at least one V. dahliae isolate, although the number of microsclerotia produced from some infections was relatively low (≤5 microsclerotia/g of dry plant). Black nightshade yielded greater numbers of microsclerotia of the V. dahliae potato isolate than any other isolate in three of four trials in the greenhouse (second trial false discovery rate, adjusted P ≤ 0.0158; third trial, P ≤ 0.0264; and fourth trial, P ≤ 0.0193). Litchi tomato yielded greater numbers of microsclerotia of the V. dahliae potato isolate than any other isolate in one of four trials (first trial, P ≤ 0.0149). A V. dahliae isolate from tomato yielded greater numbers of microsclerotia in large crabgrass and wild oat in a second trial (P ≤ 0.0158). Weeds, depending on the species, grown during and between potato crop rotations may increase the number of microsclerotia of the potato-aggressive isolates of V. dahliae.