VERTICILLIUM WILT OF CAULIFLOWER IN CALIFORNIA

Since 1990, commercial cauliflower in coastal California has been seve rely affected by a vascular wilt disease. Symptoms consist of chlorosi s, defoliation, stunting, wilting, and vascular discoloration. Disease has been widespread and has caused significant damage in summer and f all crops. Verticillium dahliae was consistently isolated from xylem t issue in stems and roots of affected plants. Techniques tested for ino culation of cauliflower plants were dipping clipped or nonclipped root s into spore suspensions, injecting spore suspensions into cauliflower stems, and planting seedlings into soil along with an agar block colo nized with microsclerotia. Only dipping roots into spore suspensions w as consistently successful in causing Verticillium wilt. Pathogenicity was established by dipping roots of 30-day-old seedlings of cauliflow er cv. White Rock into conidial suspensions (10(7) conidia per mililit er) for 5 min. Control plants were dipped into sterile distilled water . All plants were potted into autoclaved soil and incubated both in a growth chamber (20 +/- 1/15 +/- 1 C day/night regime) and in a greenho use (23 +/- 1/10 +/- 1 C day/night regime). After 4 wk, inoculated pla nts were stunted and chlorotic and V. dahliae was reisolated, whereas control plants were symptomless and V. dahliae was not reisolated. Whe n incubation temperature maxima in the greenhouse exceeded 30 C, inocu lated plants failed to show symptoms. Soil from commercial fields was assayed for microsclerotia on NP-10 selective medium using the modifie d Anderson sampler. V. dahliae was widely distributed in the Salinas V alley, with propagule densities as high as 93 microsclerotia per gram of soil. Evaluation of cauliflower cultivars in V. dahliae-infested fi elds indicated that all were susceptible. This new disease has become a major threat to cauliflower production in coastal California.