Since 1990, commercial cauliflower in coastal California has been severely affected by a vascular wilt disease. Symptoms consist of chlorosis, defoliation, stunting, wilting, and vascular discoloration. Disease has been widespread and has caused significant damage in summer and fall crops. Verticillium dahliae was consistently isolated from xylem tissue in stems and roots of affected plants. Techniques tested for inoculation of cauliflower plants were dipping clipped or nonclipped roots into spore suspensions, injecting spore suspensions into cauliflower stems, and planting seedlings into soil along with an agar block colonized with microsclerotia. Only dipping roots into spore suspensions was consistently successful in causing Verticillium wilt. Pathogenicity was established by dipping roots of 30-day-old seedlings of cauliflower cv. White Rock into conidial suspensions (10(7) conidia per mililiter) 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 greenhouse (23 +/- 1/10 +/- 1 C day/night regime). After 4 wk, inoculated plants were stunted and chlorotic and V. dahliae was reisolated, whereas control plants were symptomless and V. dahliae was not reisolated. When incubation temperature maxima in the greenhouse exceeded 30 C, inoculated plants failed to show symptoms. Soil from commercial fields was assayed for microsclerotia on NP-10 selective medium using the modified Anderson sampler. V. dahliae was widely distributed in the Salinas Valley, with propagule densities as high as 93 microsclerotia per gram of soil. Evaluation of cauliflower cultivars in V. dahliae-infested fields indicated that all were susceptible. This new disease has become a major threat to cauliflower production in coastal California.
