ABSTRACT
Wilt incited by Verticillium dahliae has recently become an important disease on cauliflower in the Salinas Valley. Although broccoli is closely related to cauliflower, wilt has not occurred on this host and V. dahliae isolates from cauliflower were weakly pathogenic on broccoli in greenhouse inoculations. In this study, the effectiveness of broccoli residues on propagule attrition in soil and wilt incidence on cauliflower was determined in a commercial field infested with V. dahliae microsclerotia. The treatments were broccoli residue with tarp, broccoli residue without tarp, chloropicrin, metham sodium, control with tarp, control without tarp, cauliflower followed by cauliflower, and cauliflower followed by fallow. Approximately 200 kg of chopped broccoli was uniformly disk incorporated into the corresponding plots (36 m2). Densities of V. dahliae microsclerotia were determined at 0, 30, 90, and 145 days after treatment in 1993 and 0, 30, 74, 109, and 140 days after treatment in 1994 using the modified Anderson sampler technique. Cauliflower cultivar White Rock was planted in all plots. Plant height, number of marketable heads, head weight, and wilt severity were determined at maturity. Even though the pre-treatment number of V. dahliae propagules in broccoli-treated and control plots were similar, by the end of the cauliflower cropping season numbers either were the same or increased in control plots compared with a reduction in numbers in broccoli residue-treated plots during both seasons. There were no differences between tarped and non-tarped plots either in broccoli residue-treated or control plots. In fumigated plots, propagules declined initially but later returned to pre-treatment levels by the end of the cropping season. Continuous cauliflower or fallowing after one cauliflower crop resulted in stable or increased levels of microsclerotia. Verticillium wilt severity was lowest in metham sodium-treated plots, followed by broccoli residue-treated plots and chloropicrin in both years. Plant height, marketable heads, and head weight were higher in broccoli-treated than in control plots. These results suggest that broccoli residues reduce V. dahliae microsclerotia in soil and wilt of cauliflower as much as or more than chloropicrin and metham sodium, and that rotation with broccoli may be a feasible approach to manage Verticillium wilt in cauliflower and other susceptible crops.
