Mechanism of broccoli-mediated Verticillium wilt reduction in cauliflower

Broccoli is resistant to Verticillium dahliae infection and does not expres s wilt symptoms. Incorporation of broccoli residues reduces soil population s of V. dahliae. The effects of broccoli residue were tested on the coloniz ation of roots by V. dahliae, plant growth response, and disease incidence of both broccoli and cauliflower in soils with different levels of Ct dahli ae inoculum and with or without fresh broccoli residue amendments. The thre e soils included a low-Verticillium soil, a high-Verticillium soil, and a b roccoli-rotation soil (soil from a field after two broccoli crops) with an average of 13, 38, and below-detectable levels of micro-sclerotia per g of soil, respectively. Cauliflower planes in broccoli-amended high-Verticilliu m soil had significantly (P less than or equal to 0.05) lower wilt incidenc e and severity than did plants in unamended soil. An immunohistochemical st aining assay utilizing a monoclonal antibody specific to V. dahliae was use d to determine colonization of the root cortex. Despite the absence of wilt symptoms, broccoli roots were colonized by V. dahliae. In high-Verticilliu m soil, the broccoli residue amendment caused a marked reduction in coloniz ation rate of V. dahliae per unit of inoculum on both cauliflower and brocc oli roots. In addition to its detrimental effects on the viability of micro sclerotia in soil, broccoli residue may also have an inhibitory effect on t he root-colonizing potential of surviving microsclerotia.