EFFECTS OF CROP-ROTATION AND IRRIGATION ON VERTICILLIUM-DAHLIAE MICROSCLEROTIA IN SOIL AND WILT IN CAULIFLOWER

Experiments were conducted in field plots to evaluate the effects of b roccoli residue on population dynamics of Verticillium dahliae in soil and on Verticillium wilt development on cauliflower under furrow and subsurface-drip irrigation and three irrigation regimes in 1994 and 19 95. Treatments were a factorial combination of three main plots (brocc oli crop grown, harvested, and residue incorporated in V. dahliae-infe sted plots; no broccoli crop or residue in infested plots;and fumigate d control plots), two subplots (furrow and subsurface-drip irrigation) , and three sub-subplots (deficit, moderate, and excessive irrigation regimes) arranged in a split-split-plot design with three replications . Soil samples collected at various times were assayed for V. dahliae propagules using the modified Anderson sampler technique. incidence an d severity of Verticillium wilt on cauliflower were assessed at 7- to 10-day intervals beginning a month after cauliflower transplanting and continuing until harvest. Number of propagules in all broccoli plots declined significantly (P < 0.05) after residue incorporation and cont inued to decline throughout the cauliflower season. The overall reduct ion in the number of propagules after two broccoli crops was approxima tely 94%, in contrast to the fivefold increase in the number of propag ules in infested main plots without broccoli after two cauliflower cro ps. Disease incidence and severity were both reduced approximately 50% (P < 0.05) in broccoli treatments compared with no broccoli treatment s. Differences between furrow and subsurface-drip irrigation were not significant, but incidence and severity were significantly (P < 0.05) lower in the deficit irrigation regime compared with the other two reg imes. Abundance of microsclerotia of V. dahliae on cauliflower roots a bout 8 weeks after cauliflower harvest was significantly (P < 0.05) lo wer in treatments with broccoli compared with treatments without brocc oli. Rotating broccoli with cauliflower and incorporating broccoli res idues into the soils is a novel means of managing Verticillium wilt on cauliflower and perhaps on other susceptible crops. This practice wou ld be successful regardless of the irrigation methods or regimes follo wed on the susceptible crops.