Interactive effects of broccoli residue and temperature on Verticillium dahliae microsclerotia in soil and on wilt in cauliflower

The effects of dry, fresh, and no broccoli amendments on Verticillium dahliae microsclerotia in soil were evaluated at 10, 15, 20, 25, 30, and 35 degrees C. Aliquots of 25 g of field soil naturally infested with V. dahliae microsclerotia (80 to 100 per gram of soil) were placed in plastic bottles to which dry (1% wt/wt), fresh (8.7% wt/wt), or no broccoli treatments were randomly assigned. One randomly chosen set of each treatment was assigned to the above temperatures and incubated for 45 days. The soil was subsequently assayed for microsclerotia using the Anderson sampler technique. The experiment was repeated nine times. At all temperatures, both dry and fresh broccoli significantly (P < 0.05) reduced the number of microsclerotia in soil compared with the unamended soil. At less than or equal to 30 degrees C fresh broccoli had a significantly (P < 0.05) greater remissive effect than dry broccoli. At 35 degrees C, however, the number of V. dahliae microsclerotia were significantly (P < 0.05) reduced after 45 days in unamended soil; both fresh and dry broccoli almost completely eliminated the pathogen from soil. Temporal dynamics of the V. dahliae microsclerotia as affected by the treatments at the above temperatures were determined in nine experiments identical to the above by sampling and assaying soil three times at 15-day intervals. Regardless of the temperature, maximum reductions in the number of microsclerotia in treatments involving broccoli occurred within 15 days, with fresh broccoli providing significantly (P < 0.05) greater reductions. Further reductions in the number of microsclerotia were not significant in broccoli treatments at the subsequent sampling dates. In unamended soil, the number of microsclerotia at all temperatures except 35 degrees C changed little throughout the experiment. The optimum temperatures for the broccoli-mediated reductions in V. dahliae microsclerotia in both experiments were 25 and 30 degrees C. Four greenhouse experiments were conducted to test the effectiveness of broccoli treatments in reducing wilt incidence in cauliflower. Consistently, cauliflower plants in the fresh broccoli treatment were taller, had greater root and shoot weights, were significantly more robust, and had the least number of infected plants (number of microsclerotia was lower) than in other treatments. The number of infected plants in the dry broccoli treatment was intermediate, similar to the number of microsclerotia. For maxima reductions in soilborne V. dahliae microsclerotia and the subsequent lower wilt incidence in cauliflower, the broccoli residue incorporation should occur when the temperatures are at least 20 degrees C.