Powdery mildew, caused by Podosphaera xanthii (syn. Sphaerotheca fuliginea auct. p.p.), is a serious and frequently occurring disease of cucurbits worldwide. Efficacy of four microbial products (i.e., Actinovate AG, Companion, BU EXP 1216C, and BU EXP 1216S), which contain microbes as the active ingredient, were evaluated on summer squash and cantaloupe against powdery mildew when applied alone or in alternation with a half-rate of conventional fungicide under greenhouse and field conditions. In greenhouse experiments, the product BU EXP 1216S significantly (P < 0.05) reduced the disease severity by nearly 70% relative to the water control. The level of control achieved was not significantly different from that obtained with Procure 480SC (triflumizole), the half-rate of conventional fungicide treatment, in two of four greenhouse experiments. Compared with the untreated water control, BU EXP 1216C and BU EXP 1216S, when applied alternately with Procure 480SC, consistently promoted plant growth measured by plant height, stem caliper, total fresh weight, and chlorophyll content in the leaves. The degree of increase was 11.6 and 11.3% in plant height, 15.6 and 19.8% in stem caliper, 25 and 40.7% in chlorophyll content, and 164 and 250% in total fresh weight, respectively. Alternating applications of these products with Procure 480SC resulted in significantly less powdery mildew disease than in the water control. In the first field trial on summer squash, all products applied individually or in alternation with Procure 480SC significantly reduced the severity of powdery mildew at the early stage (60 days after planting [DAP]) of disease development. Moreover, these alternating treatments resulted in significantly better control than with Procure 480SC alone at the late assessment stage (88 DAP). The products in alternation with Procure 480SC had a level of disease reduction equivalent to Procure 480SC alone on cantaloupe and significantly reduced disease severity in comparison with the water control. Compared with applying the microbial products alone, alternating applications of these products with Procure 480SC significantly reduced disease severity on cantaloupe and improved the marketable fruit number and weight. The data from our studies suggest that these microbial products could be effectively incorporated into disease management programs. In particular, these microbial products could be integrated into the management of powdery mildew on summer squash and cantaloupe in Florida by alternating their application with low rates of conventional fungicides, potentially reducing the development of fungicide resistance in the pathogen population.
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