Efficacy of strobilurin and triazole fungicides for the management of powdery mildew (Sphaerotheca fuliginea) in summer squash

Summer squash (Cucurbita pepo L.) is cultivated extensively in temperate and subtropical regions. Among the various challenges faced by this crop, powdery mildew stands out as one of the most damaging diseases affecting cucurbits, including summer squash. This disease inflicts significant damage on crops grown in both open-field conditions and protected environments. In the present study, we evaluated various systemic and non-systemic fungicides for the management of powdery mildew in summer squash, both under in vitro and field conditions. In our search for new alternatives to carbendazim, difenoconazole, at a concentration of 100 ppm, resulted in only 1.75% conidial germination. This was followed by hexaconazole, which achieved a germination rate of 2.53%, compared to a control group exhibiting 48.32% germination. Hexaconazole also yielded a germ tube length of 2.16 μm, while difenoconazole alone produced 2.91 μm, and the combination of difenoconazole + azoxystrobin resulted in 3.44 μm germ tube length. Under field conditions over two consecutive years, four applications of azoxystrobin at 0.1% demonstrated the highest efficacy, resulting in a disease index of 14.33% and an 81.71% reduction in disease severity compared to the untreated control, which had a disease index of 78.33%. Hexaconazole (0.05%) and difenoconazole (0.03%) were also effective, reducing the disease index to 19.67% and 22.33%, respectively, corresponding to reductions in disease severity of 74.89% and 71.49%. Notably, these fungicides significantly increased crop yield under field conditions over the two-year study period.