Improvement of the efficacy of Verticillium lecanii used in biocontrol of Sphaerotheca fuliginea by addition of oil formulations

Biocontrol of cucumber powdery mildew, Sphaerotheca fuliginea, by Verticillium lecanii is seriously hampered at low humidities. The effect is especially marked at low humidity (60% RH) during the three hours following the application of V. lecanii spores suspended in water. Formulations of V. lecanii spores in oil might improve the situation. Arachid oil (peanut oil) and two invert emulsions using either Sunspray 6N or paraffin oil were tested in formulations of V. lecanii spores. Arachid oil gave the best development of V. lecanii on mildewed cucumber leaves. V. lecanii formulated with arachid oil showed significantly better control of mildew than without. A concentration of 0.5% arachid oil was somewhat toxic to mildew but 0.05% was not. Arachid oil did not show toxicity to V. lecanii. The humidity requirements of V. lecanii formulated with and without 0.05% arachid oil were compared at 95, 90 and 85% RH. Arachid oil significantly reduced the humidity dependence of V. lecanii. Since arachid oil is safe for human consumption and not phytotoxic to cucumber leaves, low concentrations of arachid oil are recommended as an additive to increase the effectiveness of V. lecanii as a biocontrol agent of S. fuliginea.