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BioControl 44: 73–87, 1999. © 1999 Kluwer Academic Publishers. Printed in the Netherlands.

Improvement of the efficacy of Verticillium lecanii used in biocontrol of Sphaerotheca fuliginea by addition of oil formulations
M.A. VERHAAR∗, T. HIJWEGEN and J.C. ZADOKS
Laboratory of Phytopathology, Wageningen Agricultural University, P.O. Box 8025, 6700 EE Wageningen, The Netherlands; ∗ author forcorrespondence (e-mail: verhaar.m@cr-delta.nl) Received 14 December 1998; accepted in revised form 3 February 1999

Abstract. 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 inwater. 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. Aconcentration 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 cucumberleaves, low concentrations of arachid oil are recommended as an additive to increase the effectiveness of V. lecanii as a biocontrol agent of S. fuliginea. Key words: arachid oil, biocontrol, invert emulsions, vegetable oils

Introduction Verticillium lecanii has shown to be a potential biocontrol agent of cucumber powdery mildew (Sphaerotheca fuliginea (Schlecht.:Fr.) Poll.) (Hijwegen, 1988;Verhaar et al., 1996, 1997; Askary et al., 1997, 1998). High humidity conditions seemed to be of great importance to obtain good control. The humidity conditions at the plant surface are probably the most important factor influencing the germination, growth and survival of V. lecanii in this habitat (Verhaar et al., 1998). Possibly, some of the variability of biocontrol results achieved with V. lecaniican be ascribed to this factor. When V. lecanii spores are sprayed onto the leaves in a water suspension, the evaporation of the water carrier may be too fast for germination in free water so that V. lecanii

74 becomes dependent on the humidity of the air. Because the humidity conditions in Dutch glasshouses are not optimal for biocontrol by mycoparasites, the humidity requirement of V.lecanii is still a bottleneck for commercial biocontrol with this organism. Oil emulsions reduce dew dependence of mycoherbicides (Connick et al., 1991; Auld, 1993). Formulations with oils could reduce the humidity requirements of V. lecanii too. Hora Oleo 11E and paraffine oil were already used as formulations in biocontrol experiments with mycoparasites on cucumber powdery mildew (Philipp andHellstern, 1986; Philipp et al., 1990; Hijwegen, 1992; Verhaar et al., 1996) but did not give satisfactory results. For biocontrol of S. fuliginea formulations are needed that are not phytotoxic to cucumber leaves and provide moisture, possibly also nutrients, for the mycoparasites to germinate and infect the mildew. Formulations of biocontrol agents can be liquid, powdery or granular. For biocontrol ofpowdery mildew liquid oil formulations are the most interesting. Because pure oils are toxic to temperate-zone plants by interfering with respiration and transpiration, oils must be applied at controlled dosage levels. This can be achieved by emulsification. Two methods are feasible. In one the water is the external phase and oil is the internal phase (oil in water). A range of vegetable oils have...