Centre de Recherche en Horticulture, Département de Phytologie, Université Laval, Québec, Canada, G1K 7P4

Background and objectives
Because of their ectotrophic growth, powdery mildew fungi would appear as easy targets to natural enemies present on the leaf surface. A few biological control agents have been reported to attack and/or antagonize vegetative and reproductive structures of members of the Erysiphales. Among these antagonists, Pseudozyma flocculosa, (syn. Sporothrix flocculosa), Ampelomyces quisqualis, Tilletiopsis spp. and, to a lesser extent, Verticillium lecanii have been the most documented [1]. In comparative studies, P. ;flocculosa achieved the highest rate of powdery mildew control over the other known biocontrol agents under both small and commercial scale experiments [2]. However, practical application of a biocontrol agent as a control strategy requires a thorough understanding of the mechanisms of action by which the agent interferes with the target pathogen fungi and of the environmental parameters influencing their activity. The objective of our research is to understand how the mode of action of a powdery mildew antagonist influences its overall control ability and specificity, and how the environment limits or favours its development.

Results and conclusions
Our observations indicate that P. ;flocculosa attacks powdery mildews through antibiosis unlike A. ;quisqualis or V. ;lecanii which are strictly or mostly hyperparasites. In direct confrontation with susceptible fungi, P. ;flocculosa will alter their cytoplasmic activity in less than 8 ;h, which would explain why P. ;flocculosa attacks powdery mildew colonies expediently. This alteration is modulated by a perturbation of the membrane integrity and the intrinsic membrane molecular composition of fungi dictates the specificty of P. ;flocculosa. The antibiotics produced by the antagonist are rapidly biodegraded in the environment, which prevents their build-up. Through repeated and extensive exposure to the antibiotics, we are trying to determine if powdery mildew fungi, the targeted pathogens, can develop a level of resistance against P. ;flocculosa. In terms of environmental conditions, relative humidity (RH) has been widely acknowledged as the most limiting factor for efficacy of biocontrol agents of powdery mildews. While conditions of RH as high as 90% are required for certain antagonists to be active against powdery mildews, P. ; flocculosa was able to maintain its activity when RH was as low as 70%. This lower humidity requirement coupled with a rapid antibiosis-mediated mode of action are perceived as distinct advantages for obtaining maximum levels of biological control of powdery mildews.

1. Belanger RR, Dik AJ, Menzies JG, 1998. Plant-Microbe Interactions and Biological Control, pp. 89-109.
2. Dik AJ, Verhaar MA, Belanger RR, 1998. European Journal of Plant Pathology, in press.