Induced resistance in plants against insects and diseases IOBC-WPRS Bulletin Vol. 88, 2013 pp. 45-49 45 Diversity in susceptibility of Botrytis cinerea to biocontrol products inducing plant defence mechanisms Marc Bardin, Morgane Comby, Ruben Lenaerts and Philippe C. Nicot INRA, UR407 Pathologie végétale, Domaine Saint Maurice, BP94, 84143 Montfavet Cedex, France Abstract: The development of plant defence stimulants to increase host resistance represents an attractive alternative to fungicides for the protection of crops against plant pathogens. In this study we evaluated the efficiency of 14 products presumed to induce plant defence mechanisms against Botrytis cinerea on tomato and lettuce. Two days after the application of the products, tomato and lettuce leaves were inoculated with B. cinerea and incubated in conditions conducive to disease development. Out of 14 products tested, Serenade Max ® (Bacillus subtilis QST713) proved to have a significant protective efficacy against B. cinerea on both plants. To assess the presence of low susceptibility to Serenade Max in populations of B. cinerea, the protective efficacy of this product was evaluated against 20 strains differing in their geographic origin, host of isolation and level of aggressiveness. To this end, tomato and lettuce leaves were treated with Serenade Max two days before inoculation. The efficiency of the product was significantly influenced by the isolate of B. cinerea tested with protection levels ranging from 40% to 86% on tomato leaves and 0 to 80% on lettuce leaves. No correlation was observed between the level of aggressiveness of B. cinerea strains and the protection provided by the biocontrol agent. Key words: biological control, induced resistance, durability, Botrytis cinerea Introduction The development of plant defence stimulants to increase host resistance represents an attractive alternative to fungicides for the protection of crops against plant pathogens. Various biotic and abiotic agents have been shown to induce defence mechanisms in different plant species against various plant pathogens and particularly against Botrytis cinerea (Elad and Stewart, 2004; Elmer and Reglinski, 2006). Although biocontrol preparations presumed to induce plant defence mechanisms against plant disease are now marketed, little is known on the durability of efficacy of these defence-stimulating biocontrol agents. To our knowledge, the diversity of the efficacy of resistance-inducing products against plant pathogen populations has not been studied. However, as for fungicides, knowledge on the baseline sensitivity of plant pathogens to resistance-inducing compounds appears to be necessary to determine the risk of possible adaptation of pathogen populations in response to selection pressure. The purpose of the present study was (1) to evaluate the efficiency of various biocontrol preparations (microorganisms, plant extracts and organic products) presumed to induce plant defence mechanisms against B. cinerea on tomato and lettuce and (2) to estimate the diversity in susceptibility of B. cinerea to one of these biocontrol preparations (Serenade Max ® ) in order to detect any differences in sensitivity between isolates that might lead to development of resistance.