Biological Control of Fungal and Bacterial Plant Pathogens IOBC-WPRS Bulletin Vol. 86, 2013 pp. 163-168 163 Relationship between the aggressiveness of Botrytis cinerea on tomato and the efficacy of biocontrol Marc Bardin, Morgane Comby, Claire Troulet, Philippe C. Nicot INRA, UR407 Pathologie végétale, Domaine Saint Maurice, BP94, 84143 Montfavet Cedex, France e-mail: marc.bardin@avignon.inra.fr Abstract: The development of BCAs represents an attractive alternative to fungicides for the protection of crops against plant pathogens but the durability of this method has not been studied in details. The objective of the present work was to estimate the risk of loss of biocontrol efficacy towards Botrytis cinerea, by evaluating the sensibility of various isolates of the pathogen to the biocontrol agent Microdochium dimerum. The protective efficacy of M. dimerum was evaluated on tomato plants against 41 strains of B. cinerea differing in their geographic origin and host of isolation. To this end, whole tomato pruning wounds and detached stem sections were concomitantly inoculated with B. cinerea and with M. dimerum. Lesion expansion was recorded daily from the 3 rd to the 7 th day after inoculation. Due to the very high level of efficacy against all tested strains of B. cinerea when M. dimerum was used at the recommended dose, it was necessary to reduce the dose of application 10- fold to assess the diversity of sensitivity of B. cinerea to this BCA. In these conditions, a wide range of sensitivities were observed among strains of the pathogen tested with protection levels ranging from 0 to 100% (mean = 53 ± 4%; median = 49%). A correlation was observed between the level of aggressiveness of a strain to tomato and its sensitivity to the biocontrol agent (assessed by the protection level). It reveals the importance of considering several strains of the pathogen when screening for biocontrol agents, to obtain a good representation of the pathogen population and thus take into account the potential durability of biocontrol. Key words: Botrytis cinerea, biological control, durability, sensitivity, Microdochium dimerum Introduction The development of biocontrol agents (BCAs) represents an attractive alternative to fungicides for the protection of crops against plant pathogens. Various microbial agents have shown a significant efficacy in different plant species against various plant pathogens and particularly against Botrytis cinerea (Nicot et al., 2011). Although biological control against plant pathogens has been widely studied in the past years leading to the marketing of several BCAs, little is known on the durability of efficacy of this control method. Even though knowledge on the baseline sensitivity of plant pathogens to BCAs appears to be necessary to determine the risk of possible adaptation of pathogen populations in response to selection pressure exerted by the BCAs in the field. To our knowledge, the diversity of the efficacy of biocontrol agents against plant pathogens was mainly studied for BCAs that produce metabolites with a direct effect on pathogens (Ajouz et al., 2011; Buck & Jeffers, 2004; Mazzola et al., 1995; Schouten et al., 2004). Not much is known about BCAs which do not produce toxins or antagonistic compounds. The purpose of the present study was (i) to estimate the diversity in susceptibility of B. cinerea to the BCA Microdochium dimerum in order to detect any differences in sensitivity between isolates that might lead to development of resistance and (ii) to evaluate the