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Grey mould caused by the fungus Botrytis cinerea is an economically important disease in numerous crops. Biocontrol is a promising method to control the disease. Species of Streptomyces are potential biological control agents since they are ubiquitous in the environment and many of them produce various secondary metabolites with diverse biological activities including the ability to inhibit this plant pathogenic fungus. Strains RM-1-138 and RL-1-178 of S. philanthi and SS-2-243 of S. mycarofaciens, isolated from the rhizosphere soil of chili peppers grown in southern Thailand, have shown a good efficacy to control Sclerotium roljsii, Ralstonia solanacearum and Rhizoctonia solani but their effect against B. cinerea is not known. In this study we evaluated the efficacy of the three strains of Streptomyces spp. against B. cinerea in vitro and on tomato plants. Results indicated that the three strains inhibit the growth of B. cinerea in Petri plates and have a significant protective efficacy, although variable between strains of Streptomyces spp., against B. cinerea on tomato plant. To assess the possible variability in susceptibility to these antagonistic strains in populations of B. cinerea, the protective efficacy of these bacteria were evaluated against 41 strains differing in their geographic origin, host of isolation and level of aggressiveness. Results based on confrontation tests in Petri plates suggest a diversity in the sensitivity of the different strains of B. cinerea to these biological control agents. The relevance of these results to ensure durability of the efficacy of these biological control agents will be discussed.

Various biological methods, mainly based on the utilization of antagonistic microorganisms or plant extracts, have been studied to control powdery mildews. The hyperparasite fungus Ampelomyces quisqualis (AQ-10, De Sangosse) is registered in many countries to control powdery mildew on various crops, including Oidium neolycopersici on tomato in France. A plant extract from orange (Prev-Am, Vivagro) is registered to control powdery mildew and whitefly on various crops but its effect on tomato powdery mildew is not clearly established. Spray of soluble carbohydrates on leaves of various plant species proved to have an effect against sorne plant pests but their effect against plant pathogens is not well known, except for trehalose against powdery mildew of wheat. The objectives of this study were (i) to test the effect of spraying low concentration of sugars (glucose, fructose, sucrose, trehalose) on leaves of tomato against O. neolycopersici in controlled conditions and (2) to evaluate the effect of sucrose, AQ-10 and Prev-Am separately and in combinations in semi­ commercial tunnel conditions. Results revealed that spray of sugars on tomato leaves have no significant effect against powdery mildew in controlled conditions. In tunnels, the plant extract Prev-am showed a significant effect against Oidium neolycopersici on tomato. Combination of products did not enhance the efficacy of a single product. The relevance of these results to ensure the control of powdery mildew on tomato without fungicides will be discussed.

The preventive use of resistance inducers has been shown to be an interesting method to reduce dependency on pesticides for plant protection. However, little is known on possible differences in the protective effects of such methods for different varieties of a given crop. In the present study, we assessed the effect of three compounds (acibenzolar-S-méthyl, a calcium-based mineral compound and a yeast extract) for the protection of six varieties of lettuce against two major pathogens, Sclerotinia sclerotiorum and Botrytis cinerea. The compounds were sprayed on the plants three days before inoculation. A water spray was used as a control. The protective effect of the compounds was then assessed by comparing the size of lesions developing on inoculated leaves. For both pathogens, none of the compounds fully inhibited disease development. However, reduction in lesion size was observed on some of the leaves. The effect of the three compounds was different for the two pathogens. For tests with B. cinerea, effects of plant treatment were not statistically significant. In contrast, significant effects were found for five of the varieties inoculated with S. sclerotiorum. Overall, the yeast extract provided the highest level of protection against that pathogen. However, for all compounds, the extent of the protective effect depended on the variety. Furthermore, in some cases the effect of the compound was opposite to that desired and disease was more severe on treated plants than on the water control. Possible consequences for field application of such methods will be discussed.