Functional and molecular analysis of grapevine induced resistance against Plasmopara viticola in relation to the physiology of the microbial inducer

Trichoderma harzianum T39 is a commercial fungal strain active against grapevine downy mildew (Plasmopara viticola). Its biocontrol activity relies on induction of the plant defences. However, as many biocontrol agents, its use can be hampered by its variable efficacy, even for a similar plant/pathogen system. This important drawback can be a function of the conditions of application, as environmental temperature and moisture, or the formulation of the product. Another determinant of the instable efficacy could be the physiology of the microorganisms. We investigated the efficacy of T39 related to the physiological stage of conidia, in regard to all the partners of the tripartite interaction T. harzianum – P. viticola – Vitis vinifera. Greenhouse trials were conducted with T39 conidia of various ages and obtained from mycelia grown on different media. We tested i) the antagonistic activity against P. viticola inoculum and ii) their local and systemic efficacies against the pathogen. Our study revealed a trend of higher biocontrol activity by younger conidia. These results would evidence a decrease of activity and variation of plant defences activation related to the age of conidia. This information could be considered for the optimization of the production, formulation and conservation of T. harzianum T39 product of biological control. Resistance induction was characterized from a molecular point of view, analyzing the modulation of disease-related genes in plants treated or not with the biocontrol agent, before and after P. viticola inoculation. Molecular events directly activated by the inducer or in response to the pathogen are discussed.