Factors affecting the Trichoderma harzianum-induced resistance against downy mildew of grapevine and interaction between Plasmopara viticola genotypes co-inoculated in the host

Grapevine (Vitis vinifera L.) is one of the major fruit crops worldwide and varieties used for table grape or wine production are susceptible to several pathogens. Downy mildew caused by the oomycete Plasmopara viticola is an important grapevine disease that threatens leaves and young berries and, to avoid yield losses, control of the disease is based on the application of chemical fungicides. Genetic analysis of pathogen’s population structure in field indicated that germination of oospores, causing primary infections occurs for a long period, alongside secondary infection cycles, and few P. viticola genotypes are dominating during an epidemic. Alternative methods for controlling downy mildew have been studied, including the use of microbial biocontrol agents. Application of Trichoderma harzianum T39 (T39) has been shown to reduce downy mildew symptoms in grapevine by activating the plant mediated resistance mechanism. Induced resistance offers the prospect of broad-spectrum disease control using the plant’s own defenses and represents a promising low-impact tool for controlling crop diseases. However, the potential of induced resistance has yet to be fully realized, mainly due to its only partial control of the disease and its inconsistency under field conditions. Induced resistance is a plant-mediated mechanism, and its expression under field conditions is likely to be influenced by a number of factors, including environment, genotype and crop nutrition. Concerns about the impact of abiotic stresses on agriculture have been raised in the last decade, especially in light of the predicted effects of climate changes. High temperatures and drought associated with heat waves may occur with increased frequency as a result of climate change, threatening crop production and influencing interactions with both pathogenic and beneficial microorganisms. Aim of this project was to study the downy mildew disease form the pathogen and from the host plant point of views, in order to maximize the control of the disease with low-impact natural methods. To evaluate the efficacy of the T39-induced resistance under non-optimal conditions and to study pathogen’s infection dynamics of different P. viticola isolates we evaluated i) the T39-induced resistance in plants exposed to heat and drought stresses, ii) the T39-induced resistance in different grapevine cultivars and iii) the possible selection mechanisms of different P. viticola isolates. The work was structured in three distinct sections where, at first, we could assess that co-inoculated P. viticola isolates competed for the infection of the host, although being equally infective when singularly inoculated. Competition was not related to the origin of the isolate and we hypothesized that competitive selection was modulated by differences in the 2 secretion of effector molecules, which explained the establishment of dominant genotypes over an epidemic season. In other two sections we demonstrated that T39-induced resistance was found to be reduced in plants exposed to the combination of heat and drought stresses, moreover, variable levels of efficacy were observed in different grapevine cultivars. Modulation of the marker genes in the T39-induced resistance was partially attenuated in plants under heat and drought stress. The molecular mechanisms activated in response to the resistance inducer were different and complex among cultivars, indicating that specific receptors are probably involved in the regulation of the plant response. The work presented in this thesis provides a deeper understanding to current knowledge of the biology of this grapevine pathogen and of the mechanisms of the induced resistance. Different P. viticola genotypes reacted differently when co-existing on a unique substrate, indicating that plant-pathogen interactions seem to be more complex than mere colonization of the plant tissue. The environmental conditions and the plant genotype are key factors affecting the T39-induced resistance. Therefore, prevention of predominant genotypes during an epidemic season and consideration of the variable responsiveness of the plant to the resistance inducer applied are important strategies for the improvement of biocontrol methods against downy mildew of grapevine