Grapevine downy mildew development as affected by chitosan spray treatments and metabolomics implications

Chitosan has been shown to enhance grapevine tolerance toward downy mildew while reducing the environmental impact of traditional protection products. The in vitro study considered potted vines of Vitis vinifera L. cv. Merlot grafted on SO4 and K5BB rootstocks. The aim was to assess, at different phenological stages: (1) the role of chitosan chlorydrate denso treatments on the occurrence of downy mildew infection; (2) the change of the plant metabolites elicited by canopy applications of chitosan chlorydrate and subsequent downy mildew inoculation on leaf discs and berries, according to the metabolomics approach; and (3) the role of the different rootstocks on the fungal infection and the metabolomic profile. Four treatments were evaluated, as follows: a) chitosan application followed by downy mildew inoculation (CH-I); b) chitosan application without inoculation (CH-NI); c) untreated control with inoculation (UC-I); d) untreated control without inoculation (UC-NI). Leaves were sampled at the phenological stages of shoots 30 cm long (BBCH 57), fruit set (BBCH 71), and the onset of berry touch (BBCH 77), while berries were sampled at fruit set (BBCH 71).The results demonstrated that chitosan canopy treatments significantly reduced downy mildew infections on both leaves and berries. Metabolomic analysis revealed that chitosan elicited responses in secondary metabolism, with variations depending on phenological stages and rootstock combinations. The induced secondary metabolites were mainly associated with polyphenol, nitrogenous compound, and terpene pathways. These changes suggest that chitosan triggers defense-related metabolic pathways against downy mildew. Additionally, the elicitation of aromatic and antioxidant pathways in berries indicates a potential role in enhancing grape composition. Future studies should be conducted in vineyard conditions, including sensory analysis of both grapes and wines.