Foliar Application of a Tagatose-Based Product Reduces Downy Mildew Symptoms through Induction of Grapevine Resistance and Anti-Oomycete Action

Downy mildew caused by the oomycete Plasmopara viticola represents one of the most devastating diseases in vineyards. Current ways to control this disease rely mainly on fungicide applications, but agro-ecological concerns have raised interest in sustainable alternative methods. Certain rare sugars, like D-tagatose, have shown efficacy in reducing various plant diseases, including grapevine downy mildew. However, the mechanism of action of D-tagatose against grapevine downy mildew is not understood. The aim of this study was to characterize the efficacy and mechanism of action of a D-tagatose-based formulated product (IFP48) against grapevine downy mildew and compare it with the correspondent active molecule, pure D-tagatose (TAG). Whereas IFP48 root treatment provided scarce protection, the leaf treatment was the most efficient, especially at the dosage of 5 g/L. In particular, IFP48 treatment directly inhibited P. viticola sporangia germination, upregulated the expression of defense-related genes, and increased the content of stilbene phytoalexins. Conversely, the expression of defense-related genes and the content of stilbene phytoalexins were only slightly affected by TAG, suggesting that the formulation possibly improved D-tagatose effects against downy mildew in grapevine.