Trichoderma spp. volatile organic compounds protect grapevine plants by activating defence-related processes against downy mildew

Volatile organic compounds (VOCs) are produced by soil-borne microorganisms and play crucial roles in fungal interactions with plants and phytopathogens. Although VOCs have been characterized in Trichoderma spp., the mechanisms against phytopathogens strongly differ according to the strain and pathosystem. This study aimed at characterizing VOCs produced by three Trichoderma strains used as biofungicides and to investigate their effects against grapevine downy mildew (caused by Plasmopara viticola). A VOC-mediated reduction of downy mildew severity was found in leaf disks treated with Trichoderma asperellum T34 (T34), T. harzianum T39 (T39) and T. atroviride SC1 (SC1) and 31 compounds were detected by head space-solid phase microextraction gas chromatography-mass spectrometry. Among the Trichoderma VOCs annotated, -farnesene, cadinene, 1,3-octadiene, 2-pentylfuran and 6-pentyl-2H-pyran-2-one reduced downy mildew severity on grapevine leaf disks. In particular, 6-pentyl-2H-pyran-2-one and 2-pentylfuran increased the accumulation of callose and enhanced the modulation of defence-related genes after P. viticola inoculation, indicating an induction of grapevine defence mechanisms. Moreover, 6-pentyl-2H-pyran-2-one activated the hypersensitive response after P. viticola inoculation, possibly to reinforce the grapevine defence reaction. These results indicate that Trichoderma VOCs can induce grapevine resistance, and these molecules could be further applied to control grapevine downy mildew.