The good, the bad and the ugly: how volatile organic compounds (VOCs) produced by Trichoderma spp. protect grapevine plants against downy mildew

Trichoderma spp. are among the most widespread soil microorganisms and have been widely used as biological control agents against phytopathogens. Trichoderma spp. are known to produce volatile organic compounds (VOCs) that play crucial roles in the inter-kingdom communications and biological control mechanisms. Several studies highlighted the importance of Trichoderma spp. as an alternative strategy against grapevine pathogens, but no information is available on the possible biological control mechanisms mediated by Trichoderma VOCs against grapevine downy mildew (caused by Plasmopara viticola). The aim of this study was to characterize VOCs produced by three Trichoderma strains and to investigate their effects against grapevine downy mildew. VOCs produced by Trichoderma asperellum T34, T. harzianum T39, and T. atroviride SC1 reduced downy mildew symptoms on grapevine leaf disks. A head space–solid phase microextraction gas chromatography–mass spectrometry analysis revealed 31 VOCs in two independent experiments, including alkenes, ketones, pyrones, furanes and terpenes. Five reduced downy mildew symptoms 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 defense-related genes after P. viticola inoculation, indicating an induction of grapevine defense mechanisms. Moreover, one 6-pentyl-2H-pyran-2-one activated the hypersensitive response after P. viticola inoculation to reinforce the grapevine defense reaction. These results indicate that Trichoderma VOCs can induce grapevine resistance and these molecules will be further characterized to better clarify the mode of action against grapevine downy mildew.