Controlling Grapevine Trunk Diseases in California

A total of 20 vineyards belonging to 10 counties in California were sampled during summer 2019. Cordon, trunk and root tissue were taken from vines using non-destructive methods in order to isolate, analyze and study endophytic bacterial communities between healthy and diseased vines exhibiting typical trunk disease symptoms. A collection of over 1,750 isolates was obtained, from which 27.43% has been screened for their potential antifungal effect against the main GTD-causing pathogens in vitro. A set of 80 bacterial isolates was selected after a first screening against Neofusicoccum parvum, exhibiting >40% of inhibition of the pathogen mycelium. Phylogenetic analyses showed that 57 isolates belong to the genus Bacillus, 8 isolates to Variovorax, 6 isolates to Pseudomonas, 3 isolates to Stenotrophomonas, 2 isolates to Pantoea, 2 isolates to Lysobacter, 1 isolate to Lysinibacillus and 1 to Klebsiella. These isolates are currently being tested using the same methodology against other important GTD pathogens (Diplodia seriata, Eutypa lata, Diaporthe ampelina, Phaeomoniella chlamydospora, Phaeoacremonium minimum and Ilyonectria liriodendri) and a second selection will be performed with bacterial isolates exerting the higher percentages of inhibition against the majority of pathogens in order to be tested in greenhouse and field experiments. Furthermore, a fungicide trial was set over the summer of 2019 in three different nurseries (Winters, Wasco and Bakersfield), including chemical and biological products, using a vacuum chamber to infiltrate the fungicides through the vessels of dormant cuttings prior the grafting process. Fourteen treatments were included, and vines were grafted. Callus formation was evaluated 18 days after the treatments and results indicate frequencies of callusing ratings were similar among treatments, being in average 18.1% of rootstocks showing 100% of callusing, 48.0% showing 80-99%, 22,4% showing 60-79%, 7.7% showing 40-59% and 4.9% showing below 40%. Plants were further planted in pots according to each nursery protocols and kept in the facilities for approximately 3 months. In October 2019, actively growing vines were transported to the UC Davis Department of Plant Pathology field station located in Davis, CA, to be planted in the field under a completely randomized block design. In one of the nurseries, treated vines were planted in the ground after the callusing. The rooting of these plants will be evaluated in March during their transplantation to the field. Trunk disease incidence and severity will be evaluated yearly during the summer season.