UC Davis

Grapevine trunk diseases (GTDs) are caused by multiple unrelated fungal pathogens, and their management is challenging for growers around the world. The current trend of reducing the use of synthetic fungicides makes biocontrol an environmentally friendly strategy to mitigate the impact of GTDs. In this dissertation, a survey that was carried out in twenty vineyards across California revealed that a subset of endophytic and rhizospheric bacterial isolates exerted inhibitory activity in vitro against the mycelium of Neofusicoccum parvum and Diplodia seriata (Chapter II). The bacterial isolates were identified as Bacillus velezensis (n = 154), Pseudomonas spp. (n = 12) and Serratia plymuthica (n = 2). Representative isolates (n = 6) of B. velezensis, P. chlororaphis, and S. plymuthica showed consistent levels of mycelial inhibition against eight GTD-causing pathogens (N. parvum, D. seriata, Lasiodiplodia theobromae, Eutypa lata, Diaporthe ampelina, Phaeoacremonium minimum, Fomitiporia polymorpha and Ilyonectria liriodendri). The agar-diffusible metabolites and volatile organic compounds produced by the bacterial isolates inhibited the growth of N. parvum and E. lata on a concentration and on bacterial species dependent manner. Representative isolates (n = 3) of each bacterial species were further evaluated for their antagonistic efficacy against fungi under field conditions (Chapter III). The isolates were grown in the laboratory and then delivered through four ways: (i) infiltrated in dormant propagation material before grafting in nursery settings; (ii) applied as a soil drench in the vineyard; (iii) injected in the trunk and cordons; and (iv) sprayed onto dormant pruning wounds of mature vines. Results showed that the isolates of B. velezensis (UCD10631) and P. chlororaphis (UCD10763) exerted a positive effect when infiltrated in propagation material and as soil drench treatments by reducing the lesion length caused by artificially inoculated GTD-causing pathogens. A better performance was observed against E. lata and P. minimum than against N. parvum. On separate part of this work, the etiology of Aspergillus Vine Canker (AVC) and Sour Rot (SR) of grapes was reassessed due to recent changes in the taxonomy Aspergillus section Nigri (Chapter IV). Morphological and phylogenetic analyses based on nucleotide sequences of the calmodulin (CaM) gene allowed the identification of the causal agents of both AVC and SR occurring in California as A. niger, A. carbonarius, and A. tubingensis. The most prevalent species was A. tubingensis, associated to both diseases, and six isolates were equally pathogenic in healthy wood and berries of ‘Red Globe’ grapevines. These results represent the first detection of A. tubingensis causing Aspergillus Vine Canker and Sour Rot of grapes in California.