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Abstract

Grapevine crown gall (GCG) is an economically important disease affecting grapevine (Vitis vinifera) as the disease reduces both plant performance and crop quality and there is currently no effective treatment available. The bacterium responsible for GCG, Allorhizobium vitis, is primarily introduced via infected planting material. Phylogenetic research has not been performed on A. vitis strains isolated from Canadian vineyards. This thesis investigated the genetic diversity among A. vitis strains isolated from the Okanagan Valley and compared them to strains isolated globally. Multilocus sequence analysis (MLSA) placed Okanagan A. vitis strains in several groups amongst other A. vitis strains collected internationally, specifically from Australia, Europe, and the United States of America. Single nucleotide polymorphism (SNP) analysis of Okanagan A. vitis strains determined that there was no clear relationship between genetic similarity (few SNP differences) and cultivar, rootstock, or nursery of origin of the plant material the strains were isolated from. Additionally, this thesis tested the biological control ability of several bacterial and fungal isolates against A. vitis in vitro and in planta. Endophytic and rhizosphere bacteria collected from escaped vines (vines not showing disease symptoms) in GCG-diseased vineyards in British Columbia and Ontario were screened for inhibitory activity against several strains of A. vitis. Five bacterial isolates (three Bacillus spp. and two Pseudomonas spp.) and two Trichoderma spp. isolated from grapevine showed consistent inhibitory activity in vitro. However, none of the potential biological control isolates tested in greenhouse experiments using needle prick or root soak inoculation methods prevented GCG development in planta. Finally, this thesis investigated organic compost application as a means of GCG disease remediation in the field. Compost improved soil fertility and reduced two populations of pathogenic nematodes in soil (Paratylenchus and Ximphinema), pests that likely enhance grapevine root infection by A. vitis. However, visual GCG symptoms did not decrease and grapevine performance did not improve during the study period. Overall, the work outlined in this thesis contributed to a deeper understanding of the genetic diversity of A. vitis strains in the Okanagan Valley, and tested several new strategies for GCG disease prevention and management.