Bacterial canker, a disease caused by the bacteria Pseudomonas syringae pv. syringae (Pss), remains a critical economic hurdle for sweet cherry production. Young trees are most susceptible, while older trees with bacterial canker are sources of inoculum. Pss can be found throughout the environment and living on all plant surfaces. Infections can occur through multiple avenues including blossoms, wounds, and various natural openings including leaf scars. Pss at low population numbers does not cause pathogenesis but becomes an issue as populations build during periods of cool, wet weather that are typical of spring and fall in Michigan. The Pss isolated from Michigan sweet cherry flowers was found to be phylogenetically diverse, with 20 strains belonging to 3 out of the 7 clades in phylogroup (PG) 2 (PG2b, PG2c, PG2d). Virulence tests with the 20 strains on green fruit and sweet cherry wood from two sweet cherry cultivars, ‘Sweetheart’ and ‘Coral Champagne’, demonstrated a range of virulence: strains in PG2d were the most virulent, PG2b strains were moderate to avirulent, and PG2c strains were avirulent (PG2d>PG2b>PG2c). The variability of the Pss strains also was reflected in their ice nucleation activity (INA) with strains having INA+, INA−, and INA+/− phenotypes, GATTa+ + − − and GATTa+ + + − profiles, and amplification of the genes encoding for the production and secretion of the phytotoxin syringomycin being syrB+/syrD+ and syrB−/syrD+. Though much of its epiphytic lifestyle, biology, and ability to live in mixed populations with varying levels of virulence has been elucidated for Pss, very little was known about the other bacteria that exist with Pss within the sweet cherry leaf microbiome. A survey of the sweet cherry leaf microbiome of three cultivars, ‘Benton’, ‘Gold’, and ‘Sweetheart’ was conducted from Fall 2017- Fall 2019 in two regions of Michigan. There were no regional or cultivar differences in the sweet cherry leaf microbiome. The clear drivers of microbiome stability were season and Pseudomonas, as the microbiome was variable in spring and became more stable in the summer and fall. The microbiome was dominated by the Phylum Proteobacteria (67 to 100% relative abundance) and the genera Pseudomonas (26 to 95% relative abundance). The core microbiome was composed of Pseudomonas and Sphingomonas OTUs. All top taxa have the potential to be beneficial to the sweet cherry tree. As control options for the management of Pss populations are limited there is keen fruit grower interest in the potential for biological control agents. Current commercial biological controls (BCAs) moderate to avirulent Michigan Pss. The moderately virulent and avirulent Michigan Pss were surveyed via in-vitro co-inoculations for their potential to decrease growth of the virulent to moderately virulent Pss using the same method. In-vitro co-inoculations were an acceptable way to initially assess potential BCAs. All commercially available BCAs were able to reduce the mean total area of Pss growth compared to when they were next to water, with up to 50% suppression in some cases. No single commercial BCA stood out as the premier choice for Pss growth suppression. None of the Michigan Pss were found to be ideal candidates for the potential biological control of virulent Pss, with no statistical differences in growth compared to when inoculated next to water.
Read
