Phenotypic and Genetic Analysis of Epiphytic Pseudomonas syringae Populations from Sweet Cherry in Michigan

A severe outbreak of bacterial canker occurred on sweet cherry in Michigan in 2002. Blossom infection and subsequent canker formation was observed following a prolonged freeze event during bloom. Epiphytic blossom isolates of Pseudomonas syringae were recovered from 39 orchards from the three major cherry-growing areas (southwest [SW], west-central [WC], and northwest [NW]) of Michigan in 2003 and 2004. Average P. syringae populations over 2 years were 4.0, 5.1, and 4.8 log10 CFU/g of blossom tissue from the SW, WC, and NW areas, respectively. In 2003, copper-resistant P. syringae comprised 47.4, 21.1, and 3.1% of the total populations from the SW, WC, and NW areas, respectively, and levels of copper resistance were similar in 2004. Identification of 10 randomly chosen isolates from each orchard using polymerase chain reaction (PCR) assays indicated that 75 and 52% of the isolates from 2003 and 2004, respectively were P. syringae pv. syringae and that 1% and 23% of the isolates from 2003 and 2004, respectively, were P. syringae pv. morsprunorum. In addition, we were unable to determine the pathovar status of approximately 25% of the isolates each year, suggesting that a third P. syringae pathovar also was present in Michigan sweet cherry orchards. Pathogenicity on immature cherry fruit was confirmed for all P. syringae isolates. The frequency of ice nucleation was assessed for 44 individual P. syringae pv. syringae isolates, and the mean number of cells per active ice nucleus was 1,883. Extrapolating from this result, we estimated that active ice nuclei are present on most sweet cherry blossoms in Michigan orchards. Genetic fingerprinting of P. syringae pv. syringae using arbitrarily primed PCR indicated a high level of diversity and a clear differentiation of these organisms from the P. syringae isolates of unknown pathovar. A 2-year field trial evaluating the effect of dormant copper applications in spring and reduced-rate copper applications prior to bloom showed that these treatments were inconsistent in reducing P. syringae populations on blossoms.