Epidemiology and molecular characterization of Pseudomonas species on blueberry plants

Severe bacterial blight occurs periodically in British Columbia’s blueberry fields for unknown reasons. Pseudomonas syringae pv. syringae (Pss) - the putative causal organism, P. viridiflava, P. fluorescens, P. tolaasii, and P.s. pathovars aceris, tagetis, apii, and antirrhini were isolated from historically blighted fields. Three Pss isolates screened on lilac plantlets were pathogenic. A two-year field study of Pss-inoculated blueberry plants showed that Pseudomonas strains are a predominant part of blueberry stem microflora. Internal populations overwintered at low levels (10^1-10^2 CFU/g), but when wounds were present during inoculation 10^6 CFU/g survived. Maximum epiphytic populations reached 10^5 CFU/g in March-April and internal populations reached 10^6-10^7 CFU/g in Feb-May. With wounding, maximum internal populations reached 10^7-10^8 CFU/g as early as Jan-Feb. A colony blot method was developed to identify Pss from diseased tissues. Combined factors including weather, inoculum load and host phenology may contribute to the sporadic nature of bacterial blight.