Erwinia pyrifoliae as a new pathogen on strawberry in the Netherlands

During the late spring in 2013 strawberry plants (Fragaria x ananassa cv. Elsanta) were found at several locations in the Netherlands showing an intense blackening of their immature fruits, their fruit calyx and the attached stems. There were no symptoms observed on the leaves. The discoloration was also observed inside the young fruits, which presented an intense darkening or blackening of the fruit tissue at the edges and an intense shining of the fruit tissue in the middle. Fruits did not develop or were in many cases heavily malformed. In certain cases 40% of the crop was lost. The release of bacterial slime was additionally observed on the surface of the young fruits and their stems. Isolations from these symptomatic immature strawberry fruits yielded Erwinia-like colonies on YPG (Yeast Peptone Glucose) agar. Characterization of the isolates revealed Gram negative bacterial cells, giving a negative reaction on Levan, and on media containing pectin. Bacteria have the following biochemical features: positive for sorbitol, but negative for gelatine, esculin and D-raffinose. The isolates differed biochemically from E. amylovora and they were closely related to biochemical profiles of the Erwinia pyrifoliae reference strain LMG 25888. The isolates were further identified as E. pyrifoliae based on the real time PCR assay (Wensing et al., 2012). Pathogenicity of several isolates was tested and confirmed on potted strawberry plants (cv. Elsanta). Inoculation by injecting a thick bacterial suspension (108 cfu/ml) in the epicalyx of three immature strawberry fruits per plant quickly resulted in oily necrosis with abundant formation of exudate on the epicalyx and on the sepals. A few inoculated fruitlets with the isolates and E. pyrifoliae reference strain LMG 25888 developed similar symptoms to the ones observed in the original samples in three to four weeks. The bacterium was re-isolated from the affected symptomatic fruitlets and identity of the re-isolates was confirmed by PCR, indicating that E. pyrifoliae had caused the disease. Spray inoculation of strawberry flowers resulted in necrosis without ooze formation. Erwinia pyrifoliae is closely related to the main fire blight pathogen E. amylovora. The pathogen E. pyrifoliae was described for the first time in 1999 in Korea (Kim et al., 1999; 2001). E. pyrifoliae is primarily a pathogen of Asian or Nashi pear (Pyrus pyrifolia) causing fire blight on Nashi pear and is considered to have a restricted geographic distribution in East Asia (Korea and Japan). Thus far occurrence of this pathogen on strawberry has not been reported nor its presence outside Asia.