Taxonomic Position of the Causal Pathogen of Bacterial Shoot Blight of Pear

Bacteriological properties and DNA-DNA homology values were compared among the pathogen causing bacterial shoot blight of pear (BSBP) isolated in 1994–1996, Erwinia amylovora isolated outside of Japan, and other Amylovora group bacteria. Bacteriological properties of BSBP strains were identical to those of E. amylovora in the majority of tests, but differed distinctively in several tests, including hydrolysis of esculin and acid production from salicin, etc. BSBP strains differed from the others in the Amylovora group in many other tests. DNA homology among the strains of BSBP ranged from 85 to 103% and from 83 to 110% among strains of E. amylovora. In contrast, the values between BSBP strains and E. amylovora strains were 55 to 81%, while those between BSBP strains and other Amylovora group strains were 42% or less. We consider, therefore, that the BSBP pathogen may well be included in E. amylovora at the species level. E. amylovora, including BSBP strains, however, can be classified into four biovars based on differences in nine tests such as growth factor requirements and crater formation on high sucrose medium. Namely, there are two biovars from Maloideae sources, one from Rubus idaeus, and one from the source of BSBP in Hokkaido. The presence of these biovars suggests a correlation with geographical, serological, and pathogenic differentiations in the species of E. amylovora. The BSBP pathogen in Hokkaido was identified as E. amylovora bv. 4 which is distinct from E. amylovora bv. 1, 2 and 3 isolated in countries outside of Japan.