Abstract
Since April 2014, a severe canker disease has been observed on trunks, branches, shoots, leaves and flower buds of kiwifruit (Actinidia chinensis and A. deliciosa) in several kiwifruit-producing areas of Japan. In some cases, symptoms such as shoot die-back, and bleeding cankers on the trunk with red-rusty exudation are more severe than those of any other canker disease ever confirmed in Japan. Twenty-two strains of the causal bacterium, isolated from Ehime, Fukuoka, Saga, and Wakayama Prefectures, and demonstrated by inoculation and reisolation to be pathogenic on A. chinensis, were gram-negative, aerobic rods with one to two polar flagella, and formed opaque, pale-yellowish, circular colonies. On the basis of biochemical and physiological characterization, PCR assays targeting ITS, hopZ3 and hopO1-2, and a multilocus sequence analysis (MLSA) using the concatenated sequences of seven housekeeping genes (acnB, cts, gapA, gyrB, pfk, pgi and rpoD), we identified the pathogen as Pseudomonas syringae pv. actinidiae (Psa). The results of the MLSA, additional PCR tests on argK, cfl, acnB, hopH1 and hopZ5, and biochemical and physiological characterization using API 20NE clearly showed that this pathogen corresponds to biovar 3 (=Psa3), one of four known biovars (biovars 1, 2, 3 and 5) of Psa. In addition, because Psa biovar 3 strains are known to carry integrative conjugative elements (ICEs) in their chromosomes, we used PCR tests to investigate whether or not biovar 3 strains used in this study possess ICEs. Of 22 strains, 14 possessed Pac_ICE1, and the other 8, derived from Fukuoka and Saga Prefectures, had no homologs of Pac_ICE1, Pac_ICE2, or Pac_ICE3. This study is the first to show that biovar 3, divided into two groups based on the presence of Pac_ICE, is present in Japan in addition to biovars 1 and 5.
