Abstract
In Ehime Prefecture, Japan, two types of symptoms of bacterial canker on kiwifruit have been found, subsequently termed types A and B. Type A symptoms, which formed on leaves in orchards located in Iyo City, are characterized by typical lesions (leaf spots with clear yellow halos). Type B symptoms, observed on leaves in orchards in Matsuyama City and Tobe Town, include only necrotic lesions without halos. Based on genotypic and phenotypic characterizations, the pathogens isolated from the two groups of orchards were both identified as Pseudomonas syringae pv. actinidiae (Psa). In bioassay and inoculation experiments, the isolates derived from type B did not possess the ability to either produce phaseolotoxin (Ptx) or induce halos on kiwifruit leaves, although the constituent genes (amtA, ptx, ORF3 and argK) of the argK-tox cluster, which is responsible for Ptx production, were detected in these isolates by PCR. Although the coronatine-producing gene (cfl) and the effector gene (hopA1) have been reported in Psa isolated in Korea and Italy/France, respectively, we detected neither of these genes using PCR. These results suggest that a mutation(s) may have impaired Ptx production in Ptx-producing Psa, which is distributed widely in Japan, leading to the non-Ptx-producing mutant. We confirmed that such mutants are distributed exclusively in the Matsuyama-Tobe area in Ehime Prefecture, whereas Ptx-producers are prevalent in Iyo City. In addition, disease surveillance and inoculation experiments indicated that the virulence of the defective non-Ptx-producing mutant on kiwifruit branches was weaker than that of the Ptx-producers, suggesting that Ptx may function as a virulence determinant in branches.
