Pseudomonas syringae pv. actinidiae (Psa), the causal agent of bacterial canker of kiwifruit, was first reported and characterized on Actinidia deliciosa (green-fleshed kiwifruit) in Shizuoka, Japan in 1984. Canker disease was subsequently observed in China (1984), Korea (1988) and Italy (1992). In 2008, a devastating outbreak of the disease was reported in Italy on both A. deliciosa and A. chinensis (yellow-fleshed kiwifruit). Since then, it has been isolated from France, Portugal, Spain, Turkey, Chile, Switzerland, Australia and New Zealand. In New Zealand alone the cost to the industry over the next 5 years has been estimated to be between NZ$ 310 and NZ$ 410 million.
As for most pathogenic bacteria, once Psa is inside the plant, cutting out the affected parts is required to prevent the bacteria spreading throughout the plant. Psa can infect the plant from stomata, leaf abscission scars and broken trichomes. Psa has been found associated with pollen and this might help its dissemination.
The strains isolated from the recent global outbreak are different from the strains isolated earlier in Japan, Korea and Italy. Psa is now grouped within four sub-pathovar groups called biovars. Only Psa biovar 1 was present in Italy before 2008. Since 2008, all the strains isolated in Italy and in the rest of Europe belong to biovar 3. Strains of both biovar 3 and biovar 4 have been isolated from New Zealand, but only strains of biovar 3 (previously referred to as Psa-V) are economically important. Strains of biovar 4 (previously referred to as Psa- LV) are found throughout New Zealand, where they cause leaf spotting but do not cause systemic infections or plant death.