A report from the University of Otago has alleged that strains of kiwifruit disease Psa found in Europe and New Zealand likely originate from China.

Psa, known as Pseudomonas syringae pv. actinidiae, leads to bacterial canker in kiwifruit vines and has damaged key grower countries like New Zealand and Italy, particularly for more susceptible gold kiwifruit varieties.

For the research article published in the journal PLOS ONE, scientists sequenced the genomes of several strains found around the world.

"Strains from China, Italy, and Portugal have been found to belong to the same clonal lineage with only 6 single nucleotide polymorphisms (SNPs) in 3,453,192 bp and one genomic island distinguishing the Chinese strains from the European strains," the report said.

"Analysis of additional European isolates and of New Zealand isolates exploiting genome-derived markers showed that these strains belong to the same lineage as the Italian and Chinese strains.

"Results are interpreted in regard to the possible direction of movement of the pathogen between countries and suggest a possible Chinese origin of the European and New Zealand outbreaks."

The study pointed out that previous molecular analyses did not include strains from China, but for this analysis two isolates from the Chinese province of Shaanxi were sequenced.

Researchers still highlighted the exact origin of New Zealand's Psa strain remained an "open question" until complete genome sequences became available, but the similarities were very strong.

"The commercial shipping of yellow flesh kiwifruit vegetative material between China, Europe, and New Zealand during the years preceding the bacterial canker outbreaks provides a possible transmission mechanism of PSA.

"This hypothesis is also supported by the observation that the first Italian outbreak occurred on yellow flesh cultivars.

"However, a recent investigation into possible modes of PSA transmission by the New Zealand Ministry of Agriculture and Forestry deemed this scenario unlikely since all vegetative material imported into New Zealand was kept in quarantine for several years and did not show any canker symptoms."

The report said this meant other forms of transmission besides contaminated vegetative material could not be excluded as possible sources, including dissemination via pollen, equipment, or the long distance movement of the disease through the atmosphere and the water cycle or insects.

Related story: NZ scientist warns of multiple Chinese Psa strain threat

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