Scientists are now confident the kiwifruit vine disease Psa (pseudomonas syringae pv. actinidiae), which has cost New Zealand hundreds of millions of dollars, has Chinese origins.
University of Otago researchers say DNA work using advanced genomic technology showed the strain of Psa which hit New Zealand in 2010 is almost certainly Chinese in origin.
The researchers found the core genomes of the Chinese, Chilean, Italian and New Zealand strains of Psa were almost identical and probably shared a common ancestor around 10 to 15 years ago.
However, the genome sequence shows the New Zealand strains are a distinct clone, while the Italian strains form another geographical clone.
"These findings paint a clear picture of an independent Chinese origin for both the Italian and the New Zealand outbreaks and suggest the Chilean strains also come from China," Associate Professor Russell Poulter said.
The study, published on Thursday in the international science journal PloS One, backs findings the same team made in May last year, when they linked the New Zealand Psa outbreak in 2010 to bacteria found in China, but said more data was needed to confirm the link.
The researchers are now sequencing a further 20 strains, mostly from China but also Korean and Turkish strains.
They are focusing on mobile genetic elements called ICE, or integrative conjugative elements, which can transfer between cells of different bacteria strains and alter properties such as their infectiousness and resistance to antibiotics.
They have found three distinct ICEs - one shared by the New Zealand strains and three strains from China, another shared by Italian strains and a Chinese strain, and a third found in the Chilean strains.
"Some Psa may be inherently more virulent due to the particular ICE they carry. This has worrying implications as strains of kiwifruit that are resistant to one type of Psa might not be resistant to another," Dr Poulter said.
"This means strict border control by kiwifruit-producing countries is more important than ever."