Hunting the origin of kiwifruit-killing disease Psa

A researcher who has travelled the world searching for the origins of Psa says new lineages of the kiwifruit-killing disease could emerge in the future. Photo /123RF

A researcher who has travelled the world searching for the origins of Psa says new lineages of the kiwifruit-killing disease could emerge in the future.

Since it was discovered in 1984, there have been multiple outbreaks of the kiwifruit canker disease around the world - including the 2010 disaster that infected 80 per cent of orchards nationwide and cost New Zealand's industry an estimated $930 million in lost exports.

Six years on, a group of more than 200 growers are taking a High Court class action against the Ministry for Primary Industries, which has denied all of the damage claims.

Massey University researcher Dr Honour McCann said lineages responsible for all of the regional and global Psa outbreaks were very distinct from each other - but there was evidence their ancestors co-occurred for long enough to exchange DNA.

She and her colleagues therefore hypothesised the existence of a diverse source population of Psa, from which different strains had emerged to cause outbreaks over the past few decades.

"I've spent the last few years trying to identify where this source population is and what plant host it is associated with," said McCann, who is presenting her findings at the Queenstown Research Week conference.

She travelled to China suspecting the source population was linked with a breed of wild kiwifruit, Actinidia chinensis, growing there.

With colleagues from the Chinese Academy of Sciences, she visited eight provinces, isolating bacteria from wild and cultivated kiwifruit.

"We sequenced the bacterial genomes and used this sequence data to identify what lineages are present in China and construct a phylogeny, which reveals how the strains are related to each other," she said.

"Long story short: we didn't find any Psa in wild kiwifruit in China.

"Even more surprising, we only found a single lineage of Psa in Chinese kiwifruit orchards, contrasting with the situation in Korean and Japanese orchards, where multiple lineages have been identified."

This turned her attention to South Korea and Japan, where she conducted similar tests - and revealed Psa in wild kiwifruit.

"The wild host is a distant relative of the green and gold kiwifruit we're most familiar with, but you may have eaten its fruit before, as plant breeders are developing new varieties marketed under the name kiwiberry."

Based on the work, she ultimately concluded the source population of Psa infecting cultivated kiwifruit appeared to be not wild kiwifruit in China, but a distant relative of kiwifruit that was broadly distributed across China, Korea and Japan.

This had implications for future possible outbreaks - including here.

"A few years ago, when we hypothesised the existence of a source population of diverse, recombining Psa, we predicted that new lineages would likely emerge to cause disease in the future, and these would be different compared to the one that reached New Zealand in 2010.

"This prediction was proven correct when Japanese researchers found new lineages of Psa infecting kiwifruit orchards."

Understanding the location and host of the source population was essential, she said, as it would allow scientists to characterise the extent of pathogen diversity and link this with pathogen virulence.

This would allow plant breeders to develop new kiwifruit varieties that weren't resistant to a single lineage alone - as was the case with Psa-V - but have broader resistance to Psa that might challenge kiwifruit production in the future.

"This will also inform risk management strategies for the prevention of future outbreaks: the collection, movement and cultivation of the wild relative of kiwifruit should be carefully monitored as it is now known to harbour Psa."