Molecular research helps scientists combat kiwifruit disease

Pseudomonas syringae pv. actinidiae (Psa) is a bacterial pathogen that causes the widespread death of kiwifruit vines. The pathogen first appeared in this country in Te Puke during November 2010, killing gold kiwifruit orchards but leaving most crops of the mainstay green variety still farmable.

In just over a year, at least 840 orchards in the Bay of Plenty, representing a third of all kiwifruit plantings nationally, have contracted the highly virulent form of Psa (called Psa-V). More recently it has also spread over a hundred kilometres away to orchards in Tauranga, Katikati, Waihi, Whakatane, Opotiki and Pukekohe. The current cultivar of gold kiwifruit planted in New Zealand is particularly susceptible to the disease.

B3 began supporting research on Psa soon after its appearance in New Zealand, in order to assist in development of diagnostic tools to identify the virulent strain and monitor its spread. Because the Psa bacterium has variable strains, from highly virulent to completely harmless, it is important for scientists to be able to distinguish between them in order to effectively manage the disease.

At the time of the Psa outbreak in New Zealand, a molecular diagnostic technique developed by Dr Kerry Everett and her colleagues in the B3 programme was available to MAFBNZ to detect Psa. The test, although useful for incursion response, proved unable to recognise the all-important variation between Psa strains. Thus, the impetus was to fund research to underpin the development of a more precise test for Psa-V, which would enable it to be differentiated from less virulent strains. This was deemed critical for unambiguous detection of Psa-V and the search for effective management strategies to combat the pathogen.

Genetic analysis revealed that the Psa-V strain currently causing damage in New Zealand was closely related to strains associated with a similar outbreak in Italy in 2008, where economic losses started to be observed particularly in the Lazio region and have subsequently obliterated production of gold kiwifruit in this area. This analysis also found a less virulent strain (Psa-LV) present in several kiwifruit growing regions in New Zealand, which may have been present for some time.

In a united show of strength, the B3 programme has collaborated with another CRI funded research programme, Exploiting opportunities from horticultural genomics, to set up an international team of scientists led by Dr Erik Rikkerink and Dr Matt Templeton from Plant and Food Research, which includes B3 researchers Dr Andrew Pitman and Dr Mark Fiers as well as Professor Paul Rainey from the Allan Wilson Centre and Professor David Guttman from the University of Toronto. The team is using new generation DNA technology to sequence the complete genomes of 40 Psa strains from around the world in order to determine the differences between the aggressive strain and less virulent ones.  Ultimately this information will also help to determine how Psa causes disease in order to better manage the impacts.

This research builds on the B3 team’s existing work with other Pseudomonads, which includes the development of diagnostic tests to differentiate between regulated and non-regulated organisms as well as non-pathogenic variants during post-entry quarantine. These tools will assist with the importation of plants such as stonefruit.

In addition, a sister B3 project led by Dr Simon Bulman, which is focused on testing plant tissue to get a list of microorganisms associated with that plant, has been diverted to kiwifruit to determine if Psa could have been detected at the border using novel techniques.

The new DNA technology used in both projects, called Illumina sequencing, is sophisticated enough to enable scientists to quickly and cost effectively sequence entire genomes. So far twenty strains of Psa from New Zealand and overseas have been sequenced and another twenty strains are currently underway.

This genetic work is in turn being used to fast-track the rational development of resistant cultivars of kiwifruit and has already been used to develop a commercial assay to detect the virulent strain of Psa (with the testing and implementation phase funded by the Kiwifruit Vine Health industry organisation). The latest iteration of this test now provides kiwifruit growers with the fastest, most accurate test for Psa-V in the world.