Psa kiwifruit disease origins probed

A genetic analysis of several strains of the Psa bacterium, conducted by American and Italian scientists, suggests a Chinese origin for the bacterium that has caused millions of dollars of damage to the New Zealand kiwifruit industry. However, more data from affected New Zealand kiwifruit are needed to complete the picture. 

The new research, published in the journal PLoS One today, aimed to trace the genetic origins of several bacterial strains of the kiwifruit vine canker, Pseudomonas syringae pv. actinidiae (Psa). It is the first study released in a scientific journal to trace the bacterium back to its likely origin of China.

To find the root of the disease, the researchers examined how the bacteria may have evolved from the same ancestor by comparing the DNA from the different bacteria to each other. They found that the bacteria from China, Europe, and New Zealand were almost identical; but one small difference in one region of the DNA linked the New Zealand outbreak to the Chinese bacteria.

The researchers think that the most likely scenario is that the bacterium was imported from China into Italy and from China into New Zealand independently, but the concede that further data from NZ strains is required to fully confirm this.

“While our current genomic data suggest a possible Chinese origin of the European outbreak, we only have data obtained with genome-derived markers for the New Zealand outbreak.” The authors note in the article. “The origin of the New Zealand outbreak will thus remain an open question until complete genome sequences from New Zealand strains become available.”

Psa genome sequencing research being undertaken in New Zealand will provide further clues to the origins of the New Zealand outbreak.

The Science Media Centre contacted New Zealand researchers for comment on the research. Feel free to use these quotes in your reporting. If you would like to contact a New Zealand expert, please contact the SMC (04 499 5476;

Dr Margi Butler, Research Fellow, Department of Biochemistry, University of Otago, comments:

“We have previously reported that the NZ strains of Psa are very closely related to the Chinese and later Italian Psa strains and that these strains are more distantly related to the Japanese strains. These core conclusions are supported by the research provided by Mazzaglia et al [the new study]. The Mazzaglia data extends the analysis by considering Korean strains and by including the related (tea) pathogen, Ps theae. It is very pleasing that the two analyses should be in such close agreement.

“The Mazzaglia et al. analysis does not attempt a full genome sequence of any of the NZ strains and for this reason the authors concede ‘The origin of the New Zealand outbreak will thus remain an open question until complete genome sequences from New Zealand strains become available’. We have completed such analysis of four NZ strains and partial sequences from a further eight. This NZ data has enabled us to assert previously that the NZ outbreak was not derived from Europe and that the Italian outbreak was not derived from NZ. Mazzaglia et al do not attempt to address this question.

“Another important consideration is the relevance of the Chilean Psa outbreak. We are in the process of generating a full genome sequence of Chilean strains and this data (which should be completed in a few weeks) will determine the relationship between the Chilean strains and those of China, Europe and NZ. The Chilean strains are very closely related to the China/Italy/NZ strains.

“We are performing these analyses under contract to TaskForce Green an industry group established by Seeka and Eastpack. We have provided our analyses to them for release to interested parties as a matter of urgency. We have also provided data before publication to MAF NZ for their consideration as part of the investigation into the source of Psa in NZ. We will, of course, put our genome sequence data into international genome databases and will also publish our findings in the peer-reviewed literature.”

Dr Matt Templeton, Co-leader Plant Bioprotection Systems Biology, Plant & Food Research, comments:

“Psa is a major issue for the New Zealand kiwifruit industry and increasing our knowledge of the bacterium will help scientists in New Zealand and around the world develop new methods of controlling the disease.

“This new research will provide useful information for scientists looking at the origin of the bacterial disease around the world. In New Zealand, our science teams are focused on the control of the disease in our ecosystem and limiting its impact on the kiwifruit industry, one of our largest horticultural sectors.

“DNA sequence of the whole genome of Psa has enabled scientists at Plant & Food Research to identify genes controlling the virulence of this pathogen.  In the first instance this information was used to design a highly specific detection system that is now the industry standard. The current focus of scientists at Plant & Food Research is to identify the key factors that enable Psa to cause disease on kiwifruit.  Several unique factors have been found and their role in causing disease is being determined. The search is now on for finding resistance genes in our extensive kiwifruit germplasm that will target these key Psa factors.  These resistance genes will be fast-tracked into our kiwifruit breeding programme.” 

Fact box:

  • Psa is a bacterial canker of kiwifruit caused by the bacterium Pseudomonas syringae pv. Actinidiae.  
  • Psa causes a red or white bacterial slime to ooze out of the plant’s stems and branches. In the
    worst case, the entire plant wilts and dies.
  • Psa was first identified in Japan in the 1980s.
  • The first validated report of Psa in New Zealand was on 5th November, 2010 in Te Puke.
  • The strain in New Zealand, Psa-V, is particularly virulent.
  • A recent MAF investigation was unable to determine how the disease entered the country.
  • A Kiwifruit Vine Health report released today estimates that in the next 5 years Psa will:
    • Cost the industry between $310 million and $410 million
    • Result in the loss of up to 470 jobs a year between 2012 and 2015
  • According to the latest statistics (updated today, 9 May 2012):
    • A total of 1184 orchards in New Zealand are infected with Psa-V.
    • That represents 36% of all NZ kiwifruit orchards.