Combating kiwifruit Psa

Scientists around New Zealand have been working on how to combat the highly virulent pathogen Pseudomonas syringae pv. actinidiae (Psa) – a bacterial disease that has been plaguing the kiwifruit industry.

Psa was detected on a Te Puke orchard in the Bay of Plenty in late 2010 and, by October 2011, had affected 369 orchards covering 2214 hectares, with the rate of the bacterium’s spread increasing.

Effects of Psa on New Zealand’s kiwifruit industry

Dr David Ackerley from Victoria’s School of Biological Sciences is working with Seeka, New Zealand’s largest kiwifruit grower, and Professor Iain Lamont and Associate Professor Russell Poulter from the University of Otago to overcome the disease.

Psa has been brutal for our kiwifruit industry, and gold kiwifruit are particularly susceptible

Dr David Ackerley

“We only have to look overseas to see the potential damage. In the four seasons the Psa bacterium has been in Italy, it has essentially destroyed the gold variety in the country’s main growing area and is now affecting almost every orchard that’s growing green kiwifruit.”

Read more about the effects of Psa on kiwifruit.

Testing a range of antimicrobial agents

The research team is taking two approaches to find a solution to the disease.

“Our first strategy is to test a range of antimicrobial agents – substances that kill or inhibit the growth of microorganisms – to find compounds that may be suitable for use against Psa in the field,” says Dr Ackerley. “Kiwifruit crops could potentially be sprayed or even injected with these agents to help limit the spread of the disease.”

Possibility of knocking out ‘lethal’ Psa genes

“Another possible step is to identify and knock out key genes that make Psa particularly virulent, removing the ‘lethal’ genes that enable Psa to invade kiwifruit vines. You could then inoculate plants with a mild form of Psa that will dominate the surface of the plant and prevent the disease-causing strain from establishing a beachhead.

“This biocontrol strategy is particularly exciting to us and is building off the high-quality Psa genome sequence data generated by our collaborators at Otago.”

Benefits of sequencing the Psa genome

Dr Ackerley says the work to sequence the genome of Psa will have on-going benefits. “Initially, we’re aiming to combat the spread of the bacterium. Longer-term efforts may enable us to understand the precise mechanisms that make Psa such a lethal pathogen, and this could guide efforts to breed more resistant kiwifruit crops.”

Effects of intensively propagated monocultures

Professor Paul Rainey, a geneticist from Massey University in Albany, studies the evolution of plant pathogens. Professor Rainey says that modern agricultural practices may have played a role in the evolution of the virulent Psa lineage that plagues Italy and New Zealand and may have also exacerbated its rapid spread. “Modern agricultural practices based on intensively propagated monocultures, as in the kiwifruit industry, increase the risk of crop failure due to rapid spread of disease. In addition, monocultures provide opportunity for ready transmission of pathogens, which can select for virulent pathogen varieties.”

Effects of artificial pollination

Practices such as artificial pollination also pose a risk in terms of enhancing opportunities for the spread of pathogens over wide regional areas. Professor Rainey says, “We still have little understanding of Psa’s biology, epidemiology or means of control. It is likely that wind, water and insects are all involved in spread of the bacteria. The greatest hope for a solution is through resistance breeding programmes fuelled by discoveries from population genomics. Central to this goal is identification of determinants of bacterial virulence

Value of the kiwifruit industry

According to MAF’s annual report on the kiwifruit industry, New Zealand exported more than 100 million trays of kiwifruit during the year ended 31 March 2011, returning NZ$944 million. This is down on the billion dollar return recorded in the previous year due to exchange rate fluctuations.