Psa research offers hope
Cultivars from naturally resistant wild varieties could well be the key to resilience, Donna Russell
Anew research project that may help future-proof the kiwifruit industry has received a Fast Start Marsden grant. The three-year project, led by Dr Jay Jayaraman at Plant & Food Research, is investigating how the Psa plant pathogen evolves during infection of the kiwifruit plant.
Psa ( Pseudomonas syringae pv actinidiae) has been the most serious challenge to the kiwifruit industry in recent years, forcing growers to replace thousands of susceptible plants with more resistant cultivars at a huge cost to the industry.
“The industry is recovering a lot better than expected. The new varieties are performing well but still require regular copper sprays to keep the plants healthy and resistant to the bacteria,” he said.
“We want to explore alternative ways to manage the disease in future, particularly if the Psa pathogen manages to adapt to the new cultivars. As much as possible, we want to future-proof the industry to create more security for growers.”
Jayaraman and his team of researchers plan to draw on the early varieties of wild kiwifruit that have been kept at Plant & Food Research since they were collected by early botanists.
He has already noticed that some of these early wild varieties are naturally resistant to Psa.
“These varieties are closer to the wild kiwifruit. Some have small edible fruit and some are not tasty at all.”
Decades of breeding work went into creating the first commercial cultivar, which turned out to be particularly susceptible to Psa.
Kiwifruit growers now largely rely on the Zespri SunGold G3 variety, which has a vibrant yellow colour and tropical flavour, and the traditional Hayward Zespri Green variety.
The new Red19 cultivar, called Zespri Red Kiwifruit, was launched this year as a commercial crop after more than a decade of development in a partnership between
Zespri and Plant & Food Research.
The Red variety adds a sweet, juicy, raspberryflavour to the SunGold flavour and is now available to commercial growers.
Jayaraman said developing new cultivars took many years, so researchers are in a race against time to ensure any new cultivars are more resistant to Psa.
“We are all really focused and there are hundreds of scientists who are involved and want New Zealand growers to do well.”
Psa was discovered in New Zealand’s kiwifruit crops in 2010 and genomic sequencing had established that the strain had most likely arrived from China. There is only one strain in New Zealand although there are many others around the world.
Psa causes disease by secreting proteins that suppress the kiwifruit plant’s immune response. Much like in the human immune system, the kiwifruit plant identifies and responds to the pathogen resulting in leaf spotting, canker and cane dieback and vine death.
“The disease is almost everywhere in New Zealand and spreads easily through water and air, and through foot traffic and vehicles. It infects the entire plant and can kill the earlier varieties within a few weeks,” Dr Jayaraman said.
Growers have adopted careful orchard management techniques to try and contain the disease, including restricting visitors to orchards and developing biosecurity plans.
Dr Jayaraman said he hopes future cultivars will be so resistant to Psa that they can be spray-free. “That would be a dream result if that is possible.
“We would like future orchardists to have a minimal impact on the environment.”
There are hundreds of scientists who are involved and want New Zealand growers to do well.
The Marsden Fund, managed by the Royal Society Te Ap rangi on behalf of the New Zealand government, supports New Zealand’s best investigatorinitiated research in the areas of science, engineering, maths, social sciences and the humanities. This project is one of two Plant & Food Research Marsden grants awarded for this round.