Kiwifruit – learning to live with Psa

In November 2010, the bacterium Pseudomonas syringae pv. Actinidiae (Psa) was discovered in a Te Puke kiwifruit orchard. Psa had previously decimated kiwifruit crops in other parts of the world. The race was on to confirm whether the Te Puke orchard was an isolated case or whether the vine-killing disease had established itself in New Zealand.

Kiwifruit – an iconic and valuable product

Although China is the original home – and, in 2018, the largest global producer of kiwifruit – New Zealand is where the fruit was first commercially planted. We began exporting Chinese gooseberries in the 1950s at the height of the Cold War. Fruit exporters Turners and Growers decided to give the fruit a more marketable name, and the iconic kiwifruit was born. In the next few decades, many other countries also began to grow this fuzzy fruit.

Kiwifruit is a big earner for New Zealand. In 2010, the billion dollar industry accounted for 2.5% of the country’s exports, so when the bacterial disease was confirmed on 9 November 2010, there was widespread concern. Italian orchards had been wiped out by the disease in 2008, so growers and exporters were aware of the damage Psa could cause.

Psa has no risks associated with human or animal health or plants other than kiwifruit vines. When the disease was announced, some countries immediately restricted imports of the nursery stock, but fruit exports were not affected.

The question: Can we eradicate the disease?

A rapid response team from the Ministry of Agriculture and Forestry, Plant & Food Research and the marketing company Zespri took samples from the Te Puke region. The worst-affected orchards used copper-based sprays, vine removal and strict orchard hygiene practices. Psa is spread by wind and rain, so the possibility of eradication was slim.

The next step was containment. Researchers investigated antimicrobial agents and orchard management techniques to limit the spread of the disease. Neither of these strategies was successful so the teams moved to investigating ways to control Psa.

Germplasm collection, genetics and the search for new cultivars

Scientists discovered that two variants of Psa were in New Zealand – the highly virulent Psa-V and the less virulent Psa-LV. Psa-V proved to be particularly harmful to many of the country’s existing cultivars, including the lucrative Hort16A, better known as Zespri® Gold.

The kiwifruit industry, Plant & Food Research and other government agencies worked together to find new cultivars with greater resistance to Psa-V. Plant & Food Research scientists sequenced the Psa genome to identify the proteins that overwhelm kiwifruit plant defences. They also screened thousands of cultivars in their germplasm collection for plants that were Psa-V resistant and had desirable traits such as flavour, colour, storage and crop yield.

Fortunately, Plant & Food Research had three new cultivars in pre-commercial trials. The cultivar Gold3 (marketed as Zespri® SunGold) met all of the requirements, and Plant & Food Research accelerated the commercialisation process to allow growers to replace their Hort16A vines with Gold3 vines.

Investment pays off

Plant & Food Research reports that Zespri exported $2.3 billion of kiwifruit in the 2016/17 season – nearly 20% more than the 2008 season before the Psa outbreak. The rapid response of the Plant & Food Research team to the Psa crisis earned them the 2017 Prime Minister’s Science Award. It recognises the immense effort put in by the plant pathologists, breeders, orchard management specialists and others in the 100-strong team.

Psa took the industry to the brink and the industry and the science team have stepped up, and now that industry is pushing toward a $6 billion target in the future.

Dr Bruce Campbell, Chief Operating Officer, Plant & Food Research

Plant & Food Research are quick to give credit to their close association with industry partners in New Zealand and to researchers around the world for the collaborative efforts to achieve a successful result. The teams haven’t finished. Research and development programmes in New Zealand and internationally are still looking for solutions for Psa-V. In the meantime, growers have learned to live with Psa-V and to minimise its impact on their orchards.