Promising biological control agents unite to combat Psa

The battle against the devastating kiwifruit disease caused by Pseudomonas syringae pv. actinidiae (Psa) is being fought on two fronts by Plant & Food Research scientists in the Next-Generation Biopesticides research programme.

The team have harnessed the power of microorganisms to develop natural agents that can combat Psa from above- and below-ground. A combination of these natural agents may prove to be more effective at controlling this disease and improving kiwifruit vine health than using either agent alone, and field trials to test this are underway.

Psa was discovered in New Zealand in November 2010 and has spread rapidly throughout most of the country’s kiwifruit growing regions. Left untreated, the bacteria can badly damage vines and cause substantial losses of fruit. Today, 2724 orchards have been identified with Psa, representing 87% of the country’s kiwifruit orchard area.

Biological control agents (BCAs), which use living organisms or their products, are a necessity for organic kiwifruit growers who want non-chemical control options, and offer alternatives for managing plant pathogens that are resistant or tolerant to chemical pesticides.

With support from leading kiwifruit marketer Zespri International Ltd and Kiwifruit Vine Health Inc., Plant & Food Research scientists, Drs Philip Elmer and Stephen Hoyte, have identified naturally occurring BCAs that can significantly reduce Psa symptoms in kiwifruit vines. “Over the last 12 months, we have identified two new foliar-applied biological control agents that protect potted kiwifruit plants against Psa in glasshouses and outdoor experiments, and in small plot field experiments” says Dr Elmer.

The team is working with AgResearch fermentation expert David Wright to find the best and most cost-effective ways of producing these BCAs, so that these methods can be used for commercial production in the future.

One of the more challenging aspects of BCAs is formulating them for ease of use by growers. Drs Elmer and Hoyte have developed a breakthrough technology to produce a BCA that dissolves quickly in water and has long shelf life and good stability – these are very important factors for successful commercialisation. They are now investigating the use of inexpensive additives that may further improve the BCAs persistence in the field.

Lincoln University scientists Drs Robert Hill and Christine Stark, and the team at the Bio-Protection Research Centre are using a root-dwelling beneficial fungus, Trichoderma, to help in the fight against Psa.

“These fungi confer to the plants a number of real benefits,” says Dr Hill. “One is that they prime plant defence systems, so they can defend themselves better against pathogens. But they also enhance the plant’s general health and vigour.”

Results have been promising, and the research team has isolated several strains of Trichoderma that reduce symptoms of Psa disease and improve longevity of young kiwifruit seedlings. These strains are currently being tested for efficacy against Psa in potted plant trials, alone and in combination with other BCAs.

Using Trichoderma isolates from healthy plants, the team has developed the biological control product ‘TriMix-1’, which is being produced at Lincoln University. The product is currently being trialled on 2,000 hectares of kiwifruit orchards in the North Island. Further mixes are currently in the testing pipeline.

The Next Generation Biopesticides (NGB) research programme is a joint initiative between AgResearch, the Bio-Protection Research Centre, Lincoln University and Plant & Food Research. The programme aims to develop biopesticides for use in New Zealand’s pastoral, horticultural and arable sectors. NGB researchers work closely with industry representatives and partners to develop solutions for existing pests and diseases, and those emerging as a result of land use change, farming intensification, climate change and biosecurity breaches. They are developing prototype biopesticides that are capable of rapid knockdown of pests and have multiple modes of action to target pest complexes and prevent development of resistance.