NGB scientists battle kiwifruit disease

AgResearch scientists involved with the Next Generation Biopesticides (NGB) programme are playing an important part in the battle against a bacterium (Pseudomonas syringae pv. actinidiae) attacking New Zealand’s lucrative kiwifruit industry.

Dr Sean Marshall, a microbiologist with AgResearch at Lincoln, is part of a team including Emily Gerard and David Wright (also from AgResearch) plus Drs Phil Elmer, Stephen Hoyte, Kirsty Boyd-Wilson, and Pia Rhinelander investigating how the Plant & Food Research (PFR) and Zespri biocontrol agent works against Psa.

Psa, first detected in New Zealand in 2010, is a bacterium that can result in the death of kiwifruit vines. It causes leaf spotting, cane and leader dieback, flower infection and bud rot, cankers and, in extreme cases, vine death accompanied by the production of exudates (a rusty red liquid discharge).

Symptoms of Psa are usually expressed during spring and autumn with cooler temperatures, regular rain and high humidity. The disease can be spread via windborne pollen, strong winds and heavy rainfalls as well as by footwear, vehicles and orchard tools, animals and humans. The bacterium infects the plant through natural openings (e.g. stomata and leaf scars) and wounds.

Plant &Food Research scientists with support from Zespri and Kiwifruit Vine Health (KVH) recently identified a potential new biocontrol agent. Working together with PFR, David Wright is attempting to ‘scale it up’ – and get it ready to be applied in a safe, cost-effective and efficient manner. It’s feasible that once developed, the new formulation of the biocontrol agent could be sprayed in kiwifruit blocks using conventional airblast spray equipment.

But, firstly and because it’s a biological product requiring registration, Dr Marshall and colleagues are trying to understand its mode of action in reducing Psa disease development.

“It appears to inhibit the growth of Psa but we need to know how and why. By understanding that we can better understand the best times to apply it in order to maximise its efficacy against Psa and also any crop safety aspects or unintended environmental impacts.

“One possible mechanism, is that it seems to adjust its growing environment by altering the pH levels of the media in which it’s growing to a point that Psa itself can’t grow.

“If it’s a single mode of action then the potential for resistance development needs to be considered and how to use it in an integrated control program or in combination with other products. Another aspect to investigate is how can we synergise it with other additives – be they biological or chemical or cultural methods that growers can use.”

Dr Marshall says the MBIE and Zespri/KVH co-funded project is ongoing but is already identifying some promising leads about how the new biocontrol agent works.

“It also seems to be growing a biofilm but we’re not sure if this is just in lab cultures or on the plant itself. We’re also unsure whether this is going to be a physical barrier or provide some sort of anti-microbial properties.

“Early indications are that if you grow this biocontrol agent with Psa you get biofilm production, but if you grow either one of them separately you don’t get biofilm production. So, somehow the action of having them together is triggering a response and at this time we’re not sure if it’s from the biocontrol agent or the Psa itself.”

The work that the diverse team is undertaking is part of the Next-Generation Biopesticides programme that aims to find new, safe and sustainable solutions to New Zealand’s key insect pest and disease problems. Led by AgResearch, it is a partnership with collaborators Plant & Food Research, the Bio-Protection Research Centre (Lincoln University) and industry stakeholders Grasslanz, Zespri and the Foundation for Arable Research.

The Ministry of Business, Innovation and Employment is the principal funder of the project with the other project collaborators providing co-funding.