After the tomato potato psyllid arrived in 2008 its numbers quickly spread to become a big problem for potato growers and processors.
About 20% of the tiny insect’s population carries the bacteria, Candidatus Liberibacter solanacearum and when it penetrates a plant this results in unsightly and bitter-tasting blackish marks in the potato tuber.
The zebra chip disease put so much pressure on growers they formed the Canterbury Potato Liberibacter Initiative (CPLI) to combine with industry leaders and Lincoln University scientists.
About three years later, potato growing has become more viable as a result of a three-pronged control strategy.
CPLI chairman John Jackson said the work to find a solution for a disease causing heavy quality and wastage problems for potato production had made real progress.
"Obviously there are serious cost implications for farmers growing the crop. Losses could have been anywhere up to 30% if you had a problem and couldn’t control it. It’s a serious issue and some loads coming in were up at that level at its peak and then you are dealing with a lot of wastage. But it’s a lot lower now."
Diseased potatoes had to be dumped or downgraded into a lower quality product, the former agriculture director for McCain Foods Australia and New Zealand said.
Mr Jackson said the tomato potato psyllid was the major threat experienced by growers in his 35 years in the potato industry.
Farmers, commercial and seed growers, agronomists, processors, industry representatives and university researchers make up the CPLI committee. They have support from the Potatoes New Zealand team and grower and processor funding.
Mr Jackson said they initially wondered if chemical resistance was contributing to weak spraying results, but this was ruled out by resistance testing carried out by Plant & Food Research.
"We have also got Clive Kaiser on the committee who worked away and looked at chemical programmes and basically found there was a window at the start of the season where the adults come out to feed on the plants, but we weren’t controlling the eggs or the nymphs at the growth stage. And so we then went a lot earlier for controlling it than November or December and started seeing the results and also did a lot of work on soft chemistry as this doesn’t kill beneficial or predatory insects."
Lincoln University agricultural sciences department Associate Prof Clive Kaiser moved from Oregon State University to Lincoln in 2021.
He said it was vital to understand everything about the pest, including its lifecycle and where it bred, in order to control it.
A major breakthrough in the team effort was getting growers to change their pesticide spraying to target the pest earlier in its lifecycle.
For more than a decade, growers had been spraying adult psyllids, but they were only about 5% of the population with eggs making up 55% and nymphs 40%, he said.
"If you’re trying to control the population, you’re better off targeting the eggs and nymphs. By doing that, you’ll limit the number of adults, and you won’t have to spray for them."
After inspecting millions of tomato potato psyllids on African boxthorns along the beach at Rakaia River, Assoc Prof Kaiser decided the best course of action was to release biocontrol agents to feed on them.
A collection of pirate bugs, minute pirate bugs, engytatus and brown Tasman lacewing were introduced to the boxthorns and other disease hot spots at commercial fields.
Growers were taught to identify host plants and since then infected plant numbers had dropped to less than 0.1%.
Nature strips planted next to potato crops were attracting beneficial insects to feed on tomato potato psyllids.
The latest control method is to treat potato plants in the laboratory with calcium, which for some reason is unattractive to psyllids, deterring them from feeding and spreading the disease.
Assoc Prof Kaiser worked with United States researchers who repeated the trials with the same results.
Trials are expected to be carried out on commercial crops this season.
Mr Jackson said the "soft chemistry" of calcium targeted the tomato potato psyllid, hopefully without destroying beneficial insects feeding on the pest.
Results from a controlled environment trial had been outstanding so far and it was an effective and affordable treatment. Calcium propionate (E282) was in many foods in the baking, meat and juice industries and was probably one of the safest food additives, he said.
"Farmers don’t care how it works, they just want it to fix the problem ... The beauty of this is we got such good results in the glasshouse last year that we didn’t believe it and repeated it to get the same results."
He said the repelling nature of calcium had combined with biocontrol agents and a better understanding of the insect’s life cycle to give growers much better control of the tomato potato psyllid.
"So now we are probably down to below that 2% defect mark which is really good. It’s acceptable, but we’d like to get it lower to drive the cost down and use more friendly products."
The hope was DNA tests on the stomach contents of tomato potato psyllids could reveal plants it was over-wintering on to target more weeds, he said.
Assoc Prof Kaiser said the calcium treatment was too new to understand how it worked, but the controlled environment trials showed it worked well.
The idea came from previous research he had seen in Oregon’s citrus industry using different calcium and potassium salts as fungicides and bactericides.
He said three targeted sprays a year would result in more profit and far less pesticide being used which was better for people eating potatoes and better for the environment.
