How to fight Tomato Potato Psyllids (TPP): Lincoln University Research ends Zebra Chip disease threat in Canterbury

Three years ago, 5% of Canterbury’s potatoes were being lost to a disease which threatened to shut down the industry. Now the pest responsible has been driven off thanks to Te Whare Wānaka o Aoraki Lincoln University researchers. 

The story starts in 2008 when a small bug named the Tomato Potato Psyllid found its way into New Zealand. Despite being only a few millimetres in size, these insects brought with them a big problem. Within a year it floated on the wind down to the South Island, breeding in mass and feeding on our crops. 

Unfortunately for farmers, the psyllid (known as TPP) had a passenger along for the ride. The insect is host to Candidatus Liberibacter solanacearum, a type of bacteria which spreads throughout crops. The bacterium sits in the saliva of the psyllid. When the psyllid penetrates a leaf or stem with its proboscis, that infected saliva makes its way into the plant. 

That meant bad news for New Zealand’s potato industry. In potatoes that bacterium manifests as ‘zebra chip’. Diseased plants grow tubers with unattractive black marks. It’s safe to eat, but extremely bitter. If you’ve ever had a crisp or hot chip that was bitter, dark brown or black, that’s zebra chip.

Three years ago, the disease was so widespread that at one point 5.7% of Canterbury potato crops were infected. Enter, Clive Kaiser, Lincoln University Associate Professor in the Department of Agricultural Sciences. In 2021, Kaiser moved from Oregon State University to Lincoln, bringing years of specialist knowledge in integrated pest management.

When introduced to the problem, Associate Professor Kaiser tackled it with a holistic approach. He and his team set out to research TPP and teach growers how to handle it. He said it was vital to understand everything about the pest in order to control it, including its lifecycle and where it bred. This meant scouting and identifying the pest, studying the height at which they flew, regularly trapping them and inspecting them for the disease. 

A major breakthrough was getting industrial growers to change their pesticide spraying cycles to target the pest earlier in its lifecycle. For more than a decade, growers had been spraying for adult TPP, but the adults were only about 5% of the population. Eggs made up 55% and 40% nymphs, the two stages the insect went through before being fully grown.

Clive Kaiser:

"The 5% are always being replenished from those nymphs. 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."

The precision targeting has stopped the population from spreading among potato crops, but it was another control method which brought the population down.

Clive Kaiser:

"When I first got here, I wanted to understand the pest better. They told me it was on African boxthorns along the beach at Rakaia River. I was shocked at the incidence of eggs, nymphs and adults on those plants. There literally were millions. The entire bush was covered in them."

He decided the best course of action was to release biocontrol agents — predators of TPP that would target the insects and feed on them. A collection of pirate bugs, minute pirate bugs, Engytatus and brown Tasman lacewing were introduced to the boxthorns at Rakaia River, as well as nine other disease hot spots at commercial fields. He also taught growers which plants hosted TPP, many of which were weeds which were previously ignored. Since then, the number of infected plants has dropped to less than 0.01%.

It’s been sustained at those very low levels both this year and last year. Processing facilities are delighted. They thought they were going to lose their industry.

Clive Kaiser:

"It’s about doing everything you can to keep the population in check."

The latest control method is calcium. Plants treated with the mineral appear unattractive to the psyllid, deterring it from feeding and thus spreading the disease.

Clive Kaiser:

"Every instance where we did that, we didn’t find Liberibacter in the treated plants, but we always found it in the untreated ones. It appeared the insects didn’t want to feed on the plants."

Associate Professor Kaiser collaborated with researchers at the United States Department of Agriculture, who repeated the trials and recorded the same results. So far it had only been achieved in the lab, but Potatoes New Zealand wanted to fund trials in the field in the coming season, starting in October. 

The good news was that the calcium acts as a deterrent, not a method of killing the pests. As it was not considered a pesticide, the process of implementing it on a commercial scale was far smoother. If all went well farmers could be using the treatment by spring next year, he said. 

So why does calcium work?

Clive Kaiser:

"That we don’t know. It’s too new to know why the calcium is doing the job. Growers don’t care why, they just want a product that works."

The idea came from previous research he did in Oregon using different calcium and potassium salts as fungicides and bactericides.

Clive Kaiser:

"One of those calcium salts ended up being extremely potent as a broad-spectrum fungicide. It also proved fairly successful at controlling bacteria. It suggested to me it might be worthwhile looking at that to see if it could help reduce the Liberibacter in the potatoes. Fortuitously, it’s turned out to work against the insects."

Associate Professor Kaiser wants to use this research as a way to change the industry for the better. At present growers were spraying pesticides on their crops at an unstainable level. Not only did it affect the environment and the produce, but it was costing them thousands per hectare each year. He believes three targeted sprays a year would suffice, resulting in significantly more profit and far less pesticide on our food.

Clive Kaiser:

"It’s better for the people consuming them and better for the environment. Everybody wants less chemicals in their food."

Although the work has achieved significant results, he does not believe New Zealand will ever be free from Candidatus Liberibacter solanacearum. The bacterium is unculturable, meaning it has never been cultured in a lab environment. As such, lab research to try and eradicate it can’t be conducted.

Clive Kaiser:

"If we could figure out how to culture Liberibacter, that would be a real big step forward."