Friday, March 29, 2024

Zebra spuds are still a problem

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Finding a cure for ruined potatoes continues to tax growers and scientists. Tim Fulton reports.
Reading Time: 3 minutes

Zebra chip potato disease has been in New Zealand for 10 years and continues to be an unwelcome overstayer.

The potato industry produces 586,000 tonnes annually with a value of $570 million. 

About 80% is from domestic sales. Most of the exports are processed potatoes.

The industry has estimated the arrival of the tomato-potato psyllid cost it about 5% of the total value in crop losses and control costs.

Very little was known about the psyllid when it arrived and producers began crop spraying weekly in an effort to control the insect and, by association, the zebra chip disease. 

Under that regime, annual costs associated with the psyllid are estimated at $28m, including $11m in crop impact and $6 million in control costs.

We can blame the psyllid insect found in NZ in 2006 then the discovery two years later of a bacterium, Candidatus liberibacter solanacearum.

The state-owned Plant and Food Research has 21 scientists working on either TPP or the bacterium, from finding natural enemies for the psyllid to spray programmes and using genomics to compare the genetic profiles of NZ psyllid populations.

At Lincoln’s Plant and Food Research scientists are working out how the bacterium acts on the plant when transmitted by the psyllid, through its salivary glands and gut.

TPP is a small insect, about the size of an adult winged aphid. Adults and nymphs feed on the phloem in a plant by puncturing the leaf, similar to the way a mosquito bites people, so feeding damage is basically invisible. 

Both adults and nymphs can carry the disease-causing bacterium. 

However, not every TPP carries and therefore transmits the bacterium. Only a small percentage of the population tests positive for it. There is no indication on the outside of the adult and nymph of whether it is infected.

Crop protection specialist Dr Jessica Dohmen-Vereijssen says the interaction is crucial because without the psyllid, the bacterium “pretty much” comes to a dead end. 

She acknowledges that, to a lesser extent, infection can still be carried by tubers.

Dohmen-Vereijssen said a published scientific paper with input from the Ministry for Primary Industries concluded both the insect and bacterium were probably deliberately introduced, probably from California.

But growers are now most interested in the disease’s impact on their crop, she said.

To get the bottom of the complex relationship between psyllid and bacterium, researchers have looked at the psyllid’s communication (vibration) and its response to coloured light and plant odours.

Other studies have included chemical and biological controls and lures and a parasitic wasp being tried on non-crop hosts in Canterbury and Hawke’s Bay. 

The Tamarixia triozae parasitoid was imported and released last year on non-crop hosts, like African boxthorn, in Canterbury and Hawke’s Bay. It became apparent the parasitoid was susceptible to insecticides sprayed in potato or tomato crops. Therefore it wasn’t released in those crops.

The TPP and liberibacter science is a small part of Plant and Food’s research budget but it’s issue Number One for most growers, Potatoes NZ chairman Stuart Wright said.

Plant and Food and commercial agronomists are doing most of the lab and field testing to improve TPP management. They knew right from the start that while the disease couldn’t be eradicated, it can be managed.

Viewed as a whole, the zebra chip potato studies could be seen as a series of successes and failures – and each project has done its bit for the body of knowledge. Some of the research shows what approach to rule out – like the discovery that entomopathogens are not very useful out in the field and that degree days – biological development related to temperature – aren’t helpful in determining when to start spraying.

Spray programmes continue to be used to reduce the number of psyllids, to reduce the frequency and severity of zebra chip disease. The work, which started in 2009, extends to resistance management and using different spray thresholds.

There has also been work on insect mortality and transmission of bacteria for several chemical and biological insecticides as well as identification of psyllid natural enemies in potato crops.

NZ’s emerging field of zebra chip tomato-potato psyllid and bacterium research doesn’t stop there. The logbook includes symptom development and the finding a few years ago that NZ potato tubers respond differently to Liberibacter than the American ones.

Science is also looking at the effect of soil-borne diseases and the bacterium in the seed pipeline and the effect on the next generation of seed.

A spokeswoman for Plant and Food Research says the insect-pathogen relationship is one of a number of plant pests and diseases the institute is investigating.

The work ranges from management of established pests, like TPP, to developing surveillance methods and strategies to pre-empt the arrival of those that aren’t yet in NZ, such as the brown marmorated stink bug.

About 20% of the institute’s $145 million annual revenue is allocated to bioprotection research. 

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