Tomato-Potato Psyllid Research Update

Plant & Food Research’s crop protection specialist, Jessica Dohmen-Vereijssen, updates the ongoing research in Canterbury into the tomato potato psyllid, and outlines how science and industry are dealing with the pest.

The tomato potato psyllid (TPP) was first recorded in NZ in March 2006.  It breeds on plants in the Solanaceae family, such as potatoes, tomatoes, eggplants, tamarillos and capsicums.  While feeding on the leaves of plants (in both adult and immature nymph stages) the insect can transmit a bacterium, Candidatus Liberibacter solanacearum, which multiplies within the plant, reducing plant health and ultimately the quality and productivity of crops.

The infection of potatoes with Candidatus Liberibacter solanacearum causes zebra chip disease in potato tubers.  This is a particular issue with potatoes grown for processing.  The disease results in an accumulation of sugars instead of starch in the potato, which burn when the potato is fried and results in the formation of unattractive black marks or stripes on chips and fries.

Zebra chip also adds a bitter flavour to the final product, which is unacceptable to consumers, and particularly damaging for processed potato exports.

For the industry the impact has also been on ‘input’ costs to try and manage the pest.

Research on the psyllid and its impacts started in earnest in 2009, because it was not until 2008 when the new-to-science bacterium (Candidatus Liberibacter solanacearum) was discovered and the relationship between the psyllid and bacterium confirmed.  Researchers have so far been focusing their efforts on trying to control the vector – which is the psyllid.  There’s been observation of increased numbers of TPP especially in the South Island.

There are a range of research projects underway; looking at the psyllid and plant interactions with and without the bacteria present, sustainable management of the psyllid, genetics of the psyllid and the bacterium and what other host plants the psyllid uses when the potato or tomato crops aren’t available.

A Plant & Food Research crop protection specialist, Jessica Dohmen-Vereijssen, says the psyllid has proved extremely tricky to research.  Efforts to develop disruptant/attractant based tools similar to those used in other horticultural crops have so far proved unsuccessful.

There is also research going into the efficiency of a selected natural enemy in managing TPP –Tamaraxia triozae - a naturally occurring parasitoid of TPP in Mexico and the US.  Tamaraxia only attacks the psyllid at a certain stage in its lifecycle but the parasitoid could be a useful biological control agent – along with other beneficial insects such as ladybird, and larvae of lacewing and hover fly.

At the same time scientists have been trying to help growers by developing a weather-based model to inform them when TPP levels are about to increase exponentially in their region and whether insecticide control is required (after checking their crop for TPP).   

However, the industry recognises that reliance on insecticides is not sustainable long term – especially given the harm they can do to beneficial insects - and that other options are needed.  There are also programmes underway to try and help minimise the risk of insecticide resistance in TPP by helping farmers be a lot more targeted about when to use sprays and rotate mode of actions.

Researchers and industry undertook a nationwide survey of TPP using double-sided sticky traps to monitor TPP numbers.  Jessica says the survey work from 2009 through to 2012 provided a baseline to support the current impression that the wider potato industry has that psyllid numbers are increasing in the Canterbury region.

Monitoring of psyllid populations demonstrated that numbers only begin to increase from early summer.  From approximately mid-January, depending on the weather, numbers dramatically increase.

Early season fresh potatoes that are harvested in the pre-Christmas period (rarely in Canterbury, but commonly in the North Island in the Pukekohe area), do generally not need spraying to manage the tomato potato psyllid.  Understanding this, and eliminating chemical control application for early season potato crops, is estimated to have saved growers more than $300 per hectare.

Monitoring with the sticky traps and conducting plant assessments are still the best way to work out when the TPP threat is present in the crop.

Jessica says there are still many knowledge gaps and future research will aim to further understand the psyllid-bacteria-plant interactions.

One of the most promising areas of research as far as the growers and the scientists are concerned at the moment is the development of resistant or at least tolerant potato cultivars using information gained by understanding the plant’s response to the bacterium that the Tomato Potato Psyllid carries.