Taewa and psyllid resistance

New Zealand is a nation of potato lovers. We love our spuds, and for many of us, they are a key source of vitamins, minerals and phytochemicals. We’re also very efficient potato growers – growing around 525,000 tonnes of potatoes a year, of which 25% are exported.

In 2006, the tomato/potato psyllid (Bactericera cockerelli) arrived in New Zealand. Nobody knows how it got here, but it is a major threat to our horticulture industry, particularly commercial growers of potatoes and tomatoes.

New Zealand needs to find ways to reduce the risk of this insect and the damage it can cause to our potato crops. This is where Plant & Food Research scientist Aleise Puketapu comes in. Aleise had a background in research of different heirloom varieties of taewa – Māori potatoes – and was charged with investigating these to discover if any are naturally resistant to the psyllid.

The tomato/potato psyllid feeds on potato and tomato crops like an aphid – it inserts its stylet into the plant, sucking up the sap. The psyllid is a vector (carrier) for bacteria that causes Candidatus Liberibacter solanacearum (zebra chip disease). As the psyllid sucks up the sap, it injects this bacteria into the plant. The resulting disease causes discolouration of leaves, and the plant becomes stunted. New growth is affected, including production yield – the number of potatoes per plant. The disease also causes a discolouration of the carbohydrates within the plant tuber – the potato. The discolouration becomes more pronounced when the potato is fried, making it cosmetically unacceptable for commercial foodstuffs like potato chips.

Insecticide sprays can reduce the effects of the tomato/potato psyllid and zebra chip disease, but growers want a more economic and environmentally friendly option.

Aleise Puketapu from Plant & Food Research has set up a project to investigate whether any of the taewa varieties grown in New Zealand have a natural resistance to the psyllid.

Te whāinga – the goal

If Aleise can identify taewa varieties that have a natural resistance to the tomato/potato psyllid, these could be cross bred with commercial varieties to introduce better resilience.

The genetics of these rare heirloom taewa are also important to maintain genetic diversity for future pest and disease management, especially as many pests can build up a resistance to sprays and applications used to eliminate them.

In order to assess the many varieties of taewa, Aleise is working with Dr Nick Roskruge and Tāhuri Whenua National Māori Vegetable Growers Collective to source different varieties. They are helping her to reach out to different marae and whānau groups who may have rare varieties of taewa growing in their gardens.

I’ve been able to access over 30 varieties of the 70 that are known, actually gaining the support of Māori and being able to use their knowledge and combine that with modern science. It’s huge for this project.

Aleise Puketapu

Solution

At the time of filming for the Mātauranga Project, Aleise had used three taewa varieties in two trials. One trial was to look at the yields under three different insect control regimes – full spraying, mild spraying and no spraying.

The second trial uses scientific observation. Aleise takes the taewa tubers and assesses them for the level of discolouration from zebra chip disease. She looks at both the raw and fried potato.

One of the three cultivars, the tūtaekurī, was showing some resistance to the tomato/potato psyllid. This could be of huge benefit to our potato and taewa growers.

Update

The overall research programme investigating taewa resistance to Candidatus Liberibacter solanacearum (Lso) finished at the end of 2015.

These were the main outcomes of the research, based on a single year of results:

• A full or low insecticide spray regime does not fully protect against the tomato/potato psyllid (Bactericera cockerelli) nor does it guarantee an Lso-free crop. However, the spray regime did not significantly affect crop quality or yield.
• Observation of the growth habits of 17 taewa lines or varieties coupled with diagnostic DNA testing confirmed that taewa were susceptible to Lso infection. While Lso was detected in some of the North Island taewa lines, it was not found in any of the South Island lines.
• Seed tuber size can have a significant effect on yield, with larger seed tubers (40–74 g) performing better than smaller seed tubers (<40 g) when planted at the same density. This has implications for small-scale taewa growers where it is common practice to only save the smallest tubers as seed stock.

Aleise’s research has provided the first information about the incidence of the virus in taewa and has highlighted new areas for future research such as potato virus loads and the effect of such viruses on taewa production.

On completion of the programme, Research Associate Aleise Puketapu transferred to Plant & Food Research in Te Puke and is now part of the Kiwifruit Entomology team.