Archive Article: 1997/11/08

8 November 1997

New technology is being brought to bear on the £50m annual losses from potato nematodes, and on growers reliance on agrochemicals.

CONTROL of potato cyst nematode (PCN) by conventional chemicals is costing some growers more than £750/ha (£304/acre) but biotechnology may provide more environmentally-friendly solutions.

At IACR-Rothamsted in Hertfordshire, scientists are looking at breeding genetic resistance to PCN into new varieties of potato, and at a soil-dwelling fungus which could be applied to potatoes to attack the pests at a vulnerable stage.

Genetic engineering of PCN resistance has concentrated so far on defeating the rise in numbers of Globodera pallida, which has become the more numerous of the two common PCN species in the UK. The aim has been to use plant-derived genes which interfere with nematode digestion, causing the females to produce fewer eggs.

"Weve been able to introduce a partial resistance into plants," says Dr Paul Burrows, of Rothamsted. "It helps inhibit the build-up of nematode populations, giving the plants a better chance of thriving."

A lectin gene originally from the snowdrop plant is being used. Plants with the gene have 75-80% resistance when compared to control plants, and the gene also has some effect against other migratory soil nematodes.

However, a multi-gene approach is being pursued over the next two years to increase the durability and efficacy of the resistance. "Ideally, we need to find resistance which can be applied to all crops and all nematodes," says Dr Burrows. "But it wont be a complete cure. It will have to be used in conjunction with other control methods."

Consumer reaction to genetic engineering has to be considered. "Pallida-resistant plants could be available in five years time, but no one knows yet whether they will be marketable," Dr Burrows points out.

The decline in cereal cyst nematode has helped progress with biological control of PCN. A soil fungi attacking the female nematodes was identified.

Having created suitable soils artificially and encouraged female PCN numbers to build up, Dr David Crump, research nematologist at Rothamsted, and his team identified the Acremonium species as the most effective against both PCN species. It grows through the female nematode and sporulates, infecting other healthy females.

"It affects healthy females and immature eggs," he explains. "It doesnt have any effect on mature eggs, so its important to hit the right stage."

Getting the fungus into the field is still being investigated but one possible method is to put the inoculum round the tuber before planting.

A combination of differing strains may need to be used to get greater control and persistence.

This PCN control agent does not harm beneficial organisms but Dr Crump says it will have to be used with other control measures.

Hatching prevention, interference with root location and sex attractants are also being investigated at Rothamsted but are a long way from being field-ready. Trap cropping is being used by a few growers and the scientists are also refining its use.

Professor Ken Evans, project leader at Rothamsted, says it will give 80% kill providing timing is correct. "The aim is to get the nematodes to hatch and invade the roots, but not to reach maturity," he explains. "Leaving the crop just a week too long wont reduce populations, and leaving it a further two weeks will actually increase the problem.

"If G pallida is the main problem, the trap crop can be targeted to its slower hatching activity," he continues. "As soon as the white females can be seen emerging on the roots, its time to lift."

Best results have come from a crop planted in August when the soil is warm, and left to grow for 6-7 weeks. The plants are then lifted and composted. In some instances, it may be possible to market the produce.

"Where we planted an early variety in September, we had marketable yields of around 5.5t/ha of new potatoes, ready for the Christmas market," explains Prof Evans.

Work on trap cropping will continue this year, with different varieties, spacings and timings being investigated. Destruction of the crop with herbicides will also be examined. "Its important that root development isnt stopped too soon. And the plants mustnt be too widely spaced, as the nematodes have to colonise the roots for trap cropping to work."

Mapping nematode populations is also being assessed at Rothamsted. "But its not so that growers can match the map with variable rates of nematicide," cautions Prof Evans. "With PCN, its full rate or nothing."

The development of a new immunoassay test will make analysis quicker and cheaper.

"Nematicides cost so much that there are big potential savings to be made from more detailed maps of field distributions," adds Prof Evans. "And when used with our population dynamics model, we can show the effects of treatments on population densities.

"This means that the maps can be used for more than one year, and will allow more informed application of nematicide," he concludes.

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