Potato Review

32 POTATO REVIEW MARCH/APRIL 2020 VIRUS FOCUS How the potato industry must flush virus from the system E xperts are calling for better use of existing and novel virus control methods in British seed potato crops to stop the upward trend in occurrence of the past two seasons. The hot, dry summer of 2018 saw high aphid vector pressure across Britain and seed crops were left in the ground longer than usual to bulk, exposing to plants to greater virus risk. Then in 2019 the higher virus loading in input seed stocks and continued high aphid pressure resulted in 4.7% of Scottish and 17% of English seed being downgraded due to virus infection. InThe Netherlands, 38% of certified seed crops entered at S grade were downgraded. Scottish Agronomy’s Eric Anderson believes there is much more behind the numbers than high aphid pressure, as an increasingly complex situation relating to mosaic viruses unfolds (see Why is virus becoming a problem? overleaf). “It is becoming more virulent, more difficult to rogue and to be seen by the naked eye in seed certification inspections. It’s a big challenge,” said Eric. Insecticide resistance Pyrethroid insecticides have been relied upon for many years for the suppression of non-persistent mosaic viruses, as they have provided a rapid knock down of aphids and minimised transmission of PVY. Technically, seed growers can use four applications of each different pyrethroid product to a crop and up to eight applications through a season, so significant resistance selection pressure has been exerted on aphid populations in recent decades. Work by Steve Foster, entomologist at Rothamsted Research, has shown that most grain aphids carry a “kdr” genetic mutation which confers moderate resistance to pyrethroids. He urges growers to ensure that 100% recommended rates are used in insecticide programmes and where aphids aren’t controlled, send samples into Rothamsted for testing. The odds are being stacked against producers controlling potato viruses and significant changes are needed to the way crops are produced to minimise the problem. Potato Review explores what a blueprint for future strategy may look like. Similarly, peach–potato aphids (Myzus persicae) have resistance to pyrethroids and more recently, Steve has uncovered problems with the willow–carrot aphid, too. “We don’t have a ‘susceptible baseline’ for willow–carrot aphids as we do for other species, but we have been treating samples in bioassaid with full field rates of lambda- cyhalothrin and they can survive,” he told Potato Review . Researchers at Teagasc in Ireland have also found reduced sensitivity to pyrethroids in bird cherry–oat aphids, but the pest was still controlled by field rates and only serves as a warning that another species may soon be more difficult to control, too. Product losses This is against a backdrop of insecticide losses, with Plenum (pymetrozine) a recent casualty and Biscaya (thiacloprid) likely to be unavailable after 2020. These systemic insecticides are effective against pyrethroid-resistant Myzus persicae and other colonising species in potato crops, and without them, growers will be left with just three products next year: Insyst (acetamiprid) frommanufacturer Certis, Teppeki ( flonicamid) from Belchim, which is for seed crops only, and Bayer’s Movento (spirotetramat). It should be noted that the latter cannot be used before or during crop flowering, so may be of limited use in seed crops. With the options for chemical control of mosaic viruses narrowing, Eric believes fundamental changes are required to how the industry operates. “There been much talking around the issue of virus, but the industry now needs to start walking the walk,” he said. Varietal propensity In the last issue of Potato Review (January/ February, p36), the importance of testing seed crops for virus was highlighted and this should be the cornerstone of future strategy. It could be equally useful for certified seed crops and home-saved seed. Willow-carrot aphid CREDIT BLACKTHORN ARABLE