Potato Review

34 POTATO REVIEW MARCH/APRIL 2020 VIRUS FOCUS Why is virus becoming a problem? • In the mid-70s, the persistent potato leaf roll virus (PLRV) was the dominant cause of virus symptoms across the UK, but these are superseded by the non-persistent potyviruses. • ere are many di erent PVY variants, and these can recombine to produce new, more virulent strains, which outcompete others and rise to dominance. • At present, PVYN is the dominant strain in the UK. It causes milder leaf mosaic symptoms than other PVY strains, so is harder to detect in visual inspections relied upon in Certi cation schemes. • Some of the top 30 GB varieties are asymptomatic when infected with PVY, so can provide an undetected infection source for surrounding seed stocks. • A sub-strain known as PVYNTN is also on the rise, which causes severe necrosis in daughter tubers grown from infected seed. • It is estimated that PVYNTN cost ware growers an average of £50,000 in lost marketable yield where infected seed was planted in 2019. • Non-persistent viruses are more easily acquired and spread by aphids than persistent virus, making control with insecticides more challenging. •Key species common across British seed potato growing areas have developed resistance to pyrethroids, compounding the challenge for seed growers. important for his seed crops. If aphids are ying, he advocates starting programmes at 50% crop emergence and applying a pyrethroid insecticide at least every seven days thereafter until haulm destruction. As an anti-resistance strategy when colonising pyrethroid-resistant Myzus persicae are active, he uses alternative modes of action such as Insyst or Biscaya [both neonicotinoids] and Teppeki at 14-day intervals, except in high pressure, when intervals are closed to 7-days. “ e reason why we’d only use these at 14-days is because you only have four applications in total [two neonics and two Teppeki], so if you use them all at 7-day intervals you only get a month’s cover. “Later in the season once crops have nished owering, Movento is also an option if necessary,” said Graham. He adds that rapid haulm kill in seed crops will also be essential, as slower burndown or regrowth can prolong exposure to late virus infection, so options now diquat is unavailable should be carefully considered. Use wisely Nick warns that with Biscaya’s imminent withdrawal and resistance to pyrethroids in some aphid species widespread, growers only have three systemic insecticides from 2021 and these should be used with care to ensure their longevity. He adds that spraying decisions should be informed by decision support tools such as Rothamsted and SASA’s suction trap data and/ or yellow water traps run by FERA or growers themselves. “As the toolbox shrinks, we have to use them only when necessary to reduce selection pressure and maintain their e ectiveness. “Products like Insyst are the only option for controlling colonising aphids such as Myzus persicae, so their longevity is vital in keeping a lid on potato leaf roll virus (PLRV), which still remains a threat,” adds Nick. Controlling potato virus – key points • Non-persistent virus increasing in seed • Change in seed-growing practices needed to halt rise • Seed testing to be cornerstone of future strategy • Consider varietal propensity and location • Cross-rotation volunteer control vital in reducing virus reservoir • Mineral oils should play an increasing role Tuber Symptoms Photo: BLACKTHORN ARABLE Tuber Symptoms Photo: BLACKTHORN ARABLE