Potato Review

12 POTATO REVIEW MARCH/APRIL 2022 VIRUS / APHIDS What do these changes mean for aphid control programmes? After another mild winter, all the signs are that colonising aphids will arrive early again in 2022 and Eric says this is causing some concern. “You will get PVY transmission from colonising peach potato aphids and we know they are the vector of PLRV too, so susceptible varieties will be at greater risk at both ends of the season. With the chemical armoury we have now, it is impossible to cover the 8-9 weeks that seed potato crops will be exposed, so we need to look at all other tools in the box to try and mitigate risk,” he said. He adds that it is also another nail in the coffin of dual-purpose crops, as lack of season long protectionmeans cropsmust be burned down and harvested as early as possible consistent with maximising the saleable seed size fraction. Leaf roll rise The well-documented rise in non-persistent virus PVYN over the past four seasons had alerted seed growers to the threat of downgrades and potential problems for their ware-growing customers. In Scottish seed crops in 2021, downgrades attributed to non- persistent mosaic virus decreased from 5% to 4.1% while persistent PLRV increased from 3.5% to more than 5%. In 2020, most incidence of PLRV was found in just five varieties – Maris Piper, Cara, Daisy, Hermes, and Maris Peer – but in 2021 it was found in a much broader range of varieties, albeit at low levels. Eric said this is ringing more alarm bells and may be down, in part, to rogueing teams not having much experience in finding leaf roll when walking seed crops. “It’s almost like smallpox. We thought we’d pretty much eradicated it, but we are now seeing increases again and the industry needs to be re-educated on leaf roll. There may also have been some complacency, with the chemical cost of a four-spray systemic insecticide programme to control colonising aphids being over £80/ha, and some may have looked to cut corners,” he said. The rising levels of PLRV, continuing threat of PVYN and further restrictions on chemical controls has the potential to create a perfect storm for seed growers in 2022, so what should they do? The first line of defence is planting clean input stocks, with tuber testing of samples from the field prior to harvest 2021 an acknowledged way of uncovering any primary infection ahead of the growing season. When sending samples to laboratories, which in recent seasons may have been focussed on PVY, growers are urged to tick the box for the lab to run an additional analysis for PLRV. This will allow any infected stocks to be flushed out of the multiplication system. Secondly, SASA has generated good data on varietal propensity to acquire virus and where seed producers are growing high-risk varieties, they should be planted well away from other stocks. Volunteer potato control in surrounding arable fields is also fundamental. Straw mulch Scottish Agronomy conducted trials last year on the practical application of straw mulch and despite some scepticism about its value in British potato production, initial results have been very positive. It is probably most relevant to high grade Field Generation (FG) 1-3 crops with a disproportionately high number of separation beds between small areas of stocks. When spread on to potato ridges shortly before emergence, a straw mulch covers the planted beds and makes it harder for winged aphids to visually distinguish between the “It’s almost like smallpox. We thought we’d pretty much eradicated it, but we are now seeing increases again and the industry needs to be re-educated on leaf roll.” Potato leaf roll virus has increased in seed crops over the past two seasons, prompting concerns of a resurgence Spreading straw mulch at 6t/ha helped to reduce PVY by 40% and PLRV by 59%