Potato Review

36 POTATO REVIEW JANUARY/FEBRUARY 2020 VIRUS A jump in potato virus incidence in recent seasons should encourage ware growers to have seed tested at source before accepting delivery to avoid costly surprises. e summer of 2018 o ered a wakeup call for the UK potato industry, as levels of mosaic virus uncovered by seed crop inspections reached their highest for a number of years. is had a knock-on e ect in 2019, with some ware crops having in excess of 60% of plants visibly infected. Growth cracking, which can also be a symptom of infection by some mosaic viruses, was also reported in some varieties, leading to loss of marketable yield. In England, a signi cant 25% of seed stocks entered into the certi cation scheme in 2019 were downgraded because of virus or virus in adjacent crops. While the 2019 gures in Scotland were not as startling, with 4.7% of seed crops downgraded owing to virus infection, it was the highest level there since 2000, with the predominant virus strain being PVYNTN. is isn’t just a UK problem, either. In the same year, a massive 38% of Dutch certi ed seed crops entered at S grade were downgraded as a result of virus infection. Complex issue e cause is complex, but a perfect storm of changes in aphid vector pressure, insecticide resistance and loss of insecticides are all playing a part. Critically, there is also genetic variability in the PVY virus that makes it more challenging to detect in visual inspections prior to burndown allowing infection to go undetected. So, what does this mean for ware growers? For Senior Plant Virologist at agri-food science rm FERA, Adrian Fox, keeping faith and sourcing certi ed seed is the rst and most obvious piece of advice. Secondly, where there is a concern about the virus status of seed, growers should ask for it to be tested and to see the ensuing laboratory report. “ at is particularly important if the intention is to grow that seed stock on a further year or as home saved seed,” Adrian said. For seed destined for 2020 plantings, virus testing will now have to be carried out on samples taken from boxes in store. Adrian says every e ort should be made to take the 100 tubers required from as many boxes as possible to get a truly representative sample. Be vigilant on virus Post-burndown, pre-harvest In future seasons, ware growers should request a post-burndown, pre-harvest test, which is the most reliable and accurate way of determining virus levels in seed crops. Testing at this time picks up both secondary virus infection, which is present in the input stock, and any primary infection, which is spread by aphids through the crop during the growing season. To make a meaningful interpretation of results, Adrian adds that the sample has to be as representative of the eld as possible. “About 100 tubers should be taken in a “W” pattern across the eld and one tuber taken from each plant root. Don’t take the small tubers or the really big tubers. Go for the average size tuber from each plant,” said Adrian. Ware customers urged to ask seed suppliers for virus test after past year’s knock-on e ects. Adrian Fox