Potato Review

14 POTATO REVIEW JULY/AUGUST 2020 VIRUS TESTING M any 2020 seed potato crops have emerged into high aphid vector pressure and combinedwith signi cant virus carryover from 2019, risk of transmission is high. is is leading some experts to recommended additional post-burndown, pre-harvest tuber testing this autumn to identify andmanage problemstocks. Rothamsted Research suction trap data shows both the Peach-Potato aphidMyzus persicae and theWillow-Carrot aphid Cavariella aegopodii have been present in signi cant numbers across large parts of the country, encouraged by the warmand dry spring. Both these species, along with cereal aphids, are important vectors of non-persistent potato viruses, such as PVY, and seed potato growers will have been using robust insecticide programmes to limit their ability to spread virus in growing crops. However, the high pressure – combined withwidespread resistance to pyrethroids and further loss of aphicides – programmes are not guaranteed to be watertight. Compounding matters is the news that the COVID-19 pandemic has seen Animal and Plant Health Agency (APHA) inspections of most basic seed crops across England and Wales cut from two to one. Scotland’s protocol remains unchanged. is has led to worries that virus infectionmay go unnoticed in English andWelsh seed stocks and impact on further seedmultiplication or ware yield and quality in 2021. Buy certified At SASA, senior virologist Dr Christophe Lacomme says the best solution to ensure healthy seed is to purchase high-quality certi ed stocks that have been subject to robust inspection. Consider seed testing to allay potato virus fears Concerns are growing about high levels of virus transmission in potato crops this season. Potato Review asks if tuber testing can play a role in managing the problem and o ers some advice on how to get the best and most relevant results At the growing crop inspection, the tolerance within certi ed seed for PVY and PVA is 0% in pre-basic crops, and for basic seed; 0.02% in S-grade, 0.1% in SE-grade and 0.4% in E-grade, so growers should have con dence that certi ed seed has low levels of virus. “Above anything else, we would highly recommend using certi ed seed that has been inspected and graded according to the strict tolerances of the seed potato classi cation scheme,” Christophe said. However, after the high levels of virus symptoms seen inUK potato crops during 2019 and potentially high in-season transmission in 2020, growersmay want to consider post-harvest testing before planting in 2021. NickBadger of Certis, whose Tubercare initiative seeks to promote seedhealth across Britain, says the extraordinary aphidpressure across the potato growing area is potentially concerning and highlights the need for tuber testing. He says testing tubers is a goodway of seeking that extra assurance in a di cult year and provides valuable information that will helpwith decisions on the intended use of seed next spring. “ e industry has su ered signi cant yield and quality loss as a result of virus infection in recent seasons, particularly fromcertain strains of PVY. “Requesting a test by your seed supplier or arranging the test yourself is a goodway of identifying problemseed stocks early. is allows growers to react accordingly andmake the best management decisions for their potato enterprise,” explains Nick. Testing methods ere are three laboratories that provide a tuber testing service for potato growers (see panel), all covering a wide range of diseases including soil- borne pests, fungal pathogens and persistent and non-persistent viruses. For non-persistent viruses such as PVY, two test types are available. ese include a growing on test, where plants are grown from tubers to bio-amplify any virus infection. e ELISA method is then used to detect the presence of speci c viruses in the plant material. In all, the whole process takes up to six weeks fromdelivery of the sample to receiving results. e secondmethod is a direct tuber test, which uses a real-time polymerase chain reaction (PCR) to detect speci cally each virus species in tuber tissue. is process ismuch faster and take just ve days to turn results around. FERA’s principal virologist Adrian Fox says demand for real-time PCR tests is greater early (September) and late (March) in the testing seasonwhen customers require a fast turnaround. Christophe Lacomme Jane Thomas