Potato Review

www.potatoreview.com POTATO REVIEW JANUARY/FEBRUARY 2020 37 VIRUS Bigger picture While this advice can help ware growers source healthier seed, Scottish Agronomy’s Eric Anderson believes the legacy of the last two seasons should prompt more systematic action. Although these were years of exceptional aphid pressure, he says the virus problem isn’t going away and changes to the Seed Potato Classi cation Scheme (SPCS) are required. He believes the current system is not t for purpose, as crop inspections are visual and only reliable for detecting secondary infection, not primary infection that tends to be more challenging to identify within the growing season. Some varieties express virus symptoms less readily too and can be missed. Eric also raises the potential issue of there being two tests at various laboratories available to growers, with NIAB, FERA and SASA all o ering both a growing on and rapid molecular test. He questions whether all are comparable, although work carried out by Eric a number of years ago shows that both methods should provide similar results. e UK also uses a sampling rate of 100 tubers, which is divided into four tuber sub- samples or bulks. An algorithm then estimates the level of virus. is assumes that if a number of bulked samples are tested, and only a few of these are positive, then in each positive bulks there will only be a low number of positive tubers. As the number of positive bulks increases, the algorithm accounts for this by assuming that a greater number of individual tubers in each bulk are also positive. Improving confidence Eric says the tuber sampling rate needs to be increased to provide better resolution of results at low infection frequencies and give actionable data to con dently ush potyvirus out of the seed multiplication system. “It would be costly to make the wrong decision on either a false positive or false negative result for a seed stock,” he said. “For ring rot testing, samples of 400 tubers are used and for virus testing in the Netherlands, they use a minimum of 200. We need to be using at least 200 and if we want better seed health, we should potentially test more.” In addition, Eric would like to see an agreed sampling and testing protocol between the two certi cation authorities FERA and SASA, plus other testing providers such as NIAB. As with recent potato cyst nematode (PCN) testing improvements he advocates a ring test based on all participating laboratories using the same agreed methodology. is would be reinforced by a further pro ciency test where everyone tests the same material for tuber virus testing using seeded tuber virus samples. He believes there is also a case for fewer generations of certi ed seed crops, which would decrease overall virus loading in stocks. “A compulsory post-burndown, pre-harvest testing of seed might also be sensible. ere will be increased cost and value returned to the industry, but doing nothing is untenable,” said Eric. No silver bullets However, SASA Senior Virologist Christophe Lacomme cautions that compulsory testing is no silver bullet, as both virus tests have their limitations. With any test, results are only as good as the sample taken and not all sampling is carried out correctly he fears. Furthermore, primary infection is far less likely to be evenly distributed through a crop, so neither the molecular nor growing on test will give a 100% accurate prediction of nal virus levels in the following crop. “ is may be part of the reason why, in some areas such as the Netherlands, even if you have [compulsory] post-harvest testing you will still have some levels of infection, either secondary or primary, undetected. “ at is something we have to live with and the limitation of every test. “ ere is a danger in thinking introducing post-harvest testing will solve all the problems. It is more a situation of appropriate crop management, of low virus and aphid pressure and, most importantly, sourcing high quality seed to grow with the lowest virus incidence possible,” said Christophe. ➜ Eric Anderson PVY K Edward SASA Senior Virologist Christophe Lacomme

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