Potato Review

www.potatoreview.com POTATO REVIEW MARCH/APRIL 2020 33 VIRUS FOCUS A potential second step is resistant varieties. Some cultivars are disproportionately affected by virus than others, but as growers often don’t have a choice in what they grow, it is not clear how varietal propensity might be put into practice. Eric believes that one way is to keep seed production of varieties with a high propensity for PVY away from areas with high vector pressure. “That isn’t just in locations such as Yorkshire, where aphid and virus pressure is high, but also coastal parts of Scotland, such as the Black Isle, where high-grade seed is produced.” This also raises the subject of the isolation of seed crops from potential virus sources, which is something that VCS potato specialist Graham Tomalin advocates across his area of seed in East Anglia. Plan ahead For site selection, he said planning ahead is key and if growing high-grade seed, think about proximity to lower grade seed crops or ware crops that are likely to have a greater virus loading. As it won’t always be practical to completely isolate crops, he said varietal propensity should also be considered and where possible, grow a low propensity variety on sites where there is high virus risk. “In Yorkshire, it might be possible to consider altitude, with higher sites a lower risk. Although it is difficult to predict aphid flight, early- planted crops seem to experience less aphid pressure, so planting date can also help.” Carrot crops With willow–carrot aphid a very efficient vector of PVY – some evidence suggests it is equal to the most efficient Myzus persicae – it has been suggested seed should not be grown nearby carrot crops. Graham said this isn’t a big issue in England, where over-wintered strawed carrots tend to be cleared before seed crops are planted. However, Eric believes it should be a consideration, as spikes in willow–carrot aphid numbers are often seen upon removal of the straw blanket in spring. “There is often a focus on where the ware crops are in relation to seed, but is it sensible to be growing high-grade crops next to carrots? This is especially so as volunteers are hard to control in carrots and they could also be carrying virus.” Volunteers Another major consideration in getting to grips with virus is potato volunteer control. Eric confirms past work carried out on behalf of AHDB Potatoes found that the 10 most common varieties found as groundkeepers in Scotland were known to have a high propensity to carry PVY. Of the volunteers found in a given location, between 2% and 57% were infected with virus and exposes the potentially huge reservoir of infection if volunteer numbers are high. In addition, they harbour late blight, exacerbate PCN and a host of soil borne diseases such as rhizoctonia and black dot, reducing the effectiveness of crop rotation. Certis potato expert Nick Badger said that some 50% of potato crops are grown on rented land and quite often controlling volunteers isn’t high on the list of priorities for landowners between potato crops, as it can be costly. However, he is aware of some growers building long-term relationships with landlords and even helping with the cost of keeping volunteers under control in other crops and stubbles around the rotation. “It needs a cross-sector, collaborative approach to reduce this reservoir of inoculum. Volunteers need to be controlled wherever they are, whether that’s in and around potato crops, potato dumps or any other crop,” said Nick. Control programmes With pyrethroid resistance a concern, particularly in important aphid species in Scotland like the grain aphid, Eric is beginning to question their use in seed crops. Migratory or “transient” aphids carrying non-persistent viruses are very hard to control, because as soon as they land and feed, they can infect plants. Even when resistance wasn’t a factor, research showed that pyrethroids only provided a 50% “suppression” of virus and that figure is likely to be falling as they become less effective. Rather than relying on pyrethroids and taking out beneficial predators for little crop protection benefit, Eric would like to see mineral oils play more of a role. Stylet cleaning Work funded by the AHDB showed mineral oils give an additive effect to virus control when used in conjunction with pyrethroids, cleaning the aphids’ stylets of virus as they feed and minimising spread. Other countries, such as Canada, are using them every seven days from 30% emergence until burndown and have succeded in supressing non-persistent viruses in seed crops. The only stumbling block at present is that few mineral oil products are available in the UK and they can only be used up to tuber initiation, giving just a three-week window of use. “The evidence to show they work is there. Growers can then switch to some of the vegetable oils after tuber initiation,” said Eric. Many vectors While Graham acknowledges the pyrethroid resistance issue, he points out that there are more than 40 vectors of non-persistent viruses and maintaining a comprehensive aphicide programme is still ➜ Plant w Virus CREDIT BLACKTHORN ARABLE Graham Tomalin

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