Potato Review

30 POTATO REVIEW MAY/JUNE 2020 BLIGHT applications per season, to a maximum of six applications. Michael said: “It is important that we continue to demonstrate good stewardship practices to protect all the late blight chemistry products in our toolbox.” Don’t underestimate strains Frontier Agronomist Fergus Hawkins says we need to make blight programmes watertight this year, and advocates the use of products o ering curative protection from day one. He said we should not underestimate the EU 36 and 37 strains. “Following the recent triple whammy of the loss of diquat, the withdrawal of Chlorpropham (CIPC) and reports of higher aphid populations following the UK’s mild Winter, it’s fair to say that the resilience of UK potato growers and their advisors was already being severely tested, even prior to the devastating news of the Coronavirus outbreak,” he said. Although planting only began in his territory a few weeks ago, Fergus, who annually advises on 600ha of potatoes across Herefordshire, is forecasting that this season could be the most challenging ever for the industry, especially when factoring in a global pandemic that nobody saw coming. “ e higher than average rainfall in my Herefordshire territory always equates to more high risk periods for blight, especially if seasonal temperatures are high. Blight control must always begin with early planning and doing everything possible to prevent blight from entering the crop from day one. Controlling volunteers in neighbouring cereal crops or in the larger areas of conservation crops that I’ve already seen going in is a must – especially when the grower may not even be fully aware of the risk,” he said. Since gaining his BASIS quali cations four years ago via Frontier’s graduate training programme, Fergus has already seen and experienced signi cant change during a very short time. Dealing with key product losses such as linuron and diquat, coupled with two new and more aggressive blight strains, has been a major challenge, as he readily admits. “36_A2 and 37_A2 are a game changer for growers, given we know they cycle much faster at lower temperatures and that the 36 strain, in particular, produces higher numbers of sporangia per lesion. In my area there’s also a high risk of seed borne blight, particularly in earlier potato crops, so the early rosette sprays start at seven-day intervals with water volumes kept at correct levels to maximise coverage of each spray. “Product choice for these early sprays depends very much on pressure. Typically I will start with cymoxanil + mancozeb or benthiavalicarb + mancozeb. But if blight pressure is high, then it’s Ranman Top (cyazofamid) or uopicolide + propamocarb – particularly if the variety being grown has an early tuber set. In a high pressure situation I will also reduce spraying intervals to ve days,” he said. On forecasting systems Fergus’s go-to option is Blightwatch UK, combining this with local data shared with other Frontier agronomists. “ rough rapid canopy and into stable growth I want to be with oxathiapiprolin for its systemic activity alongside cymoxanil + mancozeb whilst factoring in the weather, the varieties being grown and the economics. For e ective tuber blight control you have to be aware of the variety’s own tuber blight resistance rating and its end market – green top or storage. “For late season control my key choice is Ranman Top so the plan is always to try and save two to three sprays for the end of the season. Although it’s one of only three products o ering zoospore activity, the key strengths are its rainfastness and its exibility in the tank-mix with other products. Earlier in the season you can still use it alongside rimsulfuron if top up weed control support is required but Ranman Top is also useful if temperatures drop and there is an increased risk of Zoospore activity.” With diquat gone, he believes ail and spray will now take slightly longer to completely kill green leaf and stems, increasing the risk of tuber blight so he also advises saving one spray of Ranman Top to kill any remaining zoospores post ailing. Stef Dierickx, Zonal Expert in Potatoes for global agrochemical manufacturer Belchim Crop Protection, has studied the rise in outbreaks of both the 36_A2 and 37_A2 strains in the UK as well as Germany, the Netherlands, France and Belgium. Endorsing Fergus’s approach, he highlighted the devastating a ects of the EU_36 genotype on crops and advises all UK growers to be on their guard during the season ahead. “Both these newer strains, but particularly EU_36, cycle so quickly that, if they do get into the crop, they are almost impossible to stop. Even expensive block sprays designed to stop blight in the eld will not work in many cases that I have seen. “Growers need to think about building programmes around products with curative activity to block any potential infection from latent infected tubers. During the rapid canopy phase growers must maintain a very strict protection programme employing short spraying intervals and using products at their full dose rates whilst alternating di erent modes of action. I would also advise inclusion of a curative compound such as cymoxanil in all sprays” he adds. For later season tuber control, Stef advises spraying before any forecasted rain (even if it means shortening a spraying interval) so the product is ready to give protection before leaves get wet. “For late season control after crop burndown it’s worth noting that Ranman Top and amisulbrom share the same mode of action so I would advise use of Ranman Top and uopicolide + propamocarb to help manage any remaining fungicide resistance risk more e ectively,” he said. “Think about building programmes around products with curative activity.” Stef Dierickx “Once the infection is there farmers face a significant investment to try and keep it under control.” Reuben Morris

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