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Plan 2018 potato fungicide programmes wisely Aggressive strains demand that growers are flexible in their approach to blight control for the upcoming season.

Careful planning of potato fungicide programmes is required this season to help protect crops from key diseases and manage the threat posed by the increasing prevalence of a more aggressive late blight strain. ‘Dark Green’ EU 37_A2 was first detected in the UK during 2016, when it was attributed to just 3 per cent of infected samples tested as part of AHDB Potatoes’ Fight Against Blight monitoring initiative. However, in 2017 the fit and aggressive strain, which is highly insensitive to key blight fungicide active fluazinam and linked to several tuber blight outbreaks, increased to 24 per cent of samples. Independent potato adviser John Sarup (right) – who advises from Cheshire up to the Scottish Borders – says this rise as a proportion of the UK blight genotype population is likely to continue next season and should be considered when planning a control strategy for 2018. He witnessed EU 37’s aggressiveness at the Eurofins

trials open day in Derbyshire last September, where blight products are severely tested each year in inoculated plots. “When I turned up and saw the fluazinam-treated plots looking as bad as the untreated controls, it was a bit of an eye-opener. “Bearing in mind the whole site was inoculated with Pink 6_A1 and Blue 13_A2 strains and testing showed EU 37 to be responsible for infection, it demonstrates how aggressive it must be.”

Flexible approach Certis potato expert Laurence Power says the aggressive strain demands that growers are flexible in their approach to blight control for the upcoming season, adapting product choice to risk and making the most of the tools available. He also emphasises the importance of building an anti-resistance strategy

into fungicide programmes, not relying too heavily on one mode of action throughout the season and avoiding undue selection pressure. “Programmes should also be kept tight – ideally to a minimum of sevenday spray intervals – to keep on top of the disease, as once it’s established in the crop it becomes more difficult to control,” he explains. This is something echoed by Mr Sarup and should be noted by growers in traditionally low blight pressure inland areas such as the Scottish Borders, who are more accustomed to 10-day spray intervals. “Potato production isn’t quite as concentrated there as in Lincolnshire, for example, so pressure isn’t as high, but growers might need to be prepared to change to tighter intervals in light of what’s happening with EU 37,” he adds.

Early sprays Many growers have relied on fluazinam for their first and second sprays in the past, as it has given good foliar and tuber blight control at this early stage. However, Mr Sarup has made the decision not to recommend the active at all due to resistance concerns and will be kicking off his programmes with straight multi-site active mancozeb in low-risk situations instead. Where risk is higher, he will use a cymoxanil-mancozeb co-formulation for some additional curative activity over straight mancozeb before rapid canopy expansion starts. For the second spray Valbon (benthiavalicarb-isopropyl) + mancozeb twinned with adjuvant ZinZan offers a robust and cost

effective option, with independent trials suggesting the combination is highly mobile in the plant via leaf movement and root uptake, providing both protectant and curative activity. “In addition, you’re getting a multi-site in there, which is good for resistance management and mancozeb protects against alternaria, which is key early in the programme,” says Mr Sarup.

Active choice Following Valbon at T2, Mr Sarup will use two applications of an alternative product to take him through the remainder of rapid canopy, with careful consideration paid to mixing alternative modes of action for resistance management. Where products have activity on tuber blight, they will be better saved for later season sprays as an alternative to fluazinam, rather than using them in the rapid canopy phase. At stable canopy, he will then switch back to Valbon plus ZinZan, which will be used as much as possible within the confines of the label and rotated with other actives. “I will always have at least two modes of action in every application and maintain mancozeb levels to protect against alternaria,” adds Mr Sarup. Mr Power highlights that it is important to remember the obligations for resistance management when using blight fungicides, with the carboxylic acid amide (CAA) group, which includes actives such as benthiavalicarb and dimethomorph, not permitted to make up more than 50 per cent of the programme or used in three consecutive applications. “All products need to be used and planned very carefully to avoid further problems with fungicide resistance.” ■

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