Potato Review

www.potatoreview.com POTATO REVIEW MARCH/APRIL 2023 31 BLIGHT “We are also increasingly nding volunteer potatoes in cover crops and this worrying problem threatens the uptake of “regenerative farming” where potatoes are grown. We really need some novel thinking here.” Tight intervals Looking at fungicide programmes, Martyn says it is di cult to be prescriptive for the season ahead, but the aggressive nature of dominant blight strains and resistance management should be front of mind. “Stick to tight intervals between sprays, and not simply because of the limits of product persistence, but also coverage. When applying blight fungicides, it’s surprising how much leaf is missed on each pass, so applying single products regularly is better than big mixes at wider intervals, helping to keep protection topped up, particularly on new growth,” he said. “However, there does need to be some common sense if the conditions mean risk is very low. ere may be a case for applying less often when it’s very hot like last year, but we know newer strains still cycle when it’s cold.” ‘Hit outbreaks hard’ If there is an early outbreak in a crop, Martyn says to burn patches out immediately, and treat surrounding crops quickly, as once blight gets established it is extremely di cult to stop. is could involve an application of uopicolide + propamocarb hydrochloride followed up again soon after – within three days – with another robust product such as benthiavalicarb + oxathiapiprolin. Kickback provided by fungicides is limited. “Once tubers are present and bulking, it is essential to use products with tuber blight activity, rather than waiting until the end of the season. You need to be decisive and go at it hard when there is a risk. at will allow you to back o when things have stabilised again. Otherwise, you will chase it all year.” Similarly, late in the season when weather is cooling down and conditions can be more humid, inoculum levels can soon increase on volunteers nearby, and growers must remain vigilant right to the end. Resistance management Considering product choice and resistance management, Martyn says it is vital to protect what we have, as the industry has lost many fungicide options to resistance as well as regulation. Typically, when a product is lost, the one used in its place is more costly, and with the potential of another resistance strain moving into GB, following FRAC guidance is important. at means using a diverse range of modes of action, including multisite mancozeb while it remains available, both in mixtures and in alternation. He says there is also an argument to incorporate actives now considered a resistance risk, if they are supported. is includes uazinam, in products like Shirlan, which according to DEFRA’s pesticide use survey has dropped in use since the discovery of insensitive isolate 37_A2. Martyn asks if the industry is in danger of increasing risk of insensitivity in other strains by discarding active substances too soon, when they are still active against dominant strains like 36_A2 and 6_A1, but not 37_A2, which isn’t necessarily there. “Fluazinam has good activity on sclerotinia and has a place around petal fall. It’s also active on botrytis and this disease can sometimes make it very di cult to spot late blight. We know that sclerotinia can damage stems, allowing bacterial infection in, and can hinder maleic hydrazide uptake, which is now critical in stored crops.” Cyazofamid importance He stresses the important of protecting QiI fungicides, a group which includes cyazafomid, the active in Ranman Top, as it has excellent activity against tuber blight. It could be a solution to use uazinam in mix with mancozeb to reduce pressure on cyazofamid. “We really must look after the tuber blight materials because we have so few. So much revolves around having the system to track changes in populations and we risk losing this, which is a terrible prospect for the industry,” Martyn said. On mandipropamid, he said in theory, it’s perhaps better to continue using it as adding mixing partners with Revus would be expensive and it’s not a time to be increasing costs. David agrees that actives like uazinam and mandipropamid should not necessarily be excluded from the toolbox, as sensitivity testing by Alison Lees at JHI shows they still o er good activity on key genotypes so should be used as per updated FRAC advice. However, without continued funding for Fight Against Blight, intelligence on the blight population could be limited and reduce the industry’s ability to react to issues during and between seasons. “Interestingly, lower temperature didn’t influence the sporulation capacity to the same degree, so it slows the infection, but not necessarily the spread. Ed Bingham, Potato Crop Manager for Europe, Certis Belchim With more aggressive genotypes like 36_A2 now dominant, improved attention to detail when eliminating primary inoculum sources like volunteers in other crops is critical for sustainable late blight control.