Potato Review

www.potatoreview.com POTATO REVIEW MAY/JUNE 2021 41 BLIGHT I T’S the news that growers least want to hear. Asmany contemplate having to sell last year’s crop at prices below the cost of production, the prospect of a rise inproduction costs is likely to promote further sighs of exasperation. The reason for yet another increase in production costs is the ongoing battle in protecting crops fromthe damaging effects of late blight (Phytophthora infestans).The spread of a fluazinam-resistant strain 37_A2 (often referred to as “DarkGreen 37”) in 2016 forced a change in approach as practices evolved to show greater regard for the principles of resistance management.This led to the practice ofmore complexmixes being applied as growers sought to protect products containing only onemode of actionby adding a partner belonging to a different group.The consequence of thiswas an increase in the cost of cropprotection. The uncertain future formancozeb threatens to deliver another increase in production costs, saidEricAnderson, senior agronomist at ScottishAgronomy. Analysis by ScottishAgronomy estimates that a typical 13-spray programmewill cost roughly £280/ha in 2021, an increase of 3-5%on last year. Achieving protectionwithout the use of mancozeb-containing products, however, sees this cost rise to £330/ha, an increase of 18%. “Growers are under intense pressure tomanage ONGOING CONSIDERATIONS IN BLIGHT BATTLE Limited availability of mancozeb-containing products means fungicide programmes are set to change again. Inevitably, there will be cost implications. ➜ costs and it concernsme that such increases will only add to the sense of despair felt by many. Confidence is already lowbecause of the uncertainty surrounding our tradewith the EU and the loss of the nematicide, oxamyl,” saidEric. The reliance onmancozeb inblight programmes is highlighted by statistics fromtheDEFRA PesticideUsage Survey (see table). Potatoes were last surveyed in 2018 duringwhich 127,859 hectares of ware crop received on average 10 fungicides, three herbicides and two insecticides. Mancozebwasmost often applied in amixwith cymoxanil and together this combination alone accounted for 33%of the formulation area treated. The principal issuewithmancozeb is one of product availability following the European Commission’s vote towithdraw itsmarketing authorisation in early January. It remains authorised for use inGreat Britain as the vote took place after theUK left the EUat the end of 2020. “I’mreliably informed that production of Curzate and Invader ceased before the vote onmancozeb took place and there are no plans, at least for this season, to resume production just for Great Britain,” saidEric. Thismeans growerswill have to rely onwhat is available indistribution. “There is likely to be enoughmancozeb and mancozeb-containingmixes in the supply chain for growers to be able tomeet at least part of their needs.The difficulty occurs should the 2021 season be one of highdisease pressurewhere intervals have to be reduced orwhere supply is limited.” Time for new tactics Concerns over future cropprotection strategies raise two questions. First, what effective tank-mix partner to use as an alternative; and second, at what rate can the alternative products be safely applied to provide an economically affordable mix that won’t hasten the spread of insensitive strains through greater selectionpressure? “We need to investigate effectivemixtures at less than full label rate in combinationwith appropriate nozzle choice and other application technology tomaintainprotectionwithout increasing selectionpressure,” saidEric. “Enervin (ametoctradin)may be adopted as the successor tomancozeb as a tank-mixer merely because at £12-13/ha for a full-rate application (1.2 litres/ha) it is the next least expensive product available,” saidEric. Theongoingcostofprotectingcropsagainstlate blighthasledtoare-evaluationofthecontribution thatvarietyresistancecanplayinmanagingdisease. NewresearchcarriedoutbytheScottishSociety forCropResearch(SSCR)aimedtoprovidefurther informationonvarietalresistancetolateblight. “The role of variety resistance needs reappraising. We can’tmeet the challenge of effectively Eric Anderson Alison Lees