Potato Review

42 POTATO REVIEW MAY/JUNE 2021 BLIGHT The five most used formulations applied to ware potatoes in 2018 Formulation area treated (ha) Average number of applications (where applied) Cymoxanil/mancozeb 535,594 4.66 Cyazofamid 201,330 2.47 Amisulbrom 124,772 2.58 Fluopicolide/ propamocarb hydrochloride 102,083 1.69 Cymoxanil 95,492 2.48 Source: FERA, Pesticide Usage Survey report 284 , arable crops in the UK 2018 stewardingmode of action groups for future use withoutmaking best use of varietieswith a good level of disease resistance.The SSCRwork suggests that some of themore recently introduced varieties have robust resistance, evenunder pressure from36_A2, which is one of themost aggressive strains of P. infestans currently in circulation.This suggests there is the possibility to reduce rates without compromising protection,” saidEric. “We need research to identify what is the lowest effective dose of two or more products when applied in combination. For example, what is theminimum rate of Enervin needed to give protection in combinationwith Revus (mandipropamid)?We don’t knowbecause the work hasn’t been done,” he added. The SSCR studywas an important first step to filling this knowledge gap, butmore work is needed before it canbe developed into commercial practice, he believes. The one-year SSCR study, undertakenby the JamesHutton Institute (JHI), showed that overall, the current resistance ratings for contemporary varietieswere notmore severely affected by 36_A2 thanby other strains. However, some varieties such asMarkies, the secondmost widely planted variety by crop area in 2019, were susceptible to 36_A2.Thismay partly explainwhy 36_A2 is increasing in the population saidDr AlisonLees, potato pathologist at the JHI and co-author of the SSCR report. 36_A2 is thought to bemore aggressive than other strains, but thiswouldnot account for a difference in resistance scores between varieties. This is not to say that other genotypes, such as themetalaxyl-resistant 13_A2 or 6_A1 which currently dominates theGB late blight population are no longer of significant concern. “Maris Peer andPentlandDell are both known to possess anR gene (R2) that is associatedwith resistance to 36_A2 and 6_A1. However, in inoculated trials, genotype 13_A2, which is known to overcome the R2 gene, was able to cause disease,” saidAlison. The report considers how further research could improve our understanding of hownewstrains can spread rapidly. “For 36_A2 specifically, it will be useful to look at variety resistance data togetherwith aggressiveness data and fungicide test results. Thiswould give amore comprehensive viewof the reasons behind the increase in 36_A2 in the populations comparedwith other genotypeswhich are fungicide insensitive or able to overcome a wider range of host resistance genes. AHDB is currently funding the fungicide testing,” saidAlison. In agreement with Eric, she emphasised that host resistance is underutilised in late blight IPM strategies and highlighted the importance of maintaining up-to-date information on varietal resistance using the contemporary pathogen population, such as that coming from the SSCR study. Implications for Alternaria and P. infestans A separate, but related question is how to maintainprotection against early blight (Alternaria species) before stable canopy and tuber blight after tuber initiation given that an inevitable consequence of lessmancozebwill be a greater reliance onproducts containingQuinone inside Inhibitors (QiI) andCarboxylicAcidAmides (CAA) against P. infestans in themainpart of the season. Resistancemanagement guidelines allow for QiI andCAA fungicides to formup to 50% of the intendedprogramme as long as they are applied inmixtureswith a product belonging to a differentmode of action. Nomore than three consecutive applications of amixes containing QiI products canbe applied in a programme. Ensuring crops remainprotectedwithin the constraints of resistancemanagement guidelineswill involve utilising the full spectrumof available fungicides, saidEric. “If youhave a variety that is accepted to be susceptible to early blight such asMaris Piper, Rembrandt, Markies, Royal or Taurus then you will need to be proactive in your protection, certainly before full canopy, but there are limited product options. Mancozeb products, such as Manzate 75WGwill remain the first choice, but the reality is that there are limited supplies.” TheCAA-containing products, Carial Star andAmphore Plus, bothdifenoconazole and mandipropamid, offer some protection as does Narita (difenoconazole). Signum(boscalid + pyraclostrobin) is less reliable than it oncewas as diseasemonitoring in continental Europe has identifiedA. solani strains resistant to boscalid. Similarly, A. solani isolateswith resistance to azoxystrobin are acknowledged to bewidespread. Caligula (fluopyram+ prothioconazole), a new product fromBayer, is on full release in 2021 having only been available for use in trials last season. “Manufacturer trials suggest it has activity beyond other products, but little early blight in 2020 combinedwith limitedUK trialsmeanwe are yet to see it performunder pressure. Itmay be that Caligula sets the newstandard for early blight control aftermancozeb and it will need to if we are to avoid an over-reliance onCAA- containing fungicides before stable canopy as doing sowould restrict the number of Revus (mandipropamid) applications that canbemade during themiddle of the season,” saidEric. Tuber blight changes Just three active substances are recognised as having zoospore activity and therefore protection against tuber blight: Cyazofamid and amisulbrom, bothbelonging to theQiImode of action group, andfluopicolide, which is on its own as a benzamide. Fluopicolide is only available in amixwithpropamocarb as Infinitowith the latter delivering strong anti-sporulant activity. “As a co-formulation Infinito canbe applied on its own. It’s also an integral part of any tuber blight programme alongside RanmanTop (cyazofamid). RanmanTopneeds to bemixed with a product belonging to anothermode of action to protect its performance,” saidEric. The difficulties arise, he said, where growers have appliedmore amisulbrom(as in Shinkon andGachinko) during stable canopy as this will limit the number of QiI applications that canbe used at the end of the season. “Amisulbrom is in theQiImode of action groupwith cyazofamid. One to twoQiI applications should be reserved for alternation with Infinito at the end of the season,” saidEric. Growerswill be grateful that there are no newblight strains to contendwith but the increasing complexity of building a programme that is effective, affordable and in linewith resistancemanagement guidelinesmeans 2021 is likely to bring its own challenges. How growers respond to these pressureswill test their commitment to potato production.