Potato Review

56 POTATO REVIEW MARCH/APRIL 2022 BLIGHT “Tracing clonal spread and survival of late blight is critical and late blight management strategies, especially fungicide programmes, need to be constantly updated to reflect the risk,” Eric said. “The genotype mapping performed by David and colleagues at the James Hutton Institute has been fundamental to our understanding of how the late blight threat has evolved. Without it we would not have learned about shifting sensitivities to fluazinam or metalaxyl fungicides or how new genotypes have developed other fitness advantages such as the ability to sporulate outside the previously accepted temperature range,” Eric said. Across Great Britain, the late blight population continues to change, with more recent introductions continuing to edge out previously dominant strains. Across Europe too, the emergence of new clonal P. infestans, is challenging previously dominant populations, explained David. “There have been several progressions in recent years. 36_A2, which first appeared in 2017, is now the dominant strain accounting for 40% of samples genotyped. Interestingly, it has a low incidence in Scotland, typically 7% of samples, with 6_A1 (‘Pink 6’) continuing to dominate at 30% of the population. 8_A1, which is rarely seen elsewhere in Great Britain, has become more prevalent over the past three seasons and now accounts for 15% of samples. “Since the fluazinam-insensitive strain, 37_A2 arrived in Scotland’s potato crops two years later than in England, the industry had advanced warning of new advice on fluazinam use and this kept the genotype in check and prevented the crop damage and tuber blight problems seen south of the border,” David added. “41% of samples originating from north of the highland boundary fault are novel multi- locus genotypes (MLGs).” Eric Anderson, Agronomist  Populations of Phytophthora infestans continue to change – 2021 blight was locally serious with more than 200 outbreaks as part of AHDBs last season of FAB reporting across GB. Displacement of previous genotypes (13_A2 and 6_A1) via continued expansion of the 36_A2 genotype that now comprises 40% of the sampled population. The incidence of the 37_A2 genotype with fluazinam insensitivity decreased from 10 to 9% of the population with 6_A1 reduced to 24%.  The aggressive clones are putting pressure on blight management – management mistakes may be costly  Inoculum: More than 80% of the population is clonal – with primary inoculum surviving in tubers (e.g. seed, volunteers, dumps) from the 2020 season. The remaining one fifth of blight outbreaks start from genetically diverse inoculum (on the charts termed ‘Other’) emerging from long-lived sexual oospores. All sources of primary inoculum should be managed carefully; long rotations help manage oospore risk.  Spatial variation: Local differences in genotype frequency apparent with 64%of samples being of 36_A2 inEngland comparedwith 6.5% in Scotland. Conversely, almost 42%of samples fromScotland are ‘Other’ compared to amean of 8% fromEngland andWales. The 8_A1 genotype also comprised 15%of samples in Scotland but was not reported in other British crops.  New threat: A genotype never sampled previously inGBwas found in a crop in Scotland (late August 2021); the 41_A2 type was first reported inDenmark in 2013 before spreading to other Nordic countries as well as Poland and Germany.Themigration pathway into Scotland and its potential impact are unclear. Highlights the future threats of such incursions and the need to understand the spread and impact on IPM.  Fungicide sensitivity of isolates of the 6_A1, 36_A2 and 37_A2 lineages to seven key fungicide active ingredients were tested in the laboratory. No changes in sensitivity were detected but growers should follow manufacturer’s and FRAG guidelines to protect the lifespan of active ingredients. Research needs funding The continued evolution of the late blight population has emphasised the need to review late blight management in commercial crops to ensure they are sufficiently protected, suggested Eric. In north-east Scotland, there is a significant amount of non-clonal blight isolates characterised as ‘other’ genotypes. “This is tantalising evidence for oospore activity, but these have not been visualised directly in the field. Sexual recombination has generated substantial variations with 41%of samples originating fromnorth of the highland boundary fault being novel multi-locus genotypes (MLGs).The big question is: What drives regional differences and howdoes it affect management decisions?” Eric said. Themajority of samples analysed (c. 85) were fromvarieties with a blight resistance rating of 4 which combinedwith the localised dominance of strains serves to re-enforce the need for growers to follow a robust anti- resistancemanagement policy, he suggested. “It is in every grower’s interest to ensure that FRAC resistancemanagement policies are practiced. It is essential we safeguard the efficacy of the fungicides we have if we are tomaintain the productive and profitable potato sector in Scotland,” he said. “It is also vital that this work continues to be funded, but by whom is a question of debate,” he added. Post-AHDB the developments that keep potato growers at the forefront of technology will rapidly be lost. This is an opportunity for an independent potato grower-ledmembership association to fund continuity of applied research via a voluntary membership based on potato area and to build on the close links to existing knowledge- based organisations in Scotland, perhaps through Scottish Agronomy and SRUC. Eric encouraged growers to take a positive role in the ownership of information and to determine their own destiny. “A grower- led organisation needs growers,” he said.

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