As featured in Arable Farming Magazine
Final FAB findings
by Arable Farming Magazine March issue
Last season was the final one for AHDB Potatoes-funded Fight Against Blight reporting. Teresa Rush reports.
It may have been the last season for the AHDB’s Fight against Blight (FAB) but the blight pathogen itself shows no signs of bowing out any time soon.
Late blight was a locally serious issue during the 2021 season, with more than 200 outbreaks confirmed as part of AHDB’s last season of reporting through the FAB initiative.
More than 100 so-called ‘blight scouts’ registered to take part during 2021with 67 of them actively sending in samples.
The first blight-positive sample was received on May 26, 2021, from Kent, one of a number of highly localised blight outbreaks confirmed in the south east of England.
The first positive sample in Scotland was received on July 15, 2021, from a crop in Angus.
Outbreaks in Scotland were later than average, says Dr David Cooke, plant pathology project leader at the James Hutton Institute.
In total 1,700 samples were submitted to the James Hutton Institute for testing, with 1,170 samples genotyped.
Within-season results were provided using DNA storage FTA cards, first used in 2019.
A key message from the 2021 monitoring is that populations of phytophthora infestans continue to change.
Last season saw displacement of the 13_A2 and 6_A1 genotypes via continued expansion of the 36_A2 genotype that now comprises 40% of the sampled population, says Dr Cooke.
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,” says Dr Cooke.
In terms of inoculum, more than 80% of the population is clonal, with primary inoculum surviving in tubers, including seed, volunteers and dumps from the 2020 season.
The remaining one-fifth of blight outbreaks start from genetically diverse inoculum – ‘Other’ – most likely emerging from long-lived, soil-borne sexual oospores.
“All sources of primary inoculum should be managed carefully; long rotations help manage oospore risk,” he adds.
Local differences in genotype frequency are apparent, with 64% of samples being of 36_A2 in England compared with 6.5% in Scotland.
Conversely, almost 42% of samples from Scotland are ‘Other’ compared to a mean of 8% from England and Wales.
The 8_A1 genotype also comprised 15% of samples in Scotland but was not reported in other British crops.
The 2021 FAB monitoring also revealed a new threat in the form of a genotype never sampled previously in Great Britain.
Found in a crop in Scotland in late August last year, the 41_A2 type was first reported in Denmark in 2013 before spreading to other Nordic countries, as well as Poland and Germany.
The migration pathway of 41_A2 into Scotland and its potential impact are unclear, says Dr Cooke, but its discovery highlights the future threats of such incursions and the need to understand the spread and impact on IPM.
The sensitivity to fungicides 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 guidelines from the manufacturers and the Fungicide Resistance Action Group to protect the lifespan of active ingredients,” says Dr Cooke.
He adds that cases of blight on weeds, including hairy nightshade and thorn apple, as well as volunteer potatoes, remain a cause for concern.
Most samples sent in by Blight Scouts were from varieties with a moderate blight resistance rating (4).
As to the future, while AHDB funded blight monitoring has ended, scientists are hoping to be able to continue the work in some form, says Dr Cooke.
- P. infestans population continues to change
- New clones are putting pressure on blight management – mistakes may be costly
- Clonal dominance = most inoculum surviving in tubers (seed, volunteers, dumps)
- Soil-borne oospores are still a threat – keep rotations long
- No changes in sensitivity for seven active ingredients tested
- Control issues proved related to product choice and spray interval
- 37_A2 still prevalent – do not forget advice on fluazinam use
Fungicides for late blight control
The range of fungicides currently available for control of late blight enables management strategies that mix or alternate active ingredients across the season to minimise the risks of resistance developing in the population of phytophthora infestans.
However, due to the changing nature of the P.infestans population, fungicide resistance problems have occurred in the past.
Problems with insensitivity to metalaxyl and metalaxyl-M/mefenoxam were reported from as early as 1979 and full resistance of isolates of the 13_A2 lineage (also known as Blue-13) resulted in a marked decline in the use of this active ingredient in the UK.
Resistance to fluazinam has been reported in isolates of the 33_A2 and 37_A2 genotypes that first emerged in the Netherlands in 2009 and 2013 and were later found in the UK.
Given the recent emergence of new genotypes of P.infestans, AHDB commissioned work conducted between 2019- 2021 to monitor the efficacy of fungicides belonging to a range of Fungicide Resistance Action Committee (FRAC) groups in controlling common genotypes over time (see table above).
With clonal populations exposed to active ingredients over several years, data from 2019-2021 was examined for trends in sensitivity.
The analysis revealed no evidence of any erosion of sensitivity in any active ingredient in these tests but the 2021 result for cyazofamid should be checked in 2022, says Dr Alison Lees, potato pathologist at the James Hutton Institute.
“Slight variations between years are common,” she says.
The data provides strong evidence against the incidence of cross resistance and supports the benefit to growers of following FRAG guidelines, Dr Lees adds.
GB P.infestans genotype mapping
- Genotype 36_A2 was prevalent across all but the west of England and is dominant in the east of England
- There was a decline in 37_A2 in the east of England during 2021
- In Scotland there was a higher prevalence of Clone 8_A1
- 6_A1 remains widespread but at lower frequency
- ‘Other’ widespread but most commonly sampled in Scotland
Regional P.infestans genotype mapping
- England: Highest proportion of 36_A2 (64%). Lowest proportion of ‘Other’ (7%)
- Wales: Highest proportion of 37_A2 (41%). Highest proportion of 39_A1 tomato strain (13%). No reappearance of the 42_A2 lineage present or the past four seasons
- Scotland: Relatively low proportion of 36_A2 and 37_ A2. New genotype EU_41_A2 reported in Fife in late August 2021 in a single crop of Maris Piper. Highest proportion of ‘Other’ types (41%)