Dr Alison Lees, a research leader for AHDB-funded research into late blight at the James Hutton Institute in Dundee speaks to Heather Briggs about this year's Fight Against Blight.
“We have currently received 576 samples from 86 blight outbreaks this season, with more arriving every day due to the weather conditions being favourable for late blight in large parts of the country through July and into August," she says.
Only two of the samples sent in have been negative for late blight, showing that the scouts are doing a great job as usual.
"This year, for the first time, we have been aiming to get a real-time picture of what's happening with P. infestans genotypes across GB during the growing season as we know that aggressive blight strains have evolved rapidly in recent years."
This means that the monitoring has been carried out slightly differently. In addition to the usual leaf samples, AHDB blight scouts have also been using the FTA cards supplied in their blight packs to collect samples.
Fresh, sporulating lesions are pressed on to the card before drying and posting back to the James Hutton Institute, allowing genotyping results to be returned to them within the week.
Most scouts have submitted both live samples and FTA cards and it's important to remember that the leaf samples are also very valuable to us, allowing specific characteristics of the various genotypes to be investigated – for example.
As part of the same AHDB FAB funding we are currently carrying out fungicide sensitivity tests on the most common genotypes with a range of fungicides. This will build on the fungicide testing result from last year, hopefully giving a clearer picture of any changes that may be occurring as a result of the new genotypes and helping to inform spray programmes in the future.
Alison says: "To date, of the 268 FTA samples that we have analysed so far, 41 per cent are genotype 36_A2. Earlier in the season 36_A2 was found in South-East England, but there have now also been reports of this aggressive strain in Lancashire and most recently in Scotland."
She adds that data shown by Audrey Derumier of Belchim Crop Protection at the Euroblight workshop held in York in May, reinforced the message that 36_A2 is a challenge to manage with its shorter latent period – faster lesion growth rate and higher sporulation capacity.
"Belchim showed pictures of a pot trial where potato plants were infected with genotype 13_A2 or 36_A2. Plants were sprayed at first signs of infection (i.e. testing curative activity). Visually the results were striking; the disease caused by 36_A2 was significantly more difficult to manage than that caused by the 13_A2 genotype."
The remainder of the FAB samples tested in 2019 so far are: 35 per cent genotype 6_A1, with a wide geographic range; seven per cent of genotype 37_A2 which is known to have insensitivity to fluazinam; five per cent each of genotypes 13_A2 and 8_A1; and nine per cent classified as 'other'. Isolates of genotype 41_A2 have not been identified in GB to date.
"To put these results in the context of the European situation, results of Euroblight monitoring in 2018 showed that despite a low blight pressure season new clones had displaced older genotypes across much of Europe.
"Combined, the older genotypes 6_A1, 13_A2 and 1_A1, decreased from 60 per cent to 40 per cent of the population from 2016 to 2018 while the newer genotypes 36_A2, 37_A2 and 41_A2 increased from 10 to 36 per cent of the population from 2016 to 2018.
"The spread of these clones reflects new challenges to late blight management, as they are clearly able to out-compete the existing types in the field. The additional diversity in the population across European crops increases the risk of blight management problems as new traits evolve such as virulence against novel host resistance genes and reduced sensitivity to fungicides."
New blight strains become dominant
The recently approved fungicide, Zorvec, appears to be finding an opportune moment for its slot early in the late blight management programme, reports Darryl Shailes, root crop technical manager for Hutchinsons.
At the start of the 2019 blight season, he thought blight strain 37_A2 (Dark Green) could become a real threat to this year's potato crop. The strain entered the country in 2017, and has been found to be both virulent and dominant, in addition to having reduced sensitivity to the fungicide fluazinam.
As a result, fungicide programmes, which had previously used fluazinam as a cost-effective option for tuber and foliar blight – especially against 13_A2 (previously known as Blue 13) – have had to change. "We really did not know where 37_A2 might appear this year, so we recommended avoiding fluazinam in blight spray programmes," says Darryl.
However, the first outbreaks of late blight in East Anglia found in early June were identified as being 36_A2. Although this genotype has not shown any resistance to fungicides as yet, it is known to be aggressive and causes large lesions. "All six samples we sent in for blight genotyping were identified as being 36_A2, so we made the decision to use the best materials available and recommended back-to-back Zorvec applications applied every seven days because the risk was so high."
Normally applications of Zorvec have a recommended interval of ten days, he notes. "What has been notable with these newer blight strains is how aggressive they are and how quickly they are able to become dominant.
"In 2017 some the samples we sent in were identified as being 37_A2 and nothing else, no mixed populations, which used to be frequently observed. Others were solely 6_A1 (formerly Pink 6) but never mixtures of genotype."
So far, there have been no incidents of tuber blight, but he does not advocate taking chances and is recommending Infinito (fluopicolide + propamocarb) and Ranman Top (cyazofamid).
However, changes in blight programmes can sometimes have unexpected consequences, he points out. "This year we have seen quite a lot of sclerotinia and botrytis in the crop, which previously had been controlled with fluazinam. In the future, we will need to consider using it at flowering to control these other diseases in potatoes."
Decision support also plays a key role in blight control, and Darryl – like most agronomists – has signed up to the Blight Watch service which warns when there has been a Hutton Criteria Event in any of the areas he has signed up for. This is complemented by Dacom weather stations situated out on the farms, so growers have the best possible information.
"It is likely that blight inoculum is present in compost heaps, gardens and hedgerows, so as the end of July is proving to be warm and wet, there are likely to be more outbreaks imminent. It will be interesting to see whether 36_A2 continues to be the dominant strain this year, or whether this now changes."
Late blight resistance variety trial
Late blight (Phytophthora infestans) resistance should be a key element when choosing varieties to grow, says James Lee, specialist potato agronomist and business unit manager at Produce Solutions.
In a trial set-up for an organic grower, James and his team found key evidence on varietal impact on infection. "We are trialling a number of white potato varieties, so, to easily distinguish between plots when it comes to lifting, we separate them with Desire as it is a red tuber and this makes it easier to identify the end of each plot," he explains.
"When blight entered the crop during the first half of June, all the Desire plants went down quickly and had to be pulled out. However, of the new varieties, only one became infected. The rest were all fine."
At the time of writing (the beginning of August), James has not yet found foliar blight in the conventional crops he looks after, but, interestingly, he has discovered suspected tuber blight in a crop of Pentland Dell.
"This was an easy find because it was the typical graininess in the surface which leads to bacterial break-down. The fact that tuber blight was found without foliar symptoms is concerning, so we need to find out more details."
Over the coming weeks he will be carefully checking the crop by digging out more samples, which, if suspect, will be sent to David Cooke at the James Hutton Institute for diagnosis.
Despite numerous Hutton Criteria late blight alerts suggesting blight pressure is high, there have been few infections so far, he notes. "It could be due to growers using the systemic new blight fungicide, Zorvec Enicade, which uses more than one mode of action, one of which is completely new." The product is aimed for use at the early development stage of the crop when the canopy is growing quickly.
"Everyone has been really focused and spraying has been done at 7-day intervals which is also likely to have played a key role in keeping infections down, too."
Nevertheless, this may change over the coming weeks, if blight lesions are incubating beneath the canopy, he points out.
"We need to be cautious of the new strains as they are aggressive and cause large lesions. So far the only evidence for lack of sensitivity of 36_A2 has occurred at vastly lower rates than those applied in the field and, in order to protect our current actives, at Produce Solutions we are endorsing David Cook's recommendations that growers follow the FRAC guidelines,” he emphasised. "We need to use the weapons in our armoury sensibly to maintain efficacy."
Growers can check the genotypes in their region here.