Dr David Cooke, senior plant pathologist at the James Hutton Institute, is reporting that none of the nearly 1,300 samples submitted for late blight strain analysis have been identified as the fungicide-resistant EU_43 or EU_46.
Samples were submitted during the Fight Against Blight campaign, with samples arriving in April after outbreaks in Kent. The results are a rare positive after a difficult season to control the disease.
“[April’s] early for blight,” David notes. “With a wide range of planting dates, primary inoculum from last season’s unharvested crops and dumps that survived over the winter, blight pressure has been high and made it a difficult season to manage because of crops at different growth stages.”
Dozens of samples were received each day throughout June, with David explaining that it has been one of the most intense early seasons he has experienced while managing the campaign.
“To some extent, this will have been prompted by our plea for samples to monitor for the new CAA and OSBPI fungicide-resistant strains that have been found on the continent and in Ireland, but obviously, you can only sample blight if it is present,” he says.
Fight Against Blight is only funded to test 400 samples across the season, he notes. “But we don’t like to say no to samples, and you never know when new genotypes will arrive.”
Blight strains
Within the samples received by the end of July, 72% were found to be the aggressive EU_36 strain, which has consolidated and spread across the country.
“A further 22% is EU_6, which has been around a long time, while 3% is EU_13. That’s a little bit of a surprise as it has been on a gradual decline for years to the extent we only had one sample a few years ago with this strain.”
The only newer lineage was EU_41, which was previously seen in a single outbreak in Scotland in 2021, in a Maris Piper crop that seemingly had no connection to the continent.
“This year, we’re finding it in a few outbreaks both in Scotland and south of the border,” David says.
Testing the new strain has found it is insensitive to metalaxyl, leading to speculation that it may be linked to growers spraying off potato volunteers in carrots. While the chemical is not currently widely used in potato crops, it could be an option once mancozeb’s approval lapses.
“The concern is this strain would increase the risk of using metalaxyl in potatoes.”
No let up for blight control programmes
UPL’s potato technical expert Geoff Hailstone notes that with the continued problems that EU_43 and EU_46 are causing in northwest Europe, and with a confirmed case in Ireland, growers should not let up with their blight programmes.
He believes that mancozeb, with its multi-site activity, has been a key reason for the strains not reaching UK shores. This has been combined with the mixing of at least two actives in every application and the alternation of products.
“It’s also important to use products at effective doses to help maintain at least two actives in the growth for sufficient length of time to protect each other,” he notes. “You don’t want to leave one product doing all the heavy lifting, increasing the risk of resistance build-up.”
UPL recommends its product Proxanil (propamocarb + cymoxanil) at 2 litres/ha, with a reduced rate partner, as no resistance has currently been found to either active within the product.
“In trials, using it in a mix has given on average 14% better blight control than the partner alone,” Geoff says.
With the risk of the CAA- and OSBPI-resistant strains, he suggests that propamocarb is now the only active with reliable anti-sporulant activity.
Anti-sporulants decrease sporangiophore formation, and stop the viability and dispersal of sporangia formed, reducing the spread of disease from the initial infection point.
“The kick-back activity of cymoxanil controls infections inside plant cells up to two days after the infection takes place,” he adds.
Towards the end of the season, if growers have any of the maximum four applications available, Proxanil is a very useful partner for products such as Ranman Top (cyazofamid).In Euroblight trials, that combination has provided the best tuber blight control late in the season, Geoff says.
“It’s a good place to use that mix with activity on zoospores and tuber blight, with the Proxanil protecting the single site cyazofamid from any potential resistance build-up,” he concludes.