As the 2022 maincrop begins to develop, potato growers are turning their attention to blight control programmes at a time when strains continue to evolve and the tools to control the disease are diminishing.
Last year, blight began to show itself during rain events in May, and by the time the rapid growth phase started in early June, Hutton Criteria was being met in most locations across the UK.
“We had an outbreak in the south and east, starting in Kent in mid-June,” said Craig Chisholm, Field Technical Manager for Corteva Agriscience.
“Those conditions were met pretty much continuously through the season, though the focus tended to remain in the south and east where innoculum levels meant blight pressure was greatest.”
Data from the James Hutton Institute reveals that strains of blight found in the UK continue to evolve, making for a particularly dynamic and challenging population.
The 6_A1 strain continues to account for a significant portion of the population, although it is being steadily displaced by the more aggressive 36_A2.
The resilience of 37_A2 within the UK late blight population continues to cause concern because of its insensitivity to fluazinam.
The 41_A2 strain, which has started to dominate the Nordic region, was discovered in one sample in Scotland, relatively late in the season.
“While not something to be unduly concerned about at this stage – it was a single sample in 2021 – it is another reason to maintain a tight spray interval and a robust programme in 2022” Craig says.
Meanwhile the chemistry available to growers remains limited as the number of active ingredients permitted to control late blight continues to dwindle.
“Resistance to the phenylamines has been present within the UK blight population for some years now, resulting their loss from UK programmes,” Craig says.
“Insensitivity to fluazinam restricts our options further.”
Although dithiocarbamates are currently available, their future looks uncertain.
“Dithiocarbamates are no longer authorised for use in Europe but is currently still available to us in the UK,” Craig explains. “We want to make use of that for as long as we can.”
Craig advises growers make use of mancozeb while it is still permitted too, because it provides strong protectant activity against alternaria. He advises mixing it with Corteva’s Option®, a straight cymoxanil.
“When blight pressure is high, we’ve been mixing Option to give a bit more curative activity to protectant fungicides,” he explains.
“And there's a good argument to say that we should be mixing it in with single active products as part of an anti-resistance strategy.”
The best plan of attack this season
Trials from 2021 reveal significantly better levels of blight control were achieved where Zorvec was included at a 10-day interval rather than using an alternative spray at seven days, particularly where included at both rapid growth and stable canopy phases of the crop.
“Blight came in later in our trials, and there was lower infection during the rapid growth phase,” Craig says. “So, in 2021 we saw a more pronounced benefit, in all trials, from retaining at least one or two applications of Zorvec® for use during the stable canopy phase when late blight infection was at its highest.”
Corteva’s oxathiapiprolin-based chemistry has been instrumental in meeting growers’ needs to get a start clean. Two applications in the rapid growth phase, making use of both its curative and systemic activity, provides a clean base on which to build an effective programme.
Its ability to deliver robust protection for 10 days, versus the industry-standard seven, means that if weather prevents sprayers from travelling, or capacity restricts visits to more distant crops, they will still be protected. In addition, its curative properties also deliver kickback on any latent disease in the crop.
With up to four applications permitted on a potato crop, growers are advised to apply two applications of Zorvec early in the season and retain the other two for later in the season, as blight pressure demands.
“Your strategy for using Zorvec is not going to change that much for 2022,” Craig says. “It's still an insurance policy with late blight. We want the cleanest possible start to the programme and having good curative material with systemic movement into new growth is going to be of benefit during the rapid growth phase.
“But then retaining our strongest material, that being Zorvec, for the stable canopy phase, so you have one to two sprays kept in reserve certainly makes good sense.”
Notes: This article was prepared for CPM magazine
In this video, Craig Chisholm, Field Technical Manager at Corteva Agriscience, reviews the most recent data surrounding blight strains, the weather patterns we saw in 2021 and, based on trials results, how best to use Corteva’s leading late blight fungicide, Zorvec Endavia®, to its greatest effect.