Most potato growers in Scotland have now started their programmes to protect 2022’s main crop from late blight, while dry conditions continue across the majority of the country.

Emergence is mixed so far, with some crops just appearing while early planted crops are meeting in the row. In the most forward crops, growers have turned their attention to getting the cleanest possible start to the season and already applied at least one fungicide at rosette.

With one eye on the evolution of blight strains growers will be keen to ensure a clean start and look to include a curative fungicide in the first couple of applications. A mixture containing cymoxanil would provide the curative activity required, as found in Option, from Corteva Agriscience.

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.

However, so far this year, Scotland has experienced a drier than average spring with rainfall limited in most regions throughout March, April and May.

Craig Chisholm, field technical manager for Corteva Agriscience, said: “Protecting crops from blight is a marathon not a sprint, and if we cast our minds back two years we had the same dry spring before the rain came in June. In terms of the weather, blight programmes are designed to insure growers against what’s to come rather than what’s already been.”

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, while the 41_A2 strain, which dominates the Nordic region, was discovered in one sample in Scotland, relatively late last season.

“While not something to be overly concerned about at this stage – as it was a single sample in 2021 – it is another reason to maintain a tight spray interval and a robust programme in 2022,” added Mr Chisholm.

Looking at the chemical groups available to UK growers, choices are not as great as may appear at first glance. “Resistance to the phenylamines has been present within the UK blight population for some years now, resulting their loss from UK programmes,” he argued. “Insensitivity to fluazinam restricts our options further.”

Although dithiocarbamates are currently available, at this point, their future looks limited going forward. “Dithiocarbamates are no longer authorised for use in Europe but is currently still available to us in the UK,” he explained. “We want to make use of that for as long as we can.”

He advised growers make use of mancozeb while it is still permitted too, because it provides protectant activity against alternaria, plus it should be mixed 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 said.

“And there's a good argument to say that we should be mixing it in with single active products, as a matter of course, 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 active ingredients at seven days.

This has greatest value when applied twice at rapid growth phase of the crop in order to make use of its systemic and curative activity to provide a clean start to the programme then retaining the remaining available applications for the stable canopy phase.

“Blight came in later in our trials, and there was lower infection during the rapid growth phase,” said Mr Chisholm. “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, added Mr Chisholm.

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.