Blight control strategies will have to change this season if potato growers are to combat the spread of a new aggressive, fungicide resistant strain of the disease.

According to specialists from agronomy firm, Hutchinsons, the dark green 37_A2 form of phytopthora infestans has quickly spread across Europe, reaching England two years ago when five cases were reported.

Around 20 cases were officially recorded in 2017 – mainly in the West Midlands, Yorkshire and Kent, and more recently in Suffolk – but Hutchinsons root crop technical manager, Darryl Shailes, told The SF that the figure could be higher and all crops, no matter where they are grown, are at risk.

The new strain is at least, if not more, aggressive than the dominant blue 13 and pink 6, but the crucial difference is that it appears equally aggressive on foliar and tuber blight, he warned.

“There was a massive failure of fluazinam at the Eurofins blight trials last year and dark green 37 was implicated in several cases of store breakdown.” Dark green 37 has reduced sensitivity to fluazinam, which is in many fungicides and often used towards the end of the season to reduce tuber blight.

“Fluazinam has been a cost-effective option for tuber and foliar blight, especially against the blue 13 strain, but until we have access to real-time information to determine the exact genotype of blight spores present in the field, we have to question its position in blight programmes.

“The yield loss from foliar infection of blight is often small except in the worst years, but the consequential losses from tuber blight can be massive,” he added.

Spreading threat

Dark green 37 is most likely to have reached the UK in imported seed, but also spreads via airborne spores, says John Keer, of Richard Austin Agriculture.

“We expect a progression of the new strain from hotspot areas this year, but it’s hard to predict where we’ll see it. Blue 13 and pink 6 took two or three years to spread and become more dominant, whereas other genotypes, such as A33, have remained very localised.”

Dr Keer added that despite a wide diversity of blight genotypes in the UK, usually only one is responsible for outbreaks in the field. All growers should therefore be be wary of the new strain and not assume crops are 'safe', as fluazinam will be ineffective if dark green 37 is present, he pointed out.

Control options

The increasing aggressiveness of new blight strains makes it vital to manage risk from the outset, said Mr Shailes.

All inoculum sources should be controlled, including volunteers and outgrade piles. Growing more resistant varieties affords some flexibility around blight spray timings, but with ever-changing disease populations, it is only part of risk mitigation and chemical options remain essential.

“There is a myriad of materials growers can use instead of fluazinam. As with any fungicide strategy, alternate chemistry so that you’re not relying on any one active ingredient or mode of action.”

Early-season options include fungicides based on cymoxanil, mancozeb, benthiavalicarb or fluopicolide + propamocarb; mid-season, the new oxathiapiprolin-based fungicide could fit into the rapid canopy growth stage; and once through this stage, consider products with tuber blight activity, such as those based on fluopicolide, cyazofamid or zoxamide, he said.

There may still be a case for using fluazinam-based products for sclerotinia or botrytis control in susceptible varieties around flowering, Mr Shailes added, but if so, ensure it is mixed with other actives that will effectively control blight even if dark green 37 is present.

Dr Keer suggested separating blight fungicides and desiccation completely this season, rather than mixing the desiccant with chemistry that increases the risk of vascular browning.

He recommended applying fungicide two to three days after the first desiccant. “This allows product to hit the stems and leaves lower down the canopy and opens up a range of actives for potential use without increasing browning risk.”

Herefordshire-based agronomist, Andrew Goodinson, said the presence of dark green 37 in his area will prompt big changes to blight programmes.

He will avoid using fluazinam in at least the first two blight sprays and at burn-off for anything going into store, favouring cyazofamid early in the programme and mandipropamid or fluopicolide + propamocarb later. Benthiavalicarb + mancozeb is another late-season option offering tuber blight activity and alternative chemistry, he noted.

Blight control tips:

• Minimise fluazinam use

• Use a mix of different actives and alternate through the season

• Include multi-site actives

• Manage blight sources such as volunteers and outgrade piles

• Maximise use of other integrated control options, like resistant varieties, rotational breaks, and clean seed tubers.