Discovery of a late blight strain less sensitive to a key fungicide in Scotland requires potato growers to apply strict resistance management principles when planning fungicide programmes this season.

Two blight strains, 37_A2 and 36_A2, are of concern to the UK potato industry and up until last year, positive samples of either genotype have been confined to England.

Last year, however, there was an unexpected discovery of 37_A2 in SRUC potato trials at the Auchincruive Estate, near Ayr, run by researcher and blight expert, Ruairidh Bain.

He told The Scottish Farmer that the trials were intensively inoculated with 13_A2, which took hold in plots almost immediately and expressed symptoms on a range of varieties. However, at the end of season, genotype testing of leaf blight samples by David Cooke, at James Hutton Institute (JHI), revealed that 19 of 24 samples were 6_A1, two were ‘wild-type’ 13_A2 and most importantly, three were 37_A2 – a strain less sensitive to fungicide active, fluazinam.

Dr Bain was not surprised by the presence of 6_A1, but it was a surprise to see natural infection of 37_A2, as little fluazinam was used at the site and negligible selection pressure applied. In addition, the nearest geographical recorded outbreak of 37_A2 was in the East Midlands of England.

“It tells us to expect the unexpected … I think we have to regard 37_A2 as being in Scotland now, as uncomfortable as that is because of the implications for fluazinam use.”

The advice to farmers last year was to avoid using fluazinam where the sole target is late blight and that will remain in place for 2019. With 37_A2 now in Scotland, its use to control sclerotinia in potatoes is likely to apply more selection pressure for this less sensitive genotype.

If fluazinam must be used, applications should be minimal and always in mixture with a full rate of an effective alternative late blight product with an alternative mode of action.

However, for Scottish Agronomy’s Eric Anderson, the bigger picture suggests a more broad-brush approach is required to resistance management in light of the increasing prevalence of 36_A2 in England.

The genotype jumped from 2% of the population in 2017 to 18% last year, albeit from a more limited set of infected leaf samples during the hot and dry summer across the UK.

With 37_A2 having already made the leap to Scotland, it is only be a matter of time before 36_A2 follows suit and work in The Netherlands, France and at James Hutton Institute, in Dundee, suggested it is a tougher beast to control.

Being extremely ‘fit’, 36_A2 produces large lesions that sporulate vigorously and lab tests carried out by Alison Lees, at JHI, suggested it is more difficult to kill with low doses of several key late blight fungicides.

Mr Anderson says advice from AHDB in light of the results was unchanged, but believed that with deeper consideration, this is a risky strategy for the industry to take.

He noted that the most concerning shift in sensitivity is to low rates of mandipropamid, the active in Revus. The contact-acting product commonly sits in the rapid canopy phase as a standalone spray.

As the potato plants are growing quickly at this stage, a product like Revus – which is bound to leaf wax of newly developing leaves – will be quickly diluted to the low concentrations within the canopy that struggle to control blight.

“It’s not sensible, given that information, to be applying mandipropamid without a mixing partner, such as mancozeb, during the rapid canopy phase. Scottish Agronomy will not be sanctioning sole use of carboxylic acid amide (CAA) products such as mandipropamid any longer,” pointed out Mr Anderson.

Resistance management

The type of resistance causing this shift in sensitivity in 36_A2 isolates is thought to be quantitative. An example of this type of resistance is septoria in wheat, where azole fungicides have lost efficacy slowly over time.

Until now, potato growers and agronomists have not worried about any such decline, instead focussing on building the most effective programmes at least cost.

Dr Bain said the threat from new strains meant that resistance management must come up the list of priorities to protect the active ingredients that are left. This is especially important during tuber blight protection phase from late rapid canopy onwards.

General advice includes an early start to programmes and tight spray intervals of no more than seven days through to complete haulm destruction

Any cost savings could be made by using cheaper options, such as mancozeb-based products, in low risk situations at stable canopy, rather than increasing the gap between sprays, he noted.

“Growers also need to make sure there is real diversity in fungicide programmes, ensuring there isn’t selection pressure in one direction for two consecutive intervals.”

Alternate and mix

Certis potato expert, Nick Badger, agreed with those assertions and said mixing and alternating various modes of action at each timing throughout fungicide programmes is crucial in reducing selection pressure.

He added that at least two actives should be used in each application, with an added emphasis on the inclusion of multi-sites, such as mancozeb, to support those at higher risk of sensitivity shifts.

“Co-formulated products such as Valbon, which contains benthiavalicarb and mancozeb, fit nicely into the first half of programmes and tick those resistance management boxes,” said Mr Badger.

Late blight control in 2019 and beyond:

Fluazinam-resistant strain 37_A2 now present in Scotland.

36_A2 may arrive soon and is less sensitive to low fungicide doses.

Resistance management must be prioritised in fungicide programmes.

Continue to avoid fluazinam where late blight is the sole target.

Mix and alternate modes of action at each spray timing to reduce selection pressure.

Do not rely on one mode of action in any one spray and avoid ‘blocking’.

Consider stacking multi-site fungicides such as mancozeb to protect other actives.