POTATO growers need to learn to live with fungicide-resistant late blight strains that are now established across northern Europe, with good hygiene practices and mixing and alternating of different fungicide modes of action key to maintaining control.
That is the view of Shropshire-based potato agronomy specialist Denis Buckley, who has been warning for some time that the European industry practice of blocking fungicide applications is irresponsible, and risks resistance development.
The use of blocking, whereby three or more consecutive sprays of the same fungicide are applied to a potato crop, has been widespread on the continent for many years, despite a couple of scares, one major, one minor.
The major scare came when the fluazinam-resistant EU_37_A2 genotype emerged a decade ago, with blocks of the active substance being used alone in late blight control programmes in countries like The Netherlands. This strain then spread to the UK, causing severe storage losses in many crops.
After a drop in fluazinam use the genotype never came to dominate, but the practice of blocking other fungicides continued, and the consequences are now clear to see.
European spread
The EU_43_A1 lineage was discovered in Denmark and has spread across Europe. Some 43_A1 isolates are completely resistant to CAA inhibitor fungicides like mandipropamid, in the product Revus.
In 2023, it was responsible for about 36% of late blight samples processed by the European-wide monitoring initiative Euroblight and some 43_A1 isolates have also proven to be resistant to OSPBI fungicides like oxathiapiprolin.
In the same season, newly reported genotype EU_46_A1 was responsible for 96 positive late blight samples and is also resistant to OSPBIs.
“The European industry has always been hostile to mixing and alternating, perhaps for practical simplicity, and it’s finally bitten them in the rear end.
“Having genotypes resistant to CAAs and OSPBIs is a major blow. If you take away these two important chemical groups, it puts a lot of pressure on all the others, apart from mancozeb, which is very low risk in terms of developing resistance,” says Denis.
The genotypes of concern have not yet been discovered in the UK, but Denis reminds growers that the distribution data of late blight genotypes relies on samples sent in by growers and agronomists.
Therefore, it is perfectly feasible that they are present in the UK somewhere but haven’t yet been picked up.
“Seed is a very important source and isolated outbreaks [of 43_A1] in Brittany, Portugal and Ireland have almost certainly been seed related.
“I advised growers not to buy imported seed this year, but with availability a problem, there will inevitably be stocks coming in, which carries a real risk of importing fungicide resistant strains with it,” warns Denis.
So, what does this mean for the upcoming season? Find out more in the next issue of Potato Review.
