The strain, EU_43_A1, had been identified in Denmark for the past four seasons, but never identified among the multitude of blight clones that make up the population in the UK.

Research has shown that not all EU_43_A1 are resistant to the CAA fungicide group, however five outlier isolates in Denmark had been identified as resistant last season. Sensitivity monitoring programmes of the anti-resistance action group (FRAC) in 2022 showed ‘generally a full sensitive picture for CAAs over Europe, with a few suspicious isolates under further investigation’.

European research has shown that the gene mutation associated with CAA resistance is only observed in homozygous strains of pathogens, and is therefore a recessive trait in any sexual reproduction creation of new blight clones – which significantly reduces the risk of further resistant strains developing. 

In previous reports the James Hutton Institute has highlighted how difficult it is for blight pathogens to make the transition to the UK against prevailing winds.

Syngenta Technical Manager, Andy Cunningham, advised; “Revus (mandipropamid) remains as effective against the UK blight population as has always been the case. The key challenge continues to be to assure that the CAA chemistry group efficacy can be protected as an essential tool for growers.”

He reported Syngenta’s dedicated European trial protocols, in the presence of high disease pressure and CAA resistant strains, had shown using mandipropamid in mixtures with alternative modes of action had always delivered effective control of the disease. Furthermore, the strategy reduced the level of resistance in the blight population.

The research demonstrated that adding a mixture partner was the most effective strategy, along with alternating sprays with different modes of action that was also a useful additional tool. The work also demonstrated that simply raising the dose of CAA fungicide or increasing the number of sprays would prove counterproductive, thus increasing selection and the incidence of a resistant strain. 

“Our advice is for growers and agronomists to follow the FRAC recommendations,” advocated Andy. “That includes to always mix CAA fungicides with a product with an alternative mode of action on blight, as well as to alternate sprays with different modes of action where possible. The overall number of CAA fungicides in the season-long blight programme should also be limited.

“Furthermore, maintaining spray intervals to assure preventative treatment and good application techniques to achieve full protection of the crop canopy will reduce the risk of any blight developing.”