Plan blight control to reduce resistance risk

Potato growers are urged to plan their anti-resistance strategies for blight following the failure of a major control product.

Total control failure in potato blight management product mandipropamid (Revus) was confirmed in Europe last December – prompting warnings that British farmers must revise their own strategies too.

The resistant blight genotype (EU43) is yet to be identified in the UK – but growers were told to act now to protect potato crops because blight strains that start in Europe inevitably make their way over here, says Geoff Hailstone, potato lead at UPL UK.

“Farmers need to plan their blight control pro grammes carefully,” he warns. Resistance was confirmed in a joint study by leading ag ri-chemical compa nies and researchers from Aarhus Univer sity, Denmark.

While a concern, blight resistance to fungicidal products is not a novel sit uation for UK growers. Les- sons have been learnt as a result of previous fungicide resistance issues, says Mr Hailstone.

Responsible approach

“The UK has a robust blight monitoring programme and a responsible approach to control. British producers still have plenty of options to choose from, but solo product applications must be avoided.”

Instead, growers should consult their agronomists about product mixing. “The advice should be to mix blight control products with at least one active ingredient from another group. And with each application, mixes need to be alternated.”

Growers should be mindful that all carboxylic acid amide (CAA) group fungicides, of which mandipropamid is one, could pose a threat because there is a crossresistance risk between CAA products.

CAA group products contain dimethomorph, benthiovalicarb, or mandiprop-

Potato blight remains a catastrophic disease amid, says Mr Hailstone. “We need to monitor these fungicides, as there may be additional losses from this product group.”

A prudent approach would be to consider mixing single-site products like mandipropamid with a multi-site product like Manzate 75 WG (mancozeb) or using Proxanil (propamocarb + cymoxanil) which contain actives from low-resistance-risk groups.

Later in the season, Proxanil + Ranman Top (cyazofamid) are rated most effective on the Euroblight table to control tuber blight and avoid issues in storage.

Advice to protect blight fungicide efficacy

Confirmation of a single strain blight pathogen in Denmark has reinforced advice for growers and agronomists to prevent resistance occurring in the UK.

European research has shown that the gene mutation associated with CAA resistance is only observed in homozygous strains of pathogens – 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, says Syngenta technical manager Andy Cunningham.

“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.”

Syngenta’s dedicated European trial protocols – in the presence of high disease pressure and CAA resistant strains – had shown that 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. Adding a mixture partner was the most effective, 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, says Mr Cunningham.

“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.”