UK potato growers are facing yet another hit to production as the withdrawal of mainstay potato blight fungicide mancozeb comes into effect later this year and resistance to alternative actives strikes in Europe.

Following the World Trade Organisation (WTO) consultation process, the approval of the multisite fungicide mancozeb is confirmed to expire on 31 May 2024.

The final date for the sale and supply of all products containing mancozeb will be 30 November 2024. The final storage, disposal and use-up period for products is the 30 November 2025.

See also: Scientists plan to use gene-editing to transform potato sector

Loss of vital active

“The loss of this active ingredient would leave the industry without a vital tool for disease control and fungicide resistance management,” says Geoff Hailstone, UPL potato technical lead for the UK and Ireland.

The move comes as the Health and Safety Executive (HSE) extended the expiry date for the active by one month to 31 May 2024, accounting for delays in the WTO process.

However, industry and grower representatives continue to lobby to maintain the use of the vital active ingredient.

Alternative actives for blight control are available, but confirmed cases of resistance to some key actives in Europe is creating a cause for concern.

Growers can also benefit from a new blight fungicide launched this season (see “New blight fungicide”).

ProCam agronomist Harry James highlights that fungicide resistance has not been confirmed in UK crops.

However, he says risks of resistant strains being introduced in imported seed stock are escalating due to a shortage of British seed potatoes.

“The high incidence of foliar and tuber blight in continental seed crops means there’s a real risk of resistance affecting British crops,” he says.

“For that reason, anti-resistance strategies – alternating modes of action and limiting the exposure of any single active – must be followed from the outset.”

Potato blight © Procam

Resistance in Europe

Resistance to oxathapiprolin (Zorvec) has been confirmed in the Netherlands, Belgium and Germany.

As a result, the post-Zorvec application interval is being reduced from 10 days to seven, and it must be used in alternation with a non-carboxylic acid amide (CAA) chemistry.

“This will make it more labour intensive to keep crops clean, especially as this key active no longer provides a curative effect.

“Instead, it can only be relied on to deliver preventative activity, which means sprays will need to be applied on time for crops to remain protected,” says Harry.

Potato growers are, therefore, recommended to modify blight protection programmes accordingly and seek agronomist advice to devise a strategic disease control plan.

Geoff recommends potato growers to continue to adhere to the Fungicide Resistance Action Committee (Frac) guidance, which states to mix products with different modes of action in the same application and alternate mixes in the programme.

He explains that, without mancozeb, products such as Proxanil (cymoxanil + propamocarb) will become critical and, so far, have no reported resistance issues.

“Cymoxanil is known to be one of the few actives with kickback activity and has a very low risk of developing resistance.

“It could be particularly useful in countering blight early in the season,” says Geoff.

“Propamocarb is the only active with strong anti-sporulant activity across all known strains, has good movement in the plant and is the only active in the carbamate resistance group.”

Cultural control

Harry stresses the key to reliable control and reducing resistance risk will be adapting programmes to weather conditions and using forecasting tools to determine threats.

“With careful planning, growers and agronomists should be able to achieve good levels of blight control and stay ahead of the threat of resistance.”

Cultural controls, such as selecting varieties with better natural blight resistance, and removing volunteer potatoes from dumps and other crops in the rotation should help.

“Nothing, however, will beat getting into the field and putting boots on the ground to assess the crop, the severity of disease pressure and if a blight infection has occurred,” concludes Harry.