What Mancozeb’s withdrawal date means for potato growers

British potato growers are facing another hit to production as the withdrawal of key fungicide mancozeb comes into effect later this year and resistance to alternative actives takes hold in Europe.

Following the completion of the World Trade Organization (WTO) consultation process, the fungicide mancozeb is confirmed to expire in multiple locations on May 31, 2024.

The cut-off date for sale and delivery of all products containing mancozeb is November 30, 2024. The final storage, disposal and use period for products is November 30, 2025.

See also: Scientists plan to use genetic engineering to transform the potato sector

Loss of vital active substances

“The loss of this active ingredient would leave the industry without a vital tool for disease control and fungicide resistance management,” said Geoff Hailstone, UPL Potato’s technical lead for Britain and Ireland.

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

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

Alternative actives to combat the disease are available, but confirmed cases of resistance to some key actives in Europe are a cause for concern.

Growers can also benefit from a new fire blight fungicide launched this season (see “New Fire Blight Fungicide Available for 2024”).

ProCam agronomist Harry James emphasizes that fungicide resistance has not been confirmed in British crops.

However, he says the risks of introducing resistant varieties into imported seed are increasing due to a shortage of British seed potatoes.

“The high incidence of leaf and tuber blight in continental seed crops means there is a real risk of resistance in UK crops,” he says.

“For this reason, anti-resistance strategies – alternating mechanisms of action and limiting exposure to a single active substance – should be followed from the start.”

Potato blight

Potato blight © Procam

Resistance in Europe

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

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

“This will make it more labor intensive to keep crops clean, especially as this key active substance no longer has a healing effect.

“Instead, it can only be relied on to provide preventive activities, which means timely spraying to keep crops protected.”

Potato growers are therefore recommended to adjust pest protection programs accordingly and seek advice from agronomists to develop a strategic disease control plan.

Geoff advises potato growers to continue to adhere to the Fungicide Resistance Action Committee (Frac) guidelines, which state that products with different modes of action in the same application and alternative mixtures should be mixed into the program.

He explains that products like Proxanil (cymoxanil + propamocarb) without mancozeb will become critical and that no resistance issues have been reported to date.

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

“It could be especially useful in controlling blight early in the season,” says Geoff.

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

Cultural control

Harry emphasizes that the key to reliable control and reducing resistance risk is adapting programs to weather conditions and using forecasting tools to identify threats.

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

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

“However, there is nothing like getting into the field and putting boots on the ground to assess the crop, the severity of disease pressure and whether there is a pest infestation,” concludes Harry.

New fire blight fungicide available for 2024

A new fungicide is available for the 2024 season that combines the effective CAA leaf disease active mandipropamid with amisulbrom.

Evagio Forte, launched by Syngenta, contains two different active fungicide groups, providing a valuable anti-resistance strategy.

Syngenta technical manager Andy Cunningham notes that the spray can be used from the first flowering and can be especially useful if topped up until the end of the season.

“Growers and agronomists can use three Evagio Forte applications in their blight program at a rate of 0.6 liters/ha, depending on Frac guidelines on the total number of CAA applications.

For optimal resistance management, it should be rotated with another blight product containing active ingredients from a different fungicide group,” he says.