Mancozeb renewed for use this year to curb the spread of fungicide resistance

The longstanding fungicide’s current approval is set to expire in 2024, with its future renewal being assessed by the Chemical Regulations Division (CRD). The uncertainty around Mancozeb’s availability this year meant some manufacturers were reluctant to risk being left with stocks, thus limiting supply to farmers.

However, UPL has committed to supplying the product in 2023 to ensure growers have uninterrupted access throughout the season. Geoff Hailstone, potato technical expert at UPL, commented:

“We have healthy supplies of straight mancozeb in the form of Manzate 75WG (mancozeb) and formulated with cymoxanil in Nautile DG (cymoxanil + mancozeb).

“It provides good activity on both Alternaria and late blight. Critically, it is cost-effective, persistent and has multi-site activity, making it an ideal tank mix partner.”

Mr Hailstone added Manzate can safeguard the activity of other crop protection products, most of which are single-site fungicides with a higher risk of the crop developing insensitivity or resistance.

In the wake of blight control failures last year, fungicide resistance management is in prime focus for growers this season. The genotype associated with the spread of blight in 2022, EU_43_A1, was first identified in Denmark in 2018 and subsequently accounted for 21% of samples in 2021 and 45% of Danish samples in 2022.

Moreover, almost half of the isolates of EU_43_A1 collected between 2018 and 2022 were on the variety Kuras, which has a published high resistance to foliar and tuber blight, according to the European Cultivated Potato database.

The genotype was also detected in the Netherlands and Belgium in 2022. Concerningly, researchers at the University of Aarhus and James Hutton Institute found the first five isolates, sampled from prime potato regions in Jutland, entirely resistant to mandipropamid, a member of the carboxylic acid amide (CAA) group of fungicides.

Consequently, the EU_43_A1 is the first strain of P. infestans reported to have resistance to a CAA fungicide. The proportion of resistant isolates within the genotype is not yet known, however, it is feared that the majority are affected.

Eric Anderson, senior agronomist at Scottish Agronomy, is concerned that cross-resistance to the entire chemistry group, including dimethomorph, benthiavalicarb and valifenalate, is likely.

“We rely on CAA-containing fungicides for about 50% of our blight programmes in the UK, so this news is very concerning. EU_43_A1 has not yet been discovered in the UK. However, the fact that we haven’t found it doesn’t mean it isn’t here. For this season, we must use precaution when planning our blight fungicide programmes,” he warned.

Mr Anderson believes mixing products from different mode of action groups is essential when building a fungicide programme in order to protect crops and preserve the efficacy of available products for future seasons.

“The alternation between mixes is equally essential to prevent resistance from developing,” he explained. “In this context, mancozeb will be the tank mix partner of choice this season.

“Growers should avoid using products such as Shirlan (fluazinam), Ranman Top (cyazofamid), Revus (mandipropamid) or Carial Flex (cymoxanil + mandipropamid) without the inclusion of another at an effective dose and belonging to an alternative mode of action.”

Alternaria control is another consideration, Mr Anderson added, with several fungicides intended for this purpose being in co-formulation with a CAA-containing product.

“Unless the variety has good resistance to the disease, mancozeb will be useful in suppressing Alternaria. I would advise applying 1,000g a.i/ha of mancozeb alongside a mixture of single-site chemistry,” he said.

New products are due to be launched in 2024 that will add to farmers’ potato blight armoury. In the meantime, mancozeb remains a valuable tool to help control the disease in 2023 and preserve the efficiency of existing products.