Green light for mancozeb availability to help control the spread of fungicide resistance this spring

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Mancozeb, the multisite potato blight fungicide, will continue to be available this spring. Its use will be necessary if the industry hopes to avoid further shifts in fungicide resistance, says crop protection manufacturer UPL.

The regulatory picture for the longstanding fungicide is challenging, with Mancozeb’s current approval expiring early in 2024. The active substance renewal is currently being assessed by the Chemical Regulations Division (CRD), but no conclusion is expected imminently. The uncertainty about whether it may be available this year meant some manufacturers were reluctant to risk being left with stocks. However, UPL committed to supplying mancozeb in 2023 to ensure that growers have the option to use it this season. “We have healthy supplies of straight mancozeb in the form of Manzate 75WG (mancozeb) and formulated with cymoxanil in Nautile DG (cymoxanil + mancozeb),” explains Geoff Hailstone, UPL’s potato technical expert. “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. Manzate can protect the activity of other products, most of which are single-site fungicides with a higher risk of resistance or insensitivity developing.”

Fungicide resistance management is in focus this season with last year’s blight control failures associated with the spread of blight genotype EU_43_A1.  It was first identified in Denmark in 2018 and subsequently accounted for 21% of samples in 2021 and 45% of Danish samples in 2022. 250 Samples taken from commercial fields and trials in 2022 were identified as control failures.  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.

It was also detected in the Netherlands and Belgium in 2022. Worryingly, researchers at the University of Aarhus and James Hutton Institute found the first five isolates analysed, sampled from important potato regions in Jutland, entirely resistant to mandipropamid, a member of the carboxylic acid amide (CAA) group of fungicides. EU_43_A1 is the first strain of P. infestansreported to have resistance to a CAA fungicide. We do not yet know the proportion of resistant isolates among EU_ 43_A1, but presumably the majority because this genotype has evolved rapidly from 2021 to 2022 and very rapidly at the end of the 2022 season. Cross-resistance to this whole chemistry group, including dimethomorph, benthiavalicarb and valifenalate, is likely, explains Eric Anderson, senior agronomist at Scottish Agronomy.

Concerning news

“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,” says Dr anderson.

“When building a fungicide programme, mixes of products from different mode of action groups are essential to protect crops and preserve the efficacy of these products for future seasons. The alternation between mixes is equally essential to prevent resistance from developing. 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,” he said.

“Another consideration is Alternaria control. Several fungicides that control Alternaria are 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,” adds Eric.

New products are expected to be launched in 2024, which will help add to the potato blight armoury. In the meantime, mancozeb will be a valuable tool to help control disease in 2023 and preserve the effectiveness of existing chemistry.

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