With the emergence of fungicide-resistant late blight strains in Europe, Potato Review joined a UPL-organised expert webinar on how should growers and agronomists should put together blight programmes this season.
While two new strains of late blight, EU_43_A1 and EU_46_A1, didn’t reach the UK’s shores last season, they were wreaking havoc with blight control across the channel, especially in the Netherlands.
As widely documented, their impact is likely to be seen in UK blight programmes this season, especially if their presence is confirmed in the country.
That’s because both strains can carry mutations that impact the efficacy of Carboxylic Acid Amides (CAA) and oxysterol binding protein homologue inhibition (OSBPI) fungicides. CAA fungicides include active substances such as mandipropamid, benthiavalicarb and dimethomorph, while the only OSBPI fungicide currently registered in the UK is based on oxathiapiprolin.
The discovery has led to updated Fungicide Resistance Action Committee (FRAC) guidelines for both fungicide groups, noted Eric Anderson, Senior Agronomist for Scottish Agronomy.
OSBPI fungicides should now not be used consecutively, and no more than three applications per season if more than 10 blight fungicide treatments are to be used. If 6-10 sprays are targeted through the season, then the guidance says only two OSBPI sprays should be used, and only one if five or fewer sprays will be used.
Corteva has gone further with its guidance, recommending applying Zorvec in strict alternation with effective chemistry with a different mode of action other than CAA fungicides. It also suggests that Zorvec sprays should not exceed 20% of the total number of sprays and that only two sprays per crop should be applied if the expected programme is fewer than 15 sprays, which is tougher guidance than FRAC’s.
In addition, Corteva says the spray interval after a Zorvec application should not exceed seven days to the following non-Zorvec treatment, and recommends tank mixing Zorvec products with mancozeb, Eric said.
He added that FRAC guidelines for CAA fungicides recommend using them preventatively in up to six applications, provided it makes up no more than 50% of the spray programme.
“They should also always be applied at the recommended rate and in a mixture with a different mode of action effective against late blight. Alternation of fungicides with a different mode of action is also recommended.”
The UK’s Fungicide Resistance Action Group (FRAG) follows these recommendations but recommends a strict alternation strategy: Switching to different modes of action at every application from the previous one and using a mixture, for as many applications as possible, that contains at least one active ingredient effective against all late blight strains confirmed in the UK.
If resistant strains are reported in the UK, mixtures containing affected modes of action should be applied in strict alternation, it adds.
To complicate programmes further, FRAC guidelines for QiI fungicides, such as cyazofamid (Ranman Top) and amisulbrom, say to apply no more than three sprays from this group consecutively and that it shouldn’t make up more than 50% of the intended spray programme, Eric said.
“Corteva is launching Zorvec Entecta, which contains both oxathiapiprolin and amisulbrom, which will further restrict applications of Ranman Top, and highlights that you need a mixing partner with cyazofamid and amisulbrom products, as they are also single modes of action.”
Dr Steven Kildea, Senior Research Officer for Irish advisory service Teagasc, agreed that protecting blight active ingredients by mixing and alternating as much as possible is critical.
“But you have to be careful about consecutive sprays, especially with QiIs not going over 50% of the overall programme,” he said.
Devising a blight programme for this season requires careful thought about using products according to their functional groups, Eric says. Blight fungicides fit into one or more groups, including anti-sporulants, protectants, translaminars, those with curative and systemic properties, or those giving tuber blight control.
Thinking about how to use all of those in a programme to best effect while wrapping it around an anti-resistance strategy is far from easy, he stressed.
Steven warned that it’s not all about the new resistant strains, either. “Other blight strains will need controlling while not putting them under increased pressure of developing resistance,” he said.
That could see a returning role for fluazinam as a mix partner. In Denmark, where EU43 was first detected, the use of fluazinam as a core, instead of CAA fungicides, helped last year while the Dutch were having such problems with blight control, says Dr David Cooke, senior research development scientist at the James Hutton Institute.
“But we do have to be concerned about EU_37_A2, which has shown resistance to fluazinam, although the frequency of that genotype in 2023 was low and localised, so that approach is possible.”
Steven believes Irish growers, who no longer have access to mancozeb-containing products, may well be using this approach this season. Planting in Ireland is two to three weeks behind this year, so crops will emerge into warmer weather, rapidly growing fields, and potential immediate blight pressure, he says.
With the risk of CAA-resistant blight strains in Ireland, he’s trying to avoid CAA fungicides as much as possible. “We’re looking at a Proxanil (propamocarb + cymoxanil) + Shirlan (fluazinam) mix for those initial sprays.”
In the UK, mancozeb, while still approved (see “Dates confirmed for mancozeb withdrawal”), will remain a crucial part of blight programmes, including at the start, Eric says. “It’s a multisite with no known resistance and will quash any blight present at the beginning of the season, helping to reduce amplification of any resistant strains present.”
The only true anti-sporulant active remaining is propamocarb, he says, which, in addition to being in Proxanil, is also one of the actives in Infinito (propamocarb + fluopicolide).
“You could lead with Infinito, or a Proxanil plus Enervin (ametoctradin) would be a strong, if expensive, mix when we don’t have mancozeb available.”
Mancozeb is the only true multisite fungicide approved for use in potatoes in the UK. It cannot be replaced with any similar products, and it will be a big loss for potato growers when it is no longer approved for use, Eric said.
More details that were shared at the seminar will be featured in the July issue of British Potato Review. Click here to receive a copy.
