Should a multisite fungicide be used in wheat this season?
© Gary Naylor Photography With no concrete case for routine inclusion of other multisites following the loss of chlorothalonil, the level of septoria risk should be the only deciding factor for their inclusion this spring.
The last couple of years has seen plenty of discussion about the impact of chlorothalonil’s withdrawal on disease control programmes and what, if anything, should be used in its place.
Just two conventional multisite fungicides remain, namely folpet and mancozeb, and all manufacturers can show data that shines a positive light on their respective products.
However, a pecking order has emerged from the flurry of independent trials since chlorothalonil was earmarked for withdrawal and sees folpet placed just ahead of mancozeb as a septoria protectant.
This pecking order and uncertainty over mancozeb’s future availability means folpet has taken the limelight in discussions about the future role of multisites in septoria control.
See also: Tackling rust and chocolate spot in beans post chlorothalonil
Folpet
Despite being top of the tree, folpet’s higher cost is putting some growers and agronomists off, particularly because to get close to the efficacy to 1 litre/ha of chlorothalonil, it has to be used at a full label rate of 1.5 litres/ha.
Syngenta will be marketing its own folpet brand Mirror this year and the company’s Jason Tatnell says its cost relative to chlorothalonil is now a moot point, as it has gone, and the industry look forward with the solutions available.
He accepts that growers will need to compensate with dose and that comes at a price, but technically folpet can add to septoria programmes, as shown in the latest AHDB fungicide performance work.
“If you look at what a single 1.5 litres/ha dose can do compared with a straight SDHI in fluxapyroxad or a straight azole in prothioconazole, it is comparable as a protectant,” he says.
Adas have conducted some more detailed work on folpet in combination with other products and this has led to the research group’s Jonathan Blake concluding there is a case for the inclusion of folpet.
One of the questions Mr Blake has regularly been asked is whether growers can simply up rates of SDHI and azole actives to achieve the level of disease and cost control as a three-way mix that included chlorothalonil.
Trials near Ross-on-Wye aimed to answer this question last year, with the dose response of SDHI-azole product Ascra alone and Ascra with a 1.5-litre addition of folpet product Arizona compared on the variety Kerrin (septoria resistance score 4.8).
Actives
- Ascra – bixafen + fluopyram + prothioconazole
- Arizona/Mirror – folpet
- Revystar – fluxapyroxad + mefentrifluconazole
Better control
The results showed a clear improvement in disease control where the Arizona was added, and interestingly, increasing the rate of Ascra couldn’t achieve the same control as the Ascra-Arizona mix.
He also looked at cost and how much disease control you get for each pound spent when using Ascra alone versus Ascra plus Arizona.
On a budget of £40/ha, Ascra was providing about 35-40% septoria control, while the addition of Arizona boosted control to about 45-50% in this particular trial. This also translated into increased yield, resulting in an improved margin over fungicide costs (with wheat at £165/t).
Very similar patterns were also seen when comparing Revystar with Revystar plus Arizona at 1.5 litres/ha.
Mr Blake says it has been easy to dismiss folpet when chlorothalonil was providing an eight- to 10-fold return on investment, depending on season.
While folpet provides less of a return, it still provides a decent return on investment where septoria requires control, plus gives growers an element of risk management in a high-pressure season.
“I don’t think it’s appreciated how much chlorothalonil has been providing and this has potentially lulled everyone into a false sense of security, particularly after a run of dry springs.
“If we have something approaching normal rainfall during April and May, programmes reliant on shifting SDHI-based chemistry may not provide the control we need.
“Folpet can help insure against significant septoria control failures and there is a case to include it at T1 and T2,” explains Mr Blake.
Resistance management
Mr Tatnell says this level of efficacy, combined with folpet’s multisite activity, makes a good case for its inclusion for resistance management, helping to slow the erosion in efficacy of single site mode of action like azoles and SDHIs.
He explains that a multisite like folpet takes out a base level of disease in any given scenario, reducing selection pressure across the board. That contribution will remain stable over time, as multisites are very low risk for resistance.
“While it is impossible to stop resistance developing completely unless you don’t use fungicides at all, multisites should slow the pace of development of resistance to the single site chemistry and that is the name of the game,” says Mr Tatnell.
Niab has also done plenty of work comparing multisites and investigating their impact on resistance management, and the group’s technical director Bill Clark offers a slightly different perspective.
He says folpet’s activity against septoria is not in question and the active can also provide some crop greening effect where included, but translation into yield improvement is inconsistent.
Furthermore, Mr Clark points out that folpet’s value in a resistance management strategy is also far from clear cut.
Mix and alternate
Strong evidence exists that reducing selection pressure is best achieved by mixing two or more actives with similar efficacy. Alternating different combinations through the programme is also sound advice.
Ascra and Revystar offer that desirable balance in one can and these products also have systemic activity, so offer some level of curative activity on infection already in the leaf.
Furthermore, fungicide resistance monitoring shows that there is no cross resistance between the azoles in Ascra and Revystar, so Mr Clark argues that utilising them in sequence at T1 and T2 provides another layer of resistance management.
This poses the question: does adding folpet to Ascra or Revystar further reduce selection pressure?
“It’s a complicated experiment to find that out, but the work has been done by Lise Jørgensen at Denmark’s Arhus University, and I’m afraid the answer is no,” says Mr Clark.
Risk-based decision
Asked whether he advocates using folpet in wheat fungicide programmes, Mr Clark is clear that on weaker varieties in wetter western areas it could find a home at the T1 timing, but not in other parts of the UK in most seasons.
“A 1.5-litre/ha dose of folpet will cost about £12/ha and I think anywhere else [in the UK] you would be better off spending that money on higher rates of core mixtures like Ascra or Revystar.
“Particularly at T2 where you want some eradicant activity on leaf 2. Folpet doesn’t provide that, so it’s not complimentary to the other actives either for efficacy or from a resistance management point of view,” he explains.
Views from the field: Multisite use in 2021
As things stand, Association of Independent Crop Consultants (AICC) member Jock Wilmott doesn’t plan to recommend any multisite fungicides in wheat crops this season.
Conflicting expert views on folpet have created uncertainty about a consistent return on investment for his clients and advising across Bedfordshire, Buckinghamshire and Cambridgeshire, septoria is not always severe.
Cost is also a factor, with growers having to pay about twice the price for an active ingredient that is considered about half as effective as chlorothalonil on a weight for weight basis.
However, if the season turns out to be wet and conducive to septoria progression, and where varieties are drilled early, canopies are lush and a variety is weaker, then he may reconsider.
“In those situations, I’d be talking things through its merits with the grower and if we decide to use folpet it’s going to be at T1. I think a T0 is purely a rust spray now,” he adds.
Specific use
Steve Cook of AICC/Hampshire Arable Systems is still undecided on the merit of multisite folpet in wheat crops and is only likely to use it in very specific and limited situations.
He advises across the south of England and in some of the wetter western areas of his patch, he can see a potential advantage of using folpet at T0 where risk is high and a variety vulnerable.
In these higher risk scenarios, he also admits there may also be a case for applying folpet at T1.5 to offer some protection on final leaf 2.
However, he notes that many growers – particularly as you go further west – are simply not growing susceptible varieties any longer, meaning that T0s are largely redundant except where rust is present.
“We are also trying to keep the fungicide budget similar to last year, because I don’t see why the farmer should necessarily pay more for an anti-resistance argument that is fairly poor,” says Mr Cook.
