As featured in Arable Farming Magazine
Protecting crops after the loss of chlorothalonil
by Arable Farming Magazine March issue
As the first full season approaches without multisite mainstay chlorothalonil, Martin Rickatson canvasses views on how to support cornerstone chemistry.
For many years the linchpin around which cereal fungicide programmes have been built, multisite chlorothalonil (CTL) is no more.
Having served farmers for so long, particularly as a septoria protectant, its final use date came midway through last season, May 20, 2021.
Its loss may well be felt not just in the short-term of the next few weeks and months and in current crops, but also longer term as its departure puts pressure on the longevity of mainstay single-site chemistry.
Alternatives are few and far between, and arguably none are a direct replacement with CTL’s capabilities.
But agronomists and researchers suggest they should be considered, particularly in high-pressure situations and locations, and with wheat varieties that score poorly for septoria.
Fiona Burnett, professor of applied plant pathology at SRUC, believes that in wheat there is a case for inclusion of folpet to add a multisite element to a fungicide mix.
“The loss of chlorothalonil doesn’t mean we’re out of tools to support single-site chemistry and folpet brings something useful to septoria management as part of a balanced mixture of the key fungicide groups, including azoles, SDHIs and newer chemistry such as Inatreq and Revysol.
It may have a role particularly with growers targeting lower SDHI rates for cost reasons.
“The question then is whether to use folpet three times at one litre per hectare or twice at 1.5 litres/ha.
In the northern UK crops stay green for longer, which means needing to manage septoria through to T3 and with folpet there is the option to use it at all three key timings.
“Best practice means it’s advisable to minimise inputs on lower risk varieties to protect the long-term efficacy of fungicide chemistry, but to do this across all of the SDHI, azole and multisite elements of a mix, rather than simply eliminating the multisite because the known product has gone.
“A three-way mix will present more of a challenge for septoria to overcome, particularly with a resistant variety, and will provide scope to reduce SDHI and azole rates in balanced mixtures and still achieve effective disease control.”
There are further alternative products, says Prof Burnett, but none appear to offer folpet’s capabilities.
“Mancozeb is one, but it’s evidently less effective, so its inclusion is harder to justify, even on a cost basis.
We’ve also done some work with sulphur, with some products showing promise, but whatever partner we’ve tried it with it doesn’t appear to work as consistently as folpet in Scotland.”
Inclusion of a multisite in fungicide tank mixes this season should be the norm and the dose rate of other actives should reflect fields’ individual situations, she says.
“Farmers tend to know the norm for individual fields and situations, and the variety rating, drilling date and weather are critical factors in septoria development.
A resistant variety obviously represents a lower risk, but so does a later drilling date.
The pressure an earlier-drilled crop is put under will become more evident as septoria invades.”
In barley the situation is a little more complicated, she says.
“Folpet does bring something for ramularia management, when added to otherwise robust programmes but it’s not as consistent as CTL.
In barley it should be used as part of a programme and never on its own, and only in the highest risk situations.
“There are few other multisite options here – mancozeb is not approved for use in barley, and while some different formulations of sulphur have been shown to provide elements of ramularia control, this hasn’t been consistent, and we need to understand more.
Ramularia’s severity is driven by crop stress and sulphur’s effect may often be indirect, courtesy of its primary role in nutrition, helping minimise such stress.”
The key issue with folpet is it isn’t chlorothalonil, says NIAB agronomist Patrick Stephenson and growers should not expect it to be a like-for-like substitute.
“Last season showed some occasional issues with its consistency and this may be because of application timing or the rates used.
But while I wouldn’t see a role for folpet on varieties with strong septoria resistance, I believe firmly in its value on higher-risk varieties – rated 5 and below – and more generally in the West, where cooler, damper conditions mean higher pressure.
Many Group 3 varieties are weaker due to the Cougar gene in their breeding.
“Much also depends on the other products in a programme.
Use of newer chemistry, such as Inatreq or Revysol at appropriate rates, will leave users less exposed to septoria and the loss of CTL.
“The other element is that with wheat at £200/tonne, growers need to calculate the level of investment they are prepared to make and ask whether they are better off investing more in higher rates of their main single-site fungicide or in a multisite to supplement and support it.”
Generally priced slightly lower than folpet’s £10-£11/litre, mancozeb may present an alternative multisite option, suggests Mr Stephenson.
“But having mentioned folpet’s consistency, it appears to be more consistent at higher rates than mancozeb and product availability may be more of an issue with the latter because of potato market demand.”
Sulphur is another potential consideration for its multisite fungicidal activity, he says.
“Much research into its use has been done in France, but the successful rates used on the continent are far higher than those we’d use here.
“When we’ve trialled and analysed the performance of chlorothalonil, folpet and manozeb, that was the order of efficacy.
There weren’t huge differences, but they were significant.
“The other issue with folpet may be cost – there tends to be a price pinch point on rate and here that would likely be at one litre/ ha.
However, it tends to perform more consistently at higher rates.”
Folpet also appears to work particularly well as a partner product if growers are using Ascra Xpro (bixafen + fluopyram + prothioconazole) rather than Revystar (Revysol/mefentriflu conazole) for performance and cost reasons, says Mr Stephenson.
The decision to invest in a multisite will depend on factors including variety, drilling date, region and weather, he says.
“November was kind this season, so growers who delayed drilling got crops in and away well, particularly in black-grass areas.
Drilling date has a significant influence on early septoria infection and later drilling helps, but a winter 1degC warmer than average, despite a few frosts, means additional fungicide programme investment where necessary is more than justified, given grain price rises.
“If a decision is made to add folpet for multisite activity on susceptible crops, particularly if targeting out-and-out yield, it will be best used on the leaves that matter – at T1, and potentially T2, depending on variety and pressure.”
Multiple factors, including shorter latent periods and the Cougar septoria strain, mean the septoria resilience of some wheat varieties is waning, says Andy Bailey, of Adama, which markets folpet as Arizona.
“With chlorothalonil gone, many programmes probably didn’t include a multisite at T2 last year.
That placed a lot of pressure on single-site chemistry.
Newer fungicides are very effective, but single-site modes of action carry an element of risk and need careful stewardship.
They are more than likely used in mixtures containing actives with other modes of action, but supplementing this with multisites is essential to prolonging their effective life.”
Mixing two single-site products or using a co-formulation with different modes of action aids resistance prevention, he says.
“But using two or more single-site modes of action in programmes will result in concurrent resistance development to those modes of action, so this is a delaying tactic for development of sensitivity shifts and later field resistance.”
Folpet does not interfere with uptake of curative single-site partners, even at T2, so helps maximise the single-site partner’s performance, says Mr Bailey.
“It’s a contact protectant and works very well at T1.
Even in a high-pressure T2 year like 2021, it will help single-site partners to perform curatively and bolster their protection.”
T1 is the key timing for Arizona (folpet) in any scenario, he says.
“If required, though, it can also be used at one litre/ha at T0.
T1 and T2 recommended rates are 1-1.5 litres/ha per application, although when bearing these timings in mind it’s important to remember the maximum application per crop of three litres/ha.
“We would recommend Arizona as a partner at one litre/ha at T0, T1 and T2 on wheats rated below 5 for septoria.
For moderately susceptible varieties rated 5, 1-1.5 litres/ha at T1 and T2 is advised, while for moderately resistant types rated 6, one litre/ha at T1 and at T2 should suffice.
There is value in 1-1.5 litres/ha at T1 even on varieties rated 7 and up.
They may have strong resistance, but that doesn’t mean they won’t suffer, especially in seasons with wet March-April periods.”
In barley, Arizona is currently approved for use against rhynchosporium only.
“It has an effect on ramularia and we’re hoping to also have a label recommendation for this in place for the 2023 season,” says Mr Bailey.