With the efficacy of single-site fungicides falling significantly over the last decade, and cereal diseases becoming harder to control. Alice Dyer finds out what role remaining multi-site chemistry can play in boosting control and slowing the rate of resistance.
Cereal disease control this season could be particularly challenging after two years of very low disease pressure which may have lulled growers into a false sense of security, according to Jonathan Blake, senior research scientist, ADAS.
Balancing the use of SDHI and azole fungicides to both manage resistance and control disease is one of the biggest challenges facing growers, but with the adoption of late drilling and resistant varieties, the number of applications of triazoles and SDHIs can be cut back, suggests Mr Blake.
“We need to make sure our strategies control disease first and foremost and the change in efficacy we have seen indicates that to provide a similar level of protection that we had last year or the year before, use of SDHIs and azoles should probably increase.
“But counter to that is the fact that in some circumstances we are using them when we do not need them. Where growers are sowing more resistant varieties later in the season, from mid-October, the septoria risk is significantly reduced, so the need for two SDHI/azoles is probably negligible and there is no benefit to using them twice.”
On more disease-susceptible or early sown varieties, growers should consider how to strengthen their programmes more reliably through inclusion of a range of multi-site fungicides in the programme, says Mr Blake.
“Looking at septoria control, chlorothalonil (CTL) is the most active protectant, and dose for dose mancozeb and folpet fall second, but it is not a case of just using CTL,” he says.
Mr Blake believes more growers should consider using multi-sites at T2, after a survey by ADAS revealed that only 50% of growers do.
“SDHIs and azoles are very effective, but with their decline in efficacy, there is no better time to apply protectant chemistry than when your canopy has emerged. To be reliant on chemistry that has been shown as less effective at your most important time seems foolhardy.”
Andy Bailey, fungicide technical specialist, Adama, says multi-sites can be used much more widely throughout the programme, particularly at T2.
“When you start to get these issues where diseases become less sensitive to chemistry or resistance, one of the first things affected on single-site chemistry, is its curative ability,” says Mr Bailey.
Although multi-site fungicides like folpet can only be used as a protectant, forming a barrier on the leaf to prevent fungal spores germinating, Mr Bailey says they are valuable in the overall sustainability of actives like SDHIs and azoles, which should not be applied without a multi-site in the tank where septoria is the driver.
“Multi-sites offer a dual approach,” says Mr Bailey. “They work consistently well because they are not affected by resistance. If you have a multi-site in there it will control those strains less-sensitive to the partner product and slow down the build-up each time you partner it which is why they are so vital going forward.”
However, Bill Clark, technical director at NIAB questions how useful alternative multi-sites to CTL will be, if they are not adopted across the board.
“Chlorothalonil has served us very well for a very long time – it adds to the efficacy of a fungicide spray mix and it has a good anti-resistance strategy, so it is a win/win,” he says.
The only other multi-sites on the market are folpet and mancozeb, which Mr Clark says are old products that do not offer the same level of control of septoria, giving them a questionable level of use as a tank mix.
“This is an important question because that is the only reason a farmer would use them,” says Mr Clark.
“They might be employing an anti-resistance strategy, but that has little direct benefit to the individual farmer. He is doing something to try and prolong the life of fungicides in the market but if his neighbour is not doing the same thing then any benefit would be lost because ascospores from his neighbour’s’ fields around him would infect his crop the following season and they may well be resistant because of what his neighbour did,” he adds.
Mr Clark also questions the compatibility of multi-site and single-site fungicides within a tank mix because of the way they penetrate a plant.
“With lots of new chemistry coming in 2020 it is debatable whether you need a multi-site in the tank when you have very good mixture partners like Revysol (mefentrifluconazole), Inatreq (fenpicoxamid), SDHIs and existing azoles available. These are generally better matched partners because they have similar eradicant activity, mobility and persistence, so there is an argument that says you do not need a multi-site – especially if it is not adding to efficacy.”
Possible solutions to fungicide efficacy challenges are being put through their paces at Zantra’s four UK trials sites in 2019, as they look for alternative methods of disease control, and the effectiveness of any new chemistry in future spray programmes.
Chris Bean, technical director at Zantra, explains there are some key issues that growers need to get to grips with this year, so initial answers from the trials will be helpful in finding the best way forward.
Septoria control remains an urgent challenge, says Mr Bean, given fears about the future of chlorothalonil (CTL) and the ever-present resistance threat.
“There is some new chemistry coming along which appears to be highly active against septoria and to offer better green leaf retention. Hopefully it will be approved by 2020 for use in wheat, so we are assessing its contribution to programmes.”
Zantra’s T0 timing trials are continuing to understand how CTL can be replaced, continues Mr Bean.
“CTL is under threat,” he says. “Whatever decision is made in Europe, it will either be subject to restrictions on its use or it will disappear.”
While there are existing alternatives in the shape of folpet and mancozeb, Zantra is taking the opportunity to look at other materials, including a new multi-site coded product already available in mainland Europe and a bio-elicitor.
“Bio-elicitors work by switching on the genes in the plant which resist disease,” explains Mr Bean. “They seem to have an effect and could be part of the future.”
To date, none of the alternatives have performed quite as well as CTL at T0, but mancozeb is coming a close second, followed by folpet, he reports.
The same is true with T1 mixes, while folpet was not far behind at T2 in a low disease year. “Even if they are not quite as strong as CTL, at least there are other options and we are all going to have to get our heads round how to get the best out of them.”
Another widely used fungicide, epoxiconazole, is also likely to disappear within the next two years, so Zantra is assessing how existing azoles prothioconazole and metconazole can step in and at what dose rate.
“Both are performing well – it is no longer appropriate to consider metconazole as an inferior product on septoria.”
ADAS has begun research into mixing multi-sites, which senior research scientist, Jonathan Blake says could be a solution, should the maximum rate of CTL be reduced.
“Multi-sites by definition work in many ways but it is not thought they all work in the same way. We do think there might be some complementarity effects when used together,” he explains.
“There is some interesting information that would indicate both mancozeb and folpet could work synergistically with CTL.”
This creates potential to develop a multi-site mixture, that uses a reduced amount of CTL, but still bolsters disease control according to Mr Blake.
CTL can interfere with the activity of other products on rust and head blight pathogens, but there are indications that folpet and mancozeb do not antagonise in the same way, says Mr Blake.
“Certainly with mancozeb there is evidence of head blight activity so that could be a useful multi-site to include at T3 where we are looking at additional septoria and fusarium control to complement that azole,” says Mr Blake. “Folpet may have more of a place where you are looking to target a mixture of diseases like rusts and septoria.”