Farmers urged to re-think fungicide plans to fight resistance

The trend towards increased fungicide use is accelerating the development of resistance to fungicides in septoria populations and growers need to build resistance-beating strategies to negate the threat.

About 25 years ago it was not uncommon to apply a fungicide to winter wheat at stem extension, one to the flag leaf and shut the gate until harvest. Applying an ear spray was “pushing the boat out”.

See also: Season to blame for fungicide performance disparity

Worrying discoveries

  • New isolates with “efflux” mechanism found that makes septoria less sensitive to azoles and SDHIs
  • In lab conditions, “efflux” septoria requires five to 10 times more fungicide to kill

In 2012, wheat crops received an average of three-and-a-half sprays and in 2014, an average fungicide spend of £81/ha was the highest ever.

So what is driving the trend?

Both 2012 and 2014 were extreme disease-pressure years, when winter and spring weather conspired to ramp up disease pressure to unprecedented levels.

Neil Paveley, crop protection lead at Adas, doesn’t believe the answer is quite so straightforward as saying that disease pressure is on the rise in UK fields.

“There’s always been fluctuations in septoria levels (see ‘Septoria in treated crops’, below), so it is difficult to argue that there has been a significant increase in overall disease pressure.

“Increasing grain prices during the last decade, along with the faster cycling Warrior yellow rust race and concerns about mycotoxins making an ear spray routine have all driven the frequency of treatments,” he explains.

Septoria in treated crops

Azole decline

Dr Pavely sees these as perfectly valid reasons to justify increasing fungicide use, but he is concerned that the consequences of doing so could be grave for future disease control.

It has been well documented that key fungicide group, the azoles, are in steady decline as fungicide resistance chips away at their efficacy against wheat’s number one disease, septoria, year on year.

Evidence from HGCA’s fungicide performance trials suggests that the group’s curative activity has shifted from providing about 90% control to 20%. In a protectant situation the decline isn’t quite so steep, moving from 90% to 70%, but still significant.

As this decline continues, fungicide doses and the frequency of application increase to compensate to achieve adequate control of the yield-robbing disease.

“As we increase both dose and frequency, we are driving resistance and the subsequent decline in efficacy harder and faster,” says Dr Pavely.

Perhaps more worrying is the discovery of a mechanism in the septoria pathogen called “efflux” – enabling the fungus to move a fungicide active ingredient away from the target site within the cells of the pathogen.

These isolates were found in field samples taken from HGCA variety trial sites, so are naturally occurring strains and not mutants created in laboratory conditions.

This type of resistance applies to all single site-acting fungicides, including both azoles and SDHIs. In laboratory work carried out by Bart Fraaije at Rothamsted Research, these strains take between five and 10 times the quantity of fungicide to kill them.

However, Dr Paveley notes that the discovery needs to be put into context, with observations in the laboratory often equating to only very small changes in the field.

The efflux septoria strains are likely to be unfit and find it difficult to compete with other dominant strains in a field environment, so a rapid increase in the population is unlikely in the short term.

“But it is a very strong warning shot across the bows that the septoria pathogen is evolving, will continue to evolve and will overcome SDHI fungicides in due course.

“I am flagging this up not to warn of impending disaster, but it highlights the fact that septoria will overcome the tools we have and we need to take appropriate action,” says Dr Paveley.

He adds that this is particularly necessary as the product pipeline for new fungicide modes of action is slow and it could be several years before another one comes along.

“It was around 20 years between the introduction of azoles and the strobilurins coming along, then another 20 years before the new generation SDHIs arrived.

“I hope it isn’t another 20 years before the next mode of action comes along, but it is going to be at least several years, so we need to look after what we have,” says Dr Paveley.

Building a programme

In the diagram (bottom right) it can be seen that there are four options from the standard fungicide programme of two sprays to make disease control more effective.

Options for robust fungicide sprays

These include increasing the dose, increasing the frequency of treatment, splitting the total dose into smaller and more frequent applications or mixing products together.

Frank van den Bosch at Rothamsted Research recently led a review of all global scientific research into fungicide resistance and the effect that choice of programme had on resistance development.

Consequences for fungicideresistance

The study included all diseases in all crops and from the table it can be seen that of the studies reviewed, increasing dose, frequency and splitting doses all leaned towards increasing selection pressure.

“But in experiments using fungicides in mixtures, 46 of the 53 experiments decreased the selection for less sensitive isolates of the pathogen.

“This approach overwhelmingly slows resistance development down, so applying balanced mixtures out in the field to achieve effective control will also optimise resistance management,” explains Dr Paveley.

Robust septoria control and low selection

So what is a balanced programme?

Dr Paveley explains that knowing how the septoria pathogen responds to the different chemical groups is crucial when considering how to ensure that both effective control and reduced selection pressure for resistance are achieved.

As can be seen from the table, increasing azole dose will increase selection pressure, but not as much as increasing the frequency of treatment.

SDHIs are known to be a high resistance risk fungicide group and increasing both dose and frequency significantly increases the risk of selecting for resistance.

Multisite active ingredients such as chlorothalonil are very low resistance risk, so building a programme using the multisites as the foundation is crucial, adding the other components on top as required.

“The most burden of control should be placed on the active ingredient with the lowest resistance risk, such as chlorothalonil. The azoles should be used at robust rates and confine them, where possible, to the T1 and T2 timings if practical, although this will be a difficult challenge in practice,” adds Dr Paveley.

SDHI limit

When it comes to SDHI use, which is crucial in the face of reducing azole efficacy, Dr Paveley urges the use of just one application through the programme if it is enough to achieve a good level of septoria control.

“In some cases that is not enough and two will be required in high disease seasons on susceptible varieties.

“The treatment decisions that are made on farm do drive resistance so what growers do this season will effect how fungicides work next season,” adds Dr Paveley.

Syngenta’s fungicide technical and solutions manager James Southgate agrees that knowing each chemical group’s characteristics is vital and not only how the pathogen responds to the chemical, but also where each active provides its optimum performance.

He explains all the expert advice is pointing towards adopting a preventative approach to septoria control, keeping the disease out of the crop rather than allowing it to take hold.

According to the HGCA fungicide efficacy trials, all the available fungicide products on the market are much better products at preventing disease, rather than curing it once symptoms appear.

“The most important thing is keeping disease out of the crop in the first place and this can be done by front loading the programme to stay on top from the start,” says Mr Southgate, who’s company – along with BASF – recently launched the Take Ownership Now (TON) campaign, designed to help raise awareness of the fungicide resistance issue.

How septoria takes hold

The illustration ‘How septoria takes hold’, shows how septoria infects and develops within a wheat leaf and it can be seen where each fungicide group is most effective.

Although it shows triazoles are effective from penetration through to pre-sporulation phases when curative activity is required, azole curative activity against spetoria has significantly declined, so in reality they are stronger earlier in the cycle.

SDHIs, strobilurins and multisites such as chlorothalonil all achieve their best results when used early, backing up the theory that maintaining the preventative programme with well-timed sprays is the best for disease control.

“If you’re clean from the start, you are achieving the best disease control which is best for yields and best for resistance management too,” says Mr Southgate.

Genetic resistance an “extra mode of action”

Using a variety with a solid Recommended List septoria resistance score should be considered as another mode of action in the fungicide programme, according to Dr Paveley.

Dr Neil Paveley

Dr Neil Paveley

He says there is evidence to suggest that a mixture of a resistant variety and a balanced fungicide programme does reduce the resistance selection pressure.

When strobilurin fungicides were released onto the market, they were often used with no mixing partners and they quickly broke down to resistance and are now largely ineffective against septoria.

With the newer SDHIs at a similar risk the industry learnt its lesson and ensured that it was compulsory to use them with a mixing partner.

“That was the first important step in managing fungicide resistance and the next one will be using more resistant varieties.

“It is difficult to obtain all the potential yield out of the higher yielding, less resistant types. It would be better to use more resistant types that we can get all the potential out of, even if we take a slight yield penalty. We have to look at the wider picture,” says Dr Paveley.

He adds that continuing to grow “dirty” varieties with high input fungicide programmes will drive resistance harder and losing products through resistance would be a much bigger penalty to yield.

“With UK conditions conducive to septoria, it would also put the country at a competitive disadvantage to our European neighbours, who don’t have the same septoria pressure.”

In 2014, trials were carried out using the varieties Crusoe (good resistance), Cubanita (moderate resistance) and Conqueror (low resistance) with three fungicide approaches, ranging from low input to intensive (‘Selection of azole insenstive septoria’).

Selection of azole insensitive septoria

When considering the gross margin of each variety, the low input fungicide programme on the more disease resistant variety Crusoe provided return almost equal to that of a high-input programme on the “dirty” variety Conqueror.

“It was a very high pressure season, so we were surprised that a two-spray programme performed as well as it did and achieved such a margin,” says Dr Paveley.

This season has seen a welcome shift to some of the more resistant wheat varieties on the Recommended List, with Skyfall, Crusoe and Evolution (all rated 6) as growers look to grow crops that offer an opportunity to cut costs.

Mr Southgate says this integrated approach is welcome, but reminds growers that a sensible fungicide programme is also necessary to protect the resistance genetics in the varieties too, with Rothamsted and Adas work supporting that view.

“Fungicides will help to protect the varietal resistance as much as the varieties helping to protect the fungicides that we are using.”

James Southgate

James Southgate

Bravo to multisites

In HGCA fungicide performance trials during 2014 a half dose of contact fungicide chlorothalonil was outperforming both the key azoles epoxiconazole and prothioconazole for protectant activity against septoria.

For a fungicide that is more than 50 years old, this is an incredible achievement and proves that it is at very low risk active of septoria becoming resistant to its multisite mode of action.

Syngenta’s James Southgate believes that growers should be using the active at all timings through the fungicide programme to support the azole and SDHI chemistry and help slow the build up of resistance.

“It is the most important fungicide we have for resistance management and growers should look to include it where possible.

“There is a limit of two applications of Bravo on its label, but there are other chlorothalonil-based products that can be used. However, growers must ensure that they stay within Syngenta’s guidelines of 2,000g of active per hectare in a season,” says Mr Southgate.

He adds that last year, high early yellow rust pressure led to many spraying a single azole application at T0, which will have resulted in increased selection of azole insensitive septoria isolates.

“Although growers weren’t targeting septoria it was still present and including chlorothalonil at this timing is crucial to ensure that the azole isn’t on its own, or it will be eroding the group’s efficacy even faster,” he explains.

Mr Southgate says it is also vital that growers stick to the guidelines regarding chlorothalonil usage limits to ensure that it doesn’t come onto the regulatory radar through good environmental stewardship.

“Going over the 2,000g/ha limit is likely to attract attention and we can’t afford to lose any of the multisite actives from a resistance management point of view,” he adds.

There has been a concern about using chlorothalonil with Bayer CropScience’s Aviator (bixafen + prothioconazole), but the company’s commercial technical manager in the West, Gareth Bubb, says there is no problem using the products together at T1 as long as the spray isn’t delayed.

“We believe it can reduce the curativity of the azoles and SDHIs by hindering the actives getting into the leaf, so if the timing is delayed and leaf three has been out unprotected for a period, I would leave chlorothalonil out.

“But if you are on time and in a protectant situation, it is fine to include it if you are using Aviator at T1,” he says.

Mr Bubb adds that where growers are using Aviator at T2, Bayer does not advocate using chlorothalonil with its product, with leaf two requiring some “kick-back” activity to clean up any disease present.

“The fungicide in leaf three will be running out of steam and leaf two won’t have had anything, so it will require some curative activity and we feel chlorothalonil can reduce that.”

There are two further multisite fungicide actives for use in wheat, with folpet offering some useful activity on septoria.

Multisite comparisons

The second is mancozeb, which has been available for use in the crop for some time but one recent trial has shown that it outperforms folpet, but is a little behind chlorothalonil.

UPL technical manager Pam Chambers says the company is hoping to have a liquid formulation as a more convenient-to-use alternative to the wettable powder currently available for 2016.

“All products could be subject to tighter restrictions in the future so it is vital we have alternative multisites, which is why we are doing the work with mancozeb.

“For resistance management this is vital, as we need to have products that will take the pressure off the single site actives,” adds Ms Chambers.