Crops with low disease ratings in a high rainfall area face a considerably higher risk of septoria.[/caption]
However, preventing pathogens from taking hold is an easy resolve to make, but one that’s tough to put into practice on a busy arable farm. Crop economics further exacerbate the problem – wheat prices languish close to the cost of production, while input costs are not diminishing.
That may put a strategy review on your agenda, which requires a knowledge of both the pathogen you’re facing and an understanding of the tools you have at your disposal to keep it at bay.
What’s meant by disease prevention?
Ultimately, the main aim of any effective fungicide programme is not to allow disease to get out of control. Once a pathogen takes hold, if you ask a fungicide to control it, you’re not only unlikely to succeed, you’re creating a situation in which less sensitive isolates will thrive.
But nor is it about blanket treatment. An effective strategy takes account of risk – crops with low disease ratings in a high rainfall area face a considerably higher risk of septoria than those in drier regions of East Anglia. But in parts of the south and east of England there’s a high risk of brown rust developing in some seasons, while a susceptible crop is at high risk of heavy losses from yellow rust, especially if the pathogen takes hold early on in the spring.
Effective disease prevention requires an understanding of the risk factors and then using the chemistry to protect the crop, rather than eradicate disease.
Why is prevention important?
The on-going fungicide performance trials, funded by AHDB Cereals and Oilseeds, have tracked a gradual erosion in the ability of azoles to tackle septoria. What’s significantly reduced now is the kickback activity the chemistry used to offer. This allowed flexibility between timings so that an application would eradicate a disease up to 14 days after a crop had been infected.
We’re now dependent on the persistency of SDHIs for this, but since the curative ability of azoles is relatively limited, a challenging situation puts immense pressure on the newer chemistry. That’s not a tenable situation.
The AHDB-funded trials show azoles currently offer around 60-70% average control of septoria when used in a protectant situation, while eradicant activity has dropped to just 35-40%. This means a preventative programme is not only more effective, it costs considerably less than the robust rates needed to eradicate a problem.
How do varietal disease ratings help?
Variety choice is the first step in disease management, and the good news is that there are varieties with relatively good septoria ratings now appearing on the AHDB Cereals and Oilseeds Recommended List. It’s important to look at the decimal figures, however – both Cordiale and Reflection appear to have the same rating of 5 for septoria. But Cordiale is 4.8, while Reflection stands at 5.4.
Varieties like Group 2 newcomer KWS Siskin and hard Group 4’s Graham are rated 6.7 and 6.6 respectively, offering a significant degree of protection that’s plainly evident in trials. This can help manage risk, so growers in high rainfall areas will be mindful of variety choice.
Drilling date is also an important risk factor, so varieties with robust ratings will reduce the higher risk of septoria from an early Sept-drilled crop. Likewise, a high seed rate or fertile site can produce a thick crop that will encourage disease.
So variety choice can help growers prioritise – those with large areas to cover will choose to treat high risk crops at the most ideal timing. Disease ratings are a guide, however, and varieties cannot be solely relied on to manage risk.
This was illustrated a few years ago when the new Warrior race of yellow rust infected varieties with apparently high ratings for the disease. The ratings are relative, but averaged over years, so will not reflect recent changes in pathogen population.
How does spray timing influence disease strategy?
Decades of research have established that the key timings to apply a fungicide for maximum yield is just as one of the three main leaves emerges. T2 is when the flag leaf emerges, which contributes most to yield. Leaf three emerges at T1, while leaf two appears roughly midway between.
These two key timings have been chosen because the combination of the persistence of the T1 spray and the kickback offered at T2 should keep leaf two protected. Septoria for example has a cycle of around 10-14 days, while fungicides remain effective for about two weeks.
Challenging situations reveal shortcomings in these timings, however. If the period between the T1 and T2 sprays is extended due to seasonal factors, a T1.5 timing may be necessary, in high risk situations. The lower eradicant activity offered currently by azoles increases the risk of uncontrolled infection.
What’s more, the timing of the T1 is crucial, and very hard to get right, especially on large arable units. So a T0 spray is now an established part of the programme on most farms, timed around a month earlier to keep early disease levels to a minimum. But some growers are now finding even that timing has its limitation in some situations, and a pre-T0 application is necessary.
How can timings be refined?
A new strategy, currently being adopted by a number of agronomists and progressive growers, is the leaf-layering approach. The broad aim is to keep the crop in a protectant state from before stem extension right through to canopy senescence. But there’s the same focus on timing applications to leaf emergence, with a greater emphasis on accounting for risk factors.
An overall strategy, based on background risk factors, is planned before the season starts. As each leaf emerges, you assess the background and current risk of infection to refine the course of action – whether to spray and what to use.
So high risk crops – those drilled early with poor disease ratings in high rainfall areas, for example – in high disease seasons may receive as many as six fungicide applications. But later drilled, resilient varieties in drier situations will be equally well protected with just three well judged sprays.
What’s important to note is that disease susceptibility, both through genetics and situation, will have a big influence on the scope to limit overall spend. Another key aspect in disease prevention is the success of the programme in the early part of the season – keep the crop clean up to T1 and that takes the pressure off the chemistry thereafter.
What chemistry is best for disease prevention?
A cornerstone of the programme will be the use of multi-site materials, such as chlorothalonil (CTL), folpet and possibly mancozeb. There’s no known resistance to this chemistry, and it’s effective at helping to protect the crop from infection.
Use will be tailored to risk, but up to four applications, focused on the early timings, may be used in high risk situations. Take note of label restrictions, such as no more than 2000g/ha of CTL may be applied in a season.
Azoles remain the backbone of a programme, but it can be a good idea to switch active as the season progresses. Epoxiconazole and prothioconazole are still the most effective, so best saved for the key T1 and T2 timings, although metconazole in mixture with SDHIs has been shown to be effective in recent AHDB trials.
You don’t necessarily need a septoria-active triazole at T0, so cyproconazole (included with propiconazole+ CTL in Cherokee, for example) is a good rust-focused material. Strobilurins also play an important role in both yellow and brown rust control. Tebuconazole is another useful material early in the spring. Depending on the season, the T3 spray is where you may be able to make savings.
SDHIs must be used according to FRAG advice – no more than two applications in a season and always in a mixture with at least one other mode of action. That puts the chemistry squarely at the T2 timing and most growers are finding benefits from using it at T1, too. Note that boscalid is regarded as an SDHI.
If needed, a rust-acting triazole works well as a pre-T0, while either a triazole or strobilurin (again for rust) could partner the multi-site if a T1.5 is applied.
The best way to minimise fungicide use is to have very accurate weather forecasting, while the best way to target it is to have a clear idea on how many days it will take you to spray your wheat area and prioritise fields according to risk. But these are challenging times for fungicides – the last thing you want to do is to present them with greater challenges through a poorly planned season.
Front loaded programme helps manage risk
Low grain prices make a preventive approach to disease control even more important, believes Essex farm manager, Ian Gibson of Broadacre Farming Partners – he’d rather not risk infections becoming established and losing yield as a result.
Managing 400 ha of combinable crops near Great Dunmow, he’s also taken responsibility for field operations on a further 800 ha for the first time this year. Top priority across the winter wheats will be to keep the lower leaves clean from the outset.
So early season fungicides are front-loaded against the chief concerns of septoria and rust. That begins with an “across the board” T0 of triazole plus chlorothalonil, followed by a three-pronged strategy combining new-generation SDHI plus triazole plus Bravo (chlorothalonil) at T1.
“From a septoria and rust point of view, you don’t want to be on the back foot,” explains Ian Gibson, whose soils range from London clay to organic black land.
“I think the key is to get a robust programme in place from the offset. You’re far better to start the protection early, on lower leaves, than chase disease all season.
“We do treat according to disease pressure, but we try to make the early application robust, to protect the emerging new leaf and so prevent disease from spreading to the yield-building leaves, leaf three and ultimately the flag leaf.
“Forward-loading also allows for the potential to reduce fungicide costs later in the season if disease pressure is low. However, a robust programme would always be maintained at T2, as nearly 50% of yield comes from the flag leaf, so an SDHI is also used at this timing.
“With an SDHI at T1 and T2 you’ve got extra greening to help photosynthesis fill the grains.”
Historically, Cherokee (CTL+ cyproconazole+ propiconazole) has formed the basis of the T0 application, with the switch to new-generation SDHI from older chemistry at T1 beginning in earnest in 2013. That year, early septoria concerns prompted isopyrazam plus epoxiconazole (as in Keystone and Seguris) to be used, with the aim of providing long-lasting protection across 75ha of Scout.
“The thinking was it had robust septoria control and longevity, which would keep lower leaves clean and aid reduction in splash dispersal to upper leaves,” says Ian Gibson.
Since then, isopyrazam has continued to be part of the T1. “Because it’s got that longevity it takes you through to T2. If you’re delayed, you’ve got that fall back,” he adds.
With an additional 800ha to spray this season, spray timeliness could be challenging, so he believes prevention becomes even more important. “We’re going to place increased emphasis on prevention, in case windows are stretched.
“One block in particular, which follows a river valley, tends to be prone to septoria. So again I’d like to use Keystone with 1 l/ha of Bravo, which is a cost-effective, good protectant. You just can’t afford to be complacent,” he notes.
Disease prevention: top tips
- Weigh up the risk factors – variety, weather, location, drilling date and rainfall all have a significant influence.
- Tailor the timings – use the leaf-layering approach to match applications to crop growth stage and stay in a protectant state.
- Use appropriate actives – adapt choice and rate to conditions at the time, paying particular attention to early applications.
