September 20, 2013
Managing Growing Wheat Foliar Disease Risks
Wheat growers across the UK need to make the best possible use of all the agronomic risk management tools at their disposal to combat the escalating foliar disease challenge, believes a leading agronomic researcher.
Agrii head of agronomy, Colin Lloyd sees last season providing a welcome disease respite for many, as well as a good demonstration of how well infections can be kept at bay with mainstream fungicide programmes if everything goes right. With the underlying foliar disease threat continuing to grow, though, he is adamant a far more integrated management approach is essential to avoiding the sort of losses all too apparent in 2012 in particular.
“We’re seeing worrying changes in the triazole sensitivity of Septoria tritici,” he says. “At the same time, the bulk of today’s wheat germplasm is suffering serious breakdowns in both brown and yellow rust resistance and most has questionable Septoria strength. What’s more, increasingly variable weather and fewer men and machines managing larger and larger acreages is putting ever-more pressure on spraying windows.
“Without even considering the possibility of future triazole restrictions, it’s clear we need to think well beyond the T1/T2 fungicide choice that has long been the primary focus of most disease management strategies if we are to keep our heads securely above water.”
On the evidence of more than 10 years of the company’s extensive Co-ordinated Growing Systems (COGS) trials across the country, Colin Lloyd identifies variety choice, seed treatment at T(-1), drilling date, spray timing, chemistry rates and spraying techniques as the most vital tools in modern wheat disease management alongside spray chemical choice.
Pointing to the fact that more than 40% of varieties on the current Recommended List and over 50% in NL1 and NL2 testing are based on Robigus genetics, he insists that large areas of UK wheat are likely to be particularly vulnerable to yellow rust – not to mention brown rust and Septoria – for some time to come.
“Our integrated disease control trial work across a raft of varieties and fungicide programmes every year highlights the danger,” he explains. “In 2011, for instance, the yield difference between a standard and enhanced fungicide programme with Oakley at our Marlborough AgriiFocus site was over 1t/ha. Yet it was less than 0.25 t/ha with JB Diego grown alongside it.
“This underlines just how significant variety choice can be if circumstances conspire against getting your spray programme ‘spot on’; especially if you’re growing large acreages of higher risk varieties, you fail to manage the extra risk with a T(-1) seed treatment active against foliar disease or you increase disease pressure by early drilling.”
Agrii trials over many years show that early drilling almost invariably results in greater disease pressure. Over the past four seasons, indeed, the R&D team has recorded an average 3.5% increase in Septoria tritici linked to a 0.5 t/ha yield penalty from wheat sown in the first week of September compared to late-September/early October. As a rule of thumb, Colin Lloyd suggests knocking one point off a variety’s resistance score for each foliar disease to compensate for early sowing.
Where yellow rust is concerned the trials also make a compelling case for treating less resistant varieties with a fluquinconazole-based seed dressing.
Although not a major year for rust, across 14 varieties and seven different fungicide spray programmes full rate fluquinconazole at T(-1) delivered an average 2012 first wheat yield benefit ranging from 0.15 t/ha with highly resistant, KWS Solo to 0.73t/ha with highly susceptible Torch.
“Fluquinconazole is particularly effective at keeping early yellow rust infections at bay,” Colin Lloyd observes. “This means there’s no temptation to bring your T0 forward to deal with early problems and stretch subsequent spray timings as a result. Which, in turn, leaves less opportunity for Septoria to take hold. So the T(-1) can hugely valuable in buying you extra breathing space where you’re growing varieties with less-than-ideal resistance, you’re in a problematic yellow rust area, you’re sowing early or you know your spring spraying resources may well be stretched.”
The impact of stretched spray timings is another area Agrii researchers have been investigating in recent years alongside the value of both T0 and T3 sprays.
In the absence of a T0 and with identical chemistry, they’ve found that bringing the T1 forward from the ideal timing by five days and delaying the T2 by 10 days – as can easily happen where the weather limits spray days – hit yields by around 0.4 t/ha. Their work, however, has also shown a T0 can completely reverse this loss.
“With larger acreages and increasingly variable weather, one of the most valuable roles of a T0 is the effective protection it gives against T1 and T2 timing problems,” points out Colin.
“Having said that, analysis of 10 years of all our trials work to 2011 shows an average T0 yield response of just under 0.4 t/ha “What’s more, the least T0 response we’ve ever recorded in a season has been 0.2 t/ha. At a cost of around £12/ha, this is less than half the return it gives from wheat at £150/t.
“Our work shows also shows a typical response of just under 0.5t/ha from a T3 spray,” he adds. “Again, though, this varies widely between varieties – from 0.2 t/ha to almost 0.8 t/ha. Once more it underlines the importance of both variety choice and variety-specific agronomy in foliar disease management.”
As far as fungicide selection is concerned, Agrii research further demonstrates the importance of matching chemistry to both variety and season for the greatest value.
Across five varieties over three years in six locations, the average response from a three treatment programme involving an SDHI rather than a standard triazole/strobilurin combination was just over 0.5t/ha. The extra yield benefit of the SDHI, however, ranged from less than 0.1 t/ha with the least responsive variety to a full 1t with the most responsive. And in wet seasons the average five variety response was over 0.9t/ha against just 0.2t/ha in dry seasons.
Given the particular activity of SDHIs against Septoria, Colin Lloyd finds it no surprise they deliver more in wet years. Nor that his work shows a clear relationship between SDHI response and varietal resistance to Septoria tritici. He stresses, though, that this understanding can make all the difference in deciding when and where to target the chemistry for maximum financial benefit.
Equally valuable in this respect, he has no doubt, is the intelligence in-depth Agrii trials are providing every year on specific responses to a range of dose rates, using stacked triazoles to combat changing Septoria sensitivity, and interactions between fungicides and protein quality as well as yield.
“We musn’t forget the importance of the right spraying technique either,” he says. “Our studies with different nozzles, forward speeds and water volumes at T2 highlight the considerable effect these specifics can have on spray coverage and disease development in the lower canopy, in particular.
“They show correct application is especially important with higher risk varieties, in high disease pressure seasons and where timings get stretched for one reason or other. More haste and less speed is, for instance, clearly the best approach to manage a variety like Oakley which already has disease in the lower canopy. So, here I’d be sticking with a flat fan nozzle delivering 200l/ha at 6-10 kph to ensure sufficiently good penetration. Only with more resistant varieties and lower disease pressures would I be switching to nozzles that allow faster work rates and lower water volumes.”
Stepping-up the Programme
To provide even better intelligence for agronomists and growers, Agrii is stepping-up its wheat foliar disease management research in a major way as part of the company’s market-leading R&D expansion announced in the past year.
Among the projects being added to the industry’s most extensive integrated cereal disease management trials programme from this season are:
- Barometer plot tracking of all the most important diseases across a wide range of varieties and locations to provide early warnings of disease levels and varietal resistance breakdowns.
- Early screening of all emerging varieties from all major breeders for key diseases in high pressure situations to really challenge their resistance status.
- Sophisticated modelling to adapt fungicide programmes increasingly precisely to local conditions and needs using data from an extended network of weather stations.
- More intensive development and testing of the most effective combinations of non-triazole fungicides to enhance alternative future treatment strategies.