September 10, 2019
Intelligence on Yellow Rust Concerns
Yellow rust has been a serious concern in many parts of the country in the past season, with a number of varieties proving markedly more susceptible to infection than expected; especially where early fungicide programmes were cut back in the dry spring.
A surprisingly large number of varieties maintaining solid Recommended List resistance scores of 7.0 or more in recent years have been badly affected, prompting suggestions of new races and major ‘breakdowns’ in genetic resistance.
While she has seen clear changes in yellow rust populations in 2019, they come as absolutely no surprise to leading plant pathologist, Dr Rosemary Bayles, who co-ordinates Agrii’s national cereal disease survey, established with leading wheat breeder, Bill Angus. Nor does she consider any new race or sudden ‘breakdown’ to be involved.
For the past five seasons, the former UK Cereal Pathogen Virulence Survey (UKCPVS) leader has overseen the network of 12 disease monitoring sites from Carnoustie in Scotland to Taunton in Somerset. Here, around 30 varieties – carrying known rust resistance genes and representative of the span of commercial wheat genetics – are grown untreated and exposed to natural infection in small ‘tussock’ plots alongside the company’s national and regional variety trials programme.
Regular monitoring of yellow and brown rust development in these plots allows Dr Bayles to provide Agrii agronomists and customers with the most up-to-date intelligence on variety susceptibility for the most effective risk management.
“What we’ve seen in our survey this season is simply confirmation of what we’ve been warning about for the past two years,” Dr Bayles said. “Yellow rust populations are continuing to show high levels of virulence for varieties like Solstice, Robigus, Claire and
JB Diego. In addition, we are seeing increases in virulence for a number of other varieties that are by no means new but have been uncommon in recent years.
“The most significant change we’ve seen this season is much greater virulence for the parent variety, Hereford. This virulence has been detected in the past, but the first indication we had of a possible upsurge was at a single site in 2017, where infection reached 25% on the variety.
“Having detected infections more widely in 2018, we warned that varieties with Hereford in their pedigree should be monitored especially closely in 2019. The evidence of the past season shows this warning was very timely.
“Regardless of the science around races, it underlines the critical importance of keeping a close eye on what yellow rust is doing in the field each season and basing variety ratings on this rather than multi-year averages,” stressed Dr Bayles. “It’s equally important we heed the warnings we get from single locations, rather than losing sight of these outliers in averages either.”
Based on scoring in untreated plots in the company’s parallel national and regional variety trials, Agrii Wheat Advisory List yellow rust resistance ratings for KWS Zyatt, Gleam, Shabras and Dunston – all of which have Hereford in their close pedigree – have been reduced markedly from their 2018 levels. In the case of KWS Zyatt and Dunston – both still rated as over 7.0 on the RL – to just 3.7 and 2.7 respectively.
Overall, the final update of the 2019 List shows nine of the most commonly grown varieties having current yellow rust resistance scores of less than 5.0, against six at the beginning of the season. It also shows no less than 22 of the 32 varieties rated at less than 5.0 for brown rust resistance.
“These ratings don’t mean they’re bad varieties,” insisted Agrii head of agronomy, Colin Lloyd responsible for the Advisory List which is designed to complement the RL with additional and more timely variety intelligence. “After all, we can control both yellow and brown rust well with current chemistry.
“What they really highlight, though, is that varieties need to be sown and grown with the most up-to-date understanding of their weaknesses for the least risk.
“Nor do we see all varieties with similar parentages necessarily presenting the same level of risk,” he added. “For instance, LG Sundance – which also has Hereford in its close pedigree – is continuing to stand up to yellow rust well with a rating of 9.0. It’s clearly being supported by other strengths in its ancestry.”
So, what do the current changes in yellow rust virulence identified by the Agrii national cereal disease survey mean for growers in the coming season?
Well, Rosemary Bayles – who also maintains the special Agrii yellow rust diversification programme based on variety ancestry and known susceptibility – sees it as especially important to avoid putting all one’s eggs in the same basket when it comes to varieties.
“To manage the risk, I wouldn’t want to be growing a variety with a known weakness for yellow rust alongside others with a similar parentage beginning to be more affected,” she suggested. “Especially not if my sprayer capacity wasn’t well up to the acreage.
“With so many varieties sharing a similar pedigree, it’s not always easy to know which combinations to avoid here. The diversification chart we update each year from our monitoring provides a simple way of doing this.”
“At the same time, I’d make sure to avoid sowing more susceptible varieties early,” advised Colin Lloyd. “Delaying drilling until late October to deal with black-grass sowing really helps to reduce disease pressures.
“Alongside this, the last thing I’d do with a variety of ‘yellow rust concern’ is failing to use a triazole at T0 or cut back on my T1, even in an initially dry season. As many people found to their cost in 2019, if you let the disease in early on there’s really nothing you can do to deal with the explosive infection that’s almost inevitable when you get enough moisture.
“A decent strobilurin will give you excellent yellow rust activity at a very reasonable cost here. So, it’s a hugely false economy not to use it. Equally false economy in all our trials experience is not to use the most brown-rust active SDHI at T2 wherever you’re growing one of the many susceptible varieties. We know this can do the job even on something as vulnerable as Crusoe.”