The levels of yellow rust still being found in wheat crops is forcing growers and agronomists to reconsider their strategies for the approaching T2 timing.
Even varieties with good rust-resistance ratings are carrying higher-than-expected levels of the disease, prompting questions about their ability to withstand high disease pressure and the possible emergence of a new race.
“No variety resistance score is a guarantee,” points out Jonathan Blake of Adas. “Given the right conditions for yellow rust, even high-rated varieties can get the disease.”
It is possible for the AHDB Recommended List ratings to be wrong, especially in newer varieties, he adds. “Varieties change over time and trials sites may be dominated by one race. There is also the question of differences in susceptibility at seedling and adult plant stages – some will simply grow out of it.”
He adds that it has been a good season for yellow rust so far, with most of the Adas trials sites having the disease. “The general rule is that if it is affecting the yield-forming leaves, you need to do something about it. At this stage, it is not worth taking any chances – in favourable conditions, yellow rust can cycle in 12 days.”
For the T2 spray, that may mean strengthening prothioconazole-based treatments with some strobilurin, he says. “But that applies only to susceptible varieties where yellow rust is active. There are also plenty of crops without the disease.”
The challenge for agronomists is to make sure the T2 treatment gives the required level of yellow rust control without undermining the septoria control that it also needs to provide.
“In most cases, if you do what is right for septoria, you will control yellow rust as well,” says David Parish, independent agronomist with Tag Consulting in Bedfordshire.
“Of course, there is always an option to boost the yellow rust activity provided by the planned T2 spray by adding some pyraclostrobin, for example.”
Mr Parish also took the unusual step of recommending a T1.5 spray on varieties such as Reflection and Solstice, based on either an azole or – where there was no visible rust in the crop – a strobilurin.
“It was a precautionary measure, based on the susceptible nature of some varieties and disease levels. The target was leaf 2, so it went on in the first 10 days or so of May.”
On other cleaner varieties, the T2 or flag-leaf spray will be the planned third shot at the disease. “It isn’t a difficult disease to control, especially where T0s and T1s have been well timed, but the difference this year is that we have already had to react to unexpected varieties.”
Dick Neale of Hutchinsons agrees that yellow rust is being found in a wider area and on more varieties than usual, so he stresses it has to be a consideration for T2.
Septoria risk remains
“It is also worth pointing out that the cold weather we had in March and April slowed down septoria development,” he says. “On that basis, we can expect septoria to become very evident any time now.”
David Robinson, head of Crop Dynamics, also warns that septoria is about to start appearing, as the disease is just coming out of its latent phase in perfect conditions.
“Despite all the yellow rust that’s about this year, septoria is still the greatest threat. The lack of eradicant chemistry means you can’t react to it in the same way you can with yellow rust. The rusts can be stopped in three to four days.”
As a result, a robust rate of SDHI will be essential, supported by triazole and, where necessary, a strobilurin, Mr Robinson advises.
“Farms south of the Humber should aim to cover septoria, yellow and brown rust,” he says. “North of the Humber, you don’t have to worry about brown rust, unless it becomes exceptionally warm.”
A multi-site protectant shouldn’t be required at T2 and isn’t supported with the use of some products, he cautions.
“Most have done a reasonable job with these materials already. You also don’t want to exceed the maximum limit,” says Mr Robinson.
What is happening with Reflection?
A variety-specific problem with yellow rust tends to occur every two years, seed trade sources agree.
This year’s talking point is Syngenta’s hard Group 4 variety Reflection, which is carrying much higher levels of the disease than its 6 rating would suggest – and more than others with lower scores.
With Oakley cross Denman parentage, Reflection was always likely to be susceptible to yellow rust, but commentators have been surprised by the speed of the disease’s development in the variety.
Over the next five to six weeks, the situation will become much clearer. If adult plant resistance kicks in, the variety may look quite different to the way it did as Crops went to press.
Later on in the year, the results of adult plant tests conducted by the UK Cereal Pathogen Virulence Survey will show whether a new yellow rust race is responsible for some of this season’s unexpected observations.