Yellow rust looks set to pose a threat to yields and profits once again, with the disease being spotted in winter wheat crops before Christmas.
Although the recent cold snap will have slowed it down, if warmer weather arrives in March, yellow rust levels could ramp up again.
Last season the warm spring temperatures led to the disease cycling so quickly that two weeks after T1 treatments, yellow rust was breaking out even in varieties with seemingly robust ratings of 7/8, says Prime Agriculture agronomist James Southgate, who advises across Suffolk and Essex.
“I think yellow rust was in some crops before T0.
The temperatures we had meant it was rampant and I was going back two weeks after a T1 and all of a sudden there was yellow rust breaking out on leaves that should have been protected.
“When it came around to T2 the flag leaf had emerged and wasn’t protected because that typical three-week spray window was just [too long].
“On the higher risk varieties, a T1.5 was already planned and having that in there helped no end.
Last season I didn’t see any yellow rust before Christmas, but this season I have seen a lot and I can see early season rust control being a big issue.” The main challenge is with T0 – label restrictions meaning there is nothing that can be applied to protect against rust earlier, he adds.
As a result, going back to basics to ensure the best efficacy, particularly when it comes to curative sprays, should be at the forefront of growers’ minds.
Mr Southgate says: “Especially once the plant starts to extend, getting [fungicides] into the lower leaves and trying to wipe out the inoculum down there is really quite important.
“Going quick and with low water volumes doesn’t help with trying to get the disease out of the crop.
You need a relatively fine spray and you don’t want big, coarse nozzles when it comes to fungicide control.” Formulation is also likely to play a part, says Bayer technical manager Ben Giles, with a big difference seen in curative activity between oilier tebuconazole and a suspension concentrate (SC) during last year’s drought.
He says: “We used Toledo which is much stronger and more concentrated, but is an SC formulation.
“There was a big difference, particularly in speed of curative activity between the formulations.
If you’ve got rust because of high temperatures, even tebuconazole choice can be a factor in that.” The balance between SDHI and azole contents in products varies greatly, so should be an important consideration when choosing which product to apply, he says.
Although variety resistance ratings are the best available estimate of the threat posed by yellow rust, they are only based on which races the plant was exposed to during trials.
With new strains of the disease emerging, if yellow rust is present it should be treated, says NIAB technical director Bill Clark.
“We have to predict the races which will be prevalent next year, but it’s only a best estimate.
“When you see yellow rust on 8/9 rated varieties you don’t know what the race is, so you have to assume it might be a new race.
There is masses of yellow rust out there at the moment.
The bottom line is if you look for yellow rust and you find it, you have to treat it there and then.
“I never thought I’d be recommending T1.5 for yellow rust but it’s getting to the stage where you might even have to do that where it’s really rapidly cycling.”