Flag leaf sprays could start as early as next week as crops continue to run at least 10 days ahead of their usual development, while a bridging spray could be necessary for less forward crops suffering high brown rust pressure.
An extremely warm April has encouraged an explosion in brown rust in many wheat crops, with varieties such as Alchemy and Claire particularly at risk. The disease appeared to be a problem in many areas where it would not traditionally be found, UAP‘s Chris Bean said. “I’ve had reports of it from the south-west and up into North Yorkshire.”
There was also a major problem with a potential new race infecting Robigus, Nick Myers of ProCam, suggested. “It has had a catastrophic breakdown to brown rust.”
The initial concern with Robigus had been yellow rust, he said. “We had one crop that had a T0, where we had to go in with a robust T1 because of yellow rust. Ten days later brown rust had exploded, which has dictated we had to go back in with a pre-T2 within two weeks of the T1 going on.”
Other growers may also need to go in with an early pre-flag leaf bridging treatment where T1s had gone on too early, Bill Clark, ADAS plant pathologist, said. “The main problem is the season fooled people into thinking crops were more forward than they were,” he suggested.
It meant some growers sprayed at GS30/31 rather than GS32 and left final leaf three and four unprotected. “Most triazoles are not systemic enough to move up leaves to give good control.”
That’s left some crops – typically those sprayed 10 days to a fortnight too early – with established brown rust on leaves three and four. “The yield loss so far is not that great, but leaf two is important for yield. Growers cannot wait for 10 days for flag leaf emergence before spraying.”
Instead, he advised growers to use a quarter dose triazole at GS33-37 where septoria pressure was low. “If you’re going through you must not forget about septoria – crops are still very damp in the morning, and if you’ve gone too early for rust you’ve probably gone too early for septoria too. Look to see where it is now – you might need a higher dose.”
Growers forced down that route would likely be able to delay their true flag leaf spray to combine it with an early ear spray treatment, he suggested, with strobilurin fungicides a good choice at that timing. “Crops will want the long-term protection strobs give against rusts.”
TAG agronomist David Parish agreed the rust risk brought in strobs. “They will be particularly valuable in giving protection over a longer period. And if growers are reducing triazole dose because septoria risk is low, they will be reducing rust protection, so adding a strob will help add that back in.”
But he didn’t think a bridging spray to flag leaf would be required. “I don’t perceive the risk to be quite as high. Our T1 fungicides have been timed pretty well and I’m reasonably confident they will do the job required.”
Flag leaf sprays would probably start next week in his area, he said. “It will mean the interval won’t be too great.”
Well-timed T1s were holding rust, Mr Clark agreed. But even then flag leaf sprays were likely to be needed slightly earlier – GS37-39 – than usual. “Growers will also almost certainly need to come in again for brown rust with an ear spray,” he suggested.