Skimping on T2 spray is not worth the risk
Disease pressure, particularly Septoria and yellow rust, has been exceptionally high throughout this season and many crops have excellent yield potential that needs protecting through to harvest, the firm’s technical development director David Ellerton says.
Even if earlier T0 and T1 sprays went on in good conditions at the optimum timing, Dr Ellerton warns there is still no reason to cut costs at T2 (growth stage 39), which risks undoing the earlier hard work and compromising yield.
“Get the T2 spray wrong and you’re in trouble. We’ve seen in previous years how quickly things can go wrong when the weather changes. Spring 2012 was a classic example of that, but then we were starting from a relatively low disease base after a prolonged dry spell. This year we’ve got a lot of forward, lush crops and a high level of disease already, so pressure on the flag leaf could be enormous given the right conditions.”
A well-timed robust T2 spray based on SDHI chemistry, combined with an appropriate triazole is the perfect “insurance policy” to deliver strong curative disease control and keep the yield-building flag leaf as clean as possible, Dr Ellerton advises.
Any additional cost from such an approach is outweighed by the extra curative and protectant activity that SDHI chemistry delivers over traditional triazole/ strobilurin strategies, he says.
“Ideally the flag leaf should be almost fully emerged to get decent spray coverage, but it is possible to go slightly earlier when it’s at least half emerged if necessary. The last thing you ever want to do is delay T2 and have the flag leaf out for any length of time with no protection.”
There are a number of excellent broad spectrum pre - formulated SDHI + triazole products available for use on the flag leaf. However straight SDHI formulations may also be mixed with triazole based products giving growers greater flexibility. Mixed triazole products based on epoxiconazole or prothioconazole offer advantages over straight triazoles in terms of efficacy, he says.
An “effective dose” of SDHI should be used, although Dr Ellerton acknowledges the definition of this varies between products. Rates should be adjusted, depending on disease pressure and level of curative activity required. However even the latest SDHIs don’t have the same curative ability on Septoria that triazoles had in the mid-90’s, so you’ve got to watch crop growth stages closely and be prepared to act promptly before the disease becomes established on the upper leaves.”
Bridge the gap to T2
With the T2 fungicide likely to be going on around the usual time of the second to third week in May, Essex-based Farmacy agronomist Andrew Spackman believes it may be worth applying an intermediary spray between T1 and T2 in forward crops where the interval is likely to be close or beyond the maximum four weeks.
“Anyone who applied their T1 in the first couple of weeks in April should consider going in with a triazole plus chlorothalonil mix as a holding spray around three weeks later if they think the gap to T2 is going to be stretched too far,” he says.
“You’ll probably end up going back on with the T2 quite soon after the intermediary spray, but it’s a relatively cheap solution that helps to extend the protection of leaf two and three and reduce pressure on the flag leaf spray if disease pressure is high.”
Mr Spackman says it is vital to watch how disease pressure develops closer to flag leaf emergence and be prepared to tweak tank mix partners and dose rates accordingly.
“Septoria is the biggest driver of fungicide choice this year, but don’t write off other diseases too, especially rusts. There was a lot of yellow rust in crops during the winter and although that seemed to have abated in the early part of the season, the inoculum was still present on lower leaves in many crops and built rapidly in some varieties during the stem extension phase. Brown rust too could become a concern if we see dry and warm weather over the coming weeks.”
Aggressive new races of yellow rust, such as the Warrior strain, can build up very quickly and have the potential to overcome varietal resistance, so any crop is potentially at risk, adds Dr Ellerton.
“The worst thing you can do is end up in a situation where you’re trying to get on top of rampant yellow rust, particularly if the weather goes against you and you can’t get on in the early stages of infection. “
Fulfil barley potential
Dr Ellerton says barley crops also have good yield potential this season and SDHI chemistry plus a good triazole or cyprodinil should form the basis of any T2 programme.
“All of the new SDHIs have a strong place at T2 in barley, although rates tend to be lower than at the T1 timing,” he says.
“There’s a fair amount of brown rust around at the moment and also net blotch and rhynchosporium where conditions are wetter. It’s also worth bearing Ramularia control in mind at the T2 timing.”
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