Don’t be tempted to reduce triazole doses in flag leaf sprays to add in chlorothalonil, particularly where intervals have been stretched or rust is running riot, Bob Mills, Frontier’s technical manager, warns.
The issue is to do with the effect chlorothalonil can have on triazole uptake affecting kick-back activity, he says. “The chlorothalonil formulation doesn’t necessarily assist triazoles.”
BASF trials suggest in curative situations the product can slow down or “block” the kick-back activity of triazoles.
That is not as serious a problem as some would like to make out, Mr Mills stresses. “If timings are spot-on it is not really an issue.”
But in recent seasons there has been a trend for growers to reduce triazole doses and add in chlorothalonil to top up septoria protection, he says. “Where you reduce rates the first thing that goes is kick-back and persistence.”
Both could be important this season, particularly with yellow and brown rust prominent, diseases on which chlorothalonil has little activity.
Adding in chlorothalonil must only come on top of, not at the expense of, a triazole dose, he says. “If rust continues to be an issue growers don’t want to be substituting triazole for chlorothalonil.”
That is also the case if timings are stretched. In that case growers are likely to need extra curativity from the triazole, so there could be a case for dropping chlorothalonil and spending the difference on the triazole, he says.
“At the moment timings are not looking too stretched, however,” he points out. “If flag leaf sprays go on in the next week they should be nicely within the four-week interval from T1s.”