Increase vigilance for yellow rust this spring
27th March 2023
Following a mild winter that may have caused the disease to set foot in winter wheat, growers are being urged to stay alert for yellow rust alongside Septoria tritici at the upcoming leaf three (T1) spray timing.
Yellow rust visible in wheat.
While Septoria is widespread in winter wheat, yellow rust can escalate quicker, and the mild winter means the disease may have already progressed to younger leaves, Syngenta area business manager Peter Hawkins pointed out.
“Although Septoria will be the main wheat disease growers will focus on, it’s important not to let yellow rust slip through the net at the important T1 fungicide spray timing, which is typically during April,” he said.
Although the spread of yellow rust can be reduced by frost, temperatures need to be consistently under freezing for several days for the cold to take effect. “Don’t assume March’s cold weather will have been enough to eradicate rust,” Mr Hawkins cautioned.
“Depending on the weather, yellow rust can bounce back quickly – going through its life cycle in just 10-12 days, compared with 14-28 days for Septoria tritici. This explains why it’s so important to halt it early. It can also cause big yield losses of up to 50%,” he added.
Ensuring T1 fungicides target yellow rust and not just Septoria will be an important consideration for farmers this year, Mr Hawkins noted, and delaying control until the flag leaf timing (T2) could be too late.
“AHDB dose response curves show the SDHI fungicide Elatus Era has very strong yellow rust activity, with our own trial work showing T1 is the best time to apply it against yellow rust. This is in addition to the protection it provides against Septoria tritici.
“If you check crops at T1 and think there’s no rust and don’t include a yellow rust active fungicide, you can find soon after T1 that yellow rust has suddenly appeared. I’ve seen this happen.
“You then have the conundrum of do I go back in with a T1.5 spray for yellow rust, which costs more, or not? If not, then it can cycle again before T2 in suitable weather, so you’re in a real dilemma,” Mr Hawkins explained.
“A particular issue this season is that the mild winter means disease in some fields has developed at the same rate as wheat plants have been growing. So even if a lower, older leaf has been killed by frost, the disease may have already progressed up into a younger leaf, even though you can’t see it.
“The main aim of T1 is to control disease on leaf three, which is effectively a stepping stone between any disease present on lower leaves and the upper yield-building leaves. These top three leaves contribute about 75% of final winter wheat yield,” he noted.
If the crop hasn’t been treated with a suitable T0 fungicide before T1, or if there has been a long interval since T0, Mr Hawkins recommends making sure good yellow rust activity is included in the T1 fungicide.
“Inspect crops regularly for signs of yellow rust and take into account variety resistance ratings, but remember that other factors can lead to its rapid flare up, even in more resistant varieties. With heightened Septoria tritici pressures this season, I would also urge growers to look at including the multi-site fungicide, folpet,” he added.
