Careful mixing needed to hit key wheat diseases

THE KEY issues for Scottish growers in wheat this year are septoria and yellow rust – which were present in the autumn and bubbled away over the mild winter and were still present as we came into the new year.
A spell of dry weather helped to reduce some of that infection, as lower leaves were shed at stem extension – but it hasn’t vanished and any recent rainfall will let it flair up again.
SRUC trials have reinforced the benefits of timing fungicide sprays accurately to target the important leaf layers with the T1 first timing aimed at leaf 3, which tends to emerge at second node and the second, at T2, aimed at the all important flag leaf.
Early T1 sprays at first node will tend to target leaf 4 or 5 and leave a long gap until the flag spray, when disease can take hold.
Outbreaks of yellow rust in some crops are complicating product choice at T1 (stem extension).
Where septoria risk is low then there were crops that would have got an azole plus chlorothalonil mix but the addition of a yellow rust threat in a crop makes more of a case for additional chemistry.
Adding a strobulurin helps with yellow rust protection and so would the inclusion of an SDHI. The potential advantage of an SDHI is that they will boost septoria control and also give some control of eyespot, which can also be a problem in Scottish crops. Levels were higher than usual last harvest so there is a risk for this year.
Looking forward to the flag leaf timing, then most crops will benefit from an SDHI, plus azole, plus multi-site in a three-way mix. That’s because we know that septoria is harder to manage with azoles alone and we also know that mutations conferring some reduced sensitivity to SDHIs were detected at Scottish sites last season and again at the start of this.
They are not yet likely to have a big impact on performance, but the use of balanced mixtures and crucially the inclusion of a multi-site should slow the development of these unwanted strains.
We should remember that despite the decline in the azoles efficacy against septoria they remain important in programmes. In addition to their contribution against septoria, they are the most effective group in protecting against yellow rust – so there is no case for unbalanced mixtures where the SDHI component is weightier than the azole.
That approach would hasten SDHI resistance development and would also leave the affected crop at increased risk of yellow rust so really isn’t attractive either at an individual or at an industry level.