Published on 26/09/2025
Words: Neil Waddingham, Bayer New Zealand.
Speckled Leaf Blotch (SLB) is a very important disease of winter wheat in New Zealand and one which, if not controlled, will lead to significant loss of yield.
It is also a disease that has developed resistance to several of the fungicide types we have available for its control throughout the world.
Over the past few years both Kestrel and Prosaro have delivered outstanding control of SLB in NZ. Both Kestrel and Prosaro contain two DMI fungicides, prothioconazole and tebuconazole, and it is this that ensures consistent control of SLB.
The use of ready formulated fungicides containing multiple DMI active ingredients is referred to as DMI stacking.
Fungicide resistance is increasing
While fungicide resistance doesn’t get talked about as much as herbicide resistance, it is a real and increasing threat to cereal production in NZ.
All cereal diseases are capable of developing resistance to many of the available fungicide groups (DMI, QoI (strobilurin), SDHI) with septoria leaf blotch (SLB), scald, net blotch (NB) and ramularia leaf blotch (RLS) raising the greatest concern.
How can I delay disease resistance?
Prior to spring, careful cultivar choice, planning crop rotations and ensuring good paddock hygiene can all help.
But now spring is here, it comes down to responsible fungicide use, with the key actions being to apply a fungicide programme that incorporates different fungicide modes of action, and to apply the label dose rate.
DMI fungicides are a go-to choice
When it comes to planning fungicide programmes containing different modes of action, DMI fungicides are an essential element.
Speckled leaf blotch (SLB) is probably the most important wheat disease in NZ.
Fungicides such as Prosaro and Kestrel, which contain the DMI fungicides prothioconazole and tebuconazole, deliver excellent control and have been the foundation of SLB control in NZ for many years.
DMI stacking explained
Early in the development of prothioconazole it was observed that, while prothioconazole alone provides excellent control of SLB, more effective control could be achieved by mixing prothioconazole and tebuconazole together.
The term DMI stacking was adopted to describe this mixture.
Why does it work?
In any paddock, the population of the fungal pathogen which causes SLB (Zymoseptoriatritici) is varied with different isolates varying in their susceptibility to different triazole fungicides.
By applying prothioconazole and tebuconazole in mixture, you deliver a double blow.
Controlling more elements of the SLB population achieves more complete disease control. In turn, this leaves behind less fungal pathogens that may be in the process of developing resistance.
Stacking Prosaro and Kestrel
Two popular arable fungicides which adopt DMI stacking are Prosaro and Kestrel. Both Prosaro and Kestrel contain the DMI fungicides prothioconazole and tebuconazole, formulated together into well balanced and very effective ratios that have been extensively tested under NZ conditions.
These products deliver very effective control of SLB and leaf and stripe rust in wheat while supporting a strong fungicide resistance management approach.
They are also very effective fungicides for the control of many key diseases of barley and ryegrass seed crops.
With spring around the corner, and disease development starting anytime now, now is the time to plan your fungicide programmes.
Incorporate Prosaro or Kestrel in mixture with non-DMI fungicides, such as the very effective SDHI fungicide VIMOY iblon.