Powdery mildew control in wheat
High nitrogen inputs in key wheat growing regions of Australia are leading to greater instances of powdery mildew for grain farmers seeking a bumper crop.
Elders senior agronomist Belinda Eastough explains how to protect against the disease and maintain the quality of your grain.
The fungus Blumeria graminis f.sp tritici causes powdery mildew in wheat. The disease is carried over on wheat stubble or volunteer wheat and when the season break occurs spores are released and become airborne.
The fungus appears as a whitish powdery growth on the upper surface of leaves and leaf sheaths. As the infection ages there is a yellowing of the infected tissue and the infected area turns a dull grey colour with small black specks present.
The black circular specks are the fruiting bodies. Heads may become affected and covered with masses of white powdery spores when the infection is severe.
Occurrence and seasonal conditions
Senior agronomist, Belinda Eastough, says that specific seasonal conditions, including humidity and dense crops with high nitrogen levels, can stimulate the development of powdery mildew.
“We are currently seeing high nitrogen inputs being applied to wheat in the northern wheatbelt of Western Australia due to seasonal conditions, as well as poor potassium levels increasing disease levels,” says Ms Eastough.
High humidity (85-100 per cent) is required for infection and disease build-up is most rapid at 15 to 22⁰C. Temperatures above 25⁰C retard the growth of spores.
“Wheat powdery mildew survives between crops on volunteer wheat or on wheat residues,” she says.
“In high rainfall years we are seeing an increased incidence of powdery mildew mainly due to increasingly productive crops, stubble retention or increasing varietal susceptibility.”
Rotating crop species is an effective method to decrease the likelihood of early infection.
Varietal maturity does influence the level of disease and the amount of yield reduction, so a variety maturing during a period that is favourable to disease development has a higher probability of disease loss.
Wheat varieties have different levels of resistance to powdery mildew, so if you are in an area that is prone to powdery mildew, choosing a variety with a higher level of resistance is important. Below is a table of some commonly grown wheat varieties.
|VARIETY||DISEASE RATING FOR POWDERY MILDEW|
|Chief CL Plus||S|
KEY: SVS – Susceptible to very susceptible, S – Susceptible, MSS – Moderately susceptible to susceptible, MRMS – Moderately resistant to moderately susceptible
Source: DPIRD Western Australian Crop Variety Guide 2021
Seed dressings and in-furrow treatments
There are no wheat seed dressings registered for powdery mildew control in wheat, but anecdotal evidence is that some seed dressings and in-furrow treatments may delay the onset or suppress the disease.
Powdery mildew in wheat, given the right conditions, can be very aggressive and the masses of white powdery spores can turn the leaves from yellow to brown quite quickly. The general rule of thumb is to spray at Z31-33 (early stem elongation) when more than 5 per cent of the top three leaves are affected as the disease moves very quickly in humid conditions.
If the infection is moving up the canopy late and active infections are present on the middle canopy leaves then spraying at Z39 (flag leaf emergence) may give an economic return depending on the seasonal conditions. The disease will not progress if the canopy becomes dry and warm.
Propiconazole is one of the most used actives on powdery mildew in wheat, however it is important to discuss your options with your local Elders agronomist and read the fungicide label.
Not all fungicides that are registered on barley powdery mildew are registered on wheat powdery mildew as they are a different sub species of fungi. As is often the case, the best tactical strategy is to obtain some technical advice to assist with decision making.
Elders offers a range of crop protection products.
Image credit: WA Department Primary Industries and Regional Development.