POWDERY mildew - a fungal disease mainly found in cereals - has been detected at various stages of maturity in wheat and barley crops around the State.
Planfarm agronomist Nic McKenna reported finding powdery mildew in Rockstar wheat - rated moderately susceptible, to susceptible (MSS) - in the shire of Perenjori.
The crop was sown in early April and lesions were visible on the stems but not the leaves.
DKT Rural Agencies agronomist Dan Taylor discovered mature powdery mildew pustules in regrowth wheat at heading and in Scepter wheat - rated susceptible (S) for mildew - at the four to six leaf stage near Meckering.
Factors which can favour powdery mildew:
- Mild temperatures (15-22°C)
- High humidity (in excess of 70pc).
- Dense crop canopies
- High nitrogen nutrition
- Hard rain can help suppress mildew development
The crop had been treated with Uniform fungicide in-furrow, which is not actually registered for control of mildew in wheat but is registered to suppress mildew in barley.
Powdery mildew had also been found in Buff barley (rated S for mildew) near Mukinbudin, as well as in barley crops near Northam and Wialki.
The disease itself has distinctive symptoms - fluffy, white powdery growths which become cream-grey with age - and fungal spores can be seen on leaf surfaces and under severe pressure later in the season it may also be present on leaf sheaths and in heads.
Department of Primary Industries and Regional Development (DPIRD) research scientist Andrea Hills said powdery mildew was favoured by mild temperatures (15-22°C) and high humidity (in excess of 70 per cent).
"Dense crop canopies, high nitrogen nutrition and extended periods of canopy humidity are factors which are understood to favour mildew development," Ms Hills said.
"Dry and warm weather conditions that result in periods of low canopy humidity and temperatures above 25°C can reduce disease development.
"However, hard rain can help suppress mildew development."
Barley and wheat powdery mildew are host specific and do not cross infect.
However, the reports from around the State indicate seasonal conditions have been conducive for disease development of both mildews.
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As a result, monitoring of wheat and barley crops for mildew and other diseases should be ongoing, particularly in areas where a pre-season green bridge was apparent or susceptible varieties have been sown.
Infection early in the season on susceptible and moderately susceptible varieties can significantly reduce yields by up to 25pc, however infection at later stages (after Z39) is usually less damaging.
"Seed dressing or in-furrow products applied to manage other diseases may provide some degree of protection from powdery mildew but are not registered for this use," Ms Hills said.
"Registered foliar fungicides can be used to control powdery mildew infection - application early in the epidemic before significant disease development, meaning first application around stem elongation, is most effective."
Fungicide resistance
There are currently no records of fungicide resistance in wheat powdery mildew in Western Australia.
However, reports of mutations affecting Group 11 and Group 3 fungicides in the Eastern States show the need for appropriate fungicide resistance management strategies in WA.
Since 2012 barley powdery mildew in WA has exhibited resistance to some DMI fungicide products and as part of a disease resistance management strategy growers should avoid using compromised fungicides.
"The compromised triazole ingredients include tebuconazole (resistant), flutriafol and propiconazole (reduced sensitivity)," said DPIRD research scientist Kithsiri Jayasena.
"These fungicides (particularly tebuconazole) have reduced field efficacy and are not recommended for barley powdery mildew control as stand-alone products.
"As good practice all DMI fungicide actives should be rotated within and across seasons, repeated use of the same active ingredient will increase the selection pressure on the fungicide resistant strains of powdery mildew."
One way to reduce resistance development of the pathogen is to use fungicide mixtures with different modes of action - 13 compounds from four modes of action are registered in Australia for powdery mildew control.
Dr Jayasena said fungicides containing both a strobilurin (Group 11) and an uncompromised triazole (Group 3) active ingredient or succinate dehydrogenase (SDHI Group 7) base should have uncompromised activity against powdery mildew.
"Use one application per season of each of these strobilurin or SDHI based fungicide mixtures as part of a fungicide resistance management strategy within the disease control program," he said.
"A powdery mildew specific multisite amine (Group 5), spiroxamine (for example Prosper 500 EC) is also registered."
