Powdery mildew found in Geraldton wheat crops

One report found powdery mildew in Vixen wheat (rated susceptible) at Binnu, in a crop which was at ear emergence (GS50-59) and had been extremely dry post sowing but received 30 millimetres of rain towards the end of July.

Infection was also reported on a few wheat varieties at Eradu, with the most severe infection on a Vixen crop that was at the booting growth stage (GS40-49).

The crops were on sandplain soils and symptoms were being seen regardless of previous crop type including wheat grown on canola stubble, lupin stubble and not any worse on wheat stubble.

On Twitter a grower had reported powdery mildew in Scepter near Dalwallinu and there were also anecdotal reports that it was being found in wheat at Mingenew, so growers across the region have been encouraged to monitor their cereal crops.

Department of Primary Industries and Regional Development (DPIRD) research scientist Ciara Beard said powdery mildew was more likely to advance rapidly on more susceptible wheat varieties.

"Control of powdery mildew is more difficult in advanced crops so prioritise inspecting crops that are not yet at head emergence growth stage," Ms Beard said.

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"Crops sown at high density and/or with high nitrogen application may be at higher risk and crops with good yield potential are more likely to be economical to spray."

The disease can be both stubble borne and carried over a green bridge and it produces masses of wind-borne spores which can be carried over long distances.

This year it has possibly arrived in the north via wind borne spores from the Kwinana port zone where it has been reported in wheat and wheat regrowth since June.

Powdery mildew infection usually starts low in the canopy and under severe disease pressure can appear on stems and heads.

Growers should look for fluffy, white powdery growths of fungal spores on the 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.

Ms Beard said a powdery mildew-infected crop may appear yellow from a distance.

"Temperatures of 15-22°C favour the disease in conjunction with high humidity and while rain does not spread the disease, it can create extended periods of canopy humidity which favour disease development," she said.

"Under favourable conditions, the infection cycle can take as little as seven days causing rapid build-up in crops, so growers and consultants can expect to see more powdery mildew if humid and damp conditions continue.

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"Mildew can also disappear rapidly in dry, hot weather and very heavy rain can also wash spores away."

Powdery mildew can cause significant yield and quality reductions, however the presence of mildew infection does not always result in a yield loss or in an economic response to fungicide application.

Infection early in the season can significantly reduce yield in susceptible varieties (by up to 25 per cent), but after Z39 it usually causes less impact (5-25pc yield loss).

Infection of glumes and awns can reduce grain weight and grain number, and subsequently yield.

Significant impact from head infection is more likely when infection is severe (more than 20pc glume area affected) but fungicide application is generally only economical if applied before head infection occurs.

Head infection is most likely when mildew is present in the crop canopy and mild humid weather continues after heads have emerged.

DPIRD research scientist Geoff Thomas said research had shown it's crucial to control the disease before it becomes too severe and develops in the upper canopy and on heads as then it is very difficult to control.

"A registered foliar fungicide can reduce the disease impact, but growers need to consider the weather outlook, variety susceptibility, growth stage, the level of disease present in the crop, presence of other diseases and crop yield potential when deciding whether an economic response to fungicide application is likely," Mr Thomas said.

"If wheat powdery mildew is present in a susceptible variety that is not at head emergence, is increasing in the canopy and the weather outlook is favourable, it is recommended growers intervene with a well-timed application of registered foliar fungicide to stop disease reaching damaging levels and moving onto the flag leaf and head.

"Fungicide application once all leaves have emerged, after Z39, is a good strategy so maximum canopy area is protected."

Fungicide applied following head emergence and prior to severe infection can reduce risk and impact of head infection, however in terms of yield responses, trial results have been variable.

A period of warm dry weather can also quickly eliminate powdery mildew infection on wheat heads and eliminate the value of fungicide.

Spraying after heads have emerged, prior to the end of flowering and before significant head infection is more likely to achieve an economic return if a wet spring or longer season is expected.

"Fungicide application at a registered rate should protect sprayed leaves for a period of about four weeks and slow disease development in the crop, however it is unlikely to totally eradicate disease and infection of unprotected foliage can still occur," Mr Thomas said.

"Yield response to fungicide application is not guaranteed - if disease is not severe or diminishes naturally then fungicide is unlikely to provide significant yield benefit.

"A second fungicide application has generally not been found to be justified or economical in DPIRD trials conducted to date but may be considered in longer season environments."

Wheat powdery mildew is at high risk of developing fungicide resistance so growers are encouraged to use fungicides wisely.

As good practice, all fungicide actives should be rotated within and across seasons.