Grains Research and Development

Date: 03.03.2014

Shift in powdery mildew resistance ratings

Author: Tom Dixon

With good monitoring and effective fungicides, growers can avoid significant losses from powdery mildew in barley.

Image of powdery mildew on a barley leaf

Powdery mildew appears as white, fluffy patches on leaf surfaces, and colonies produce many windborne spores.

PHOTO: Queensland DAFF

Barley is a staple of northern farming systems, but mild, humid conditions threaten outbreaks of the fungal disease powdery mildew.

Many varieties carry genetic resistance to the fungus and, until recently, the popular variety ShepherdPBR logo was considered resistant.

Researchers, however, have found new strains of powdery mildew with the ability to infect commercial varieties of barley, such as ShepherdPBR logo, and are urging growers to be aware that yields can be affected by the disease.

Diseases such as leaf rust and net blotches will reduce yields more, but a severe powdery mildew infection can reduce yields by as much as 15 per cent.

High risk/low losses

Barley powdery mildew is a windborne fungal disease that survives on stubble, plant residues and barley volunteers.

Mildew that over-summers on stubble releases spores during cool, wet periods in autumn and infects young barley plants.
The disease can increase rapidly. Spore germination, infection and subsequent spore production may take as little as six days.

However, it is not all bad news for growers. “In a favourable season, powdery mildew will be a risk, but will not automatically have a yield impact,” says Greg Platz, principal pathologist at the Queensland Department of Agriculture, Fisheries and Forestry (DAFF) Hermitage Research Facility in Warwick. “Yield losses generally range from five to 15 per cent from the heaviest disease levels,” he says.

Queensland DAFF scientists are keen to help growers understand the risks of powdery mildew in barley: “We want to create awareness of powdery mildew but not alarm,” Mr Platz says.

And awareness in the Eastern Darling Downs is growing, says agronomist Jeff Stone from Pursehouse Rural who is based in Pittsworth, Queensland.

“Growers are now thinking about protecting barley yield and quality in relation to disease, because of the yield losses suffered in 2010,” Mr Stone says.

Taking control

For growers in the Eastern Darling Downs, who mainly crop CommanderPBR logo, ShepherdPBR logo and Oxford varieties, the coming season is likely to be similar to previous years for powdery mildew.

CommanderPBR logo and Oxford varieties have good levels of resistance to mildew and should not need fungicide.

However, laboratory studies in 2012 identified a strain of powdery mildew that can infect the ShepherdPBR logo variety and its susceptibility was confirmed by field observations last year. The updated resistance rating for ShepherdPBR logo is now Moderately Susceptible to Susceptible (MS-S).

The last two seasons on the Eastern Darling Downs have been characterised by winters conducive to early disease build-up, but dry springs restricted its spread. Added to this, improved disease resistance in newer varieties and better disease management by growers have minimised losses to powdery mildew in barley crops.

Apart from choosing and planting resistant varieties of barley, growers can control powdery mildew with fungicides.

Up-front fungicides

Barley growers on the Eastern Darling Downs have readily adopted the use of the fungicide flutriafol since 2010 as an up-front treatment for barley diseases. As a result, he has seen very little powdery mildew in barley over the past four seasons.

Mr Stone says most growers that he sees apply flutriafol with starter fertiliser or, to a lesser extent, in furrow injection at planting.

“Flutriafol is good insurance against powdery mildew, giving long disease protection at a low cost, and with careful monitoring has allowed us to put off foliar fungicide treatments to awns-visible stage,” he says.

These up-front treatments can see crops through their most vulnerable stages, giving protection for  six to 12 weeks from sowing.

“People are controlling mildew well, particularly the larger barley growers,” Mr Stone says. “They are being proactive with disease management because they don’t want to be dealing with low disease levels early in winter that lead to later disease build-up when weather conditions become favourable, including powdery mildew.”

Fungicide resistance: growers should avoid using repeat applications of the same DMI fungicide in any one season 

Plant pathologist Greg Platz says that proactive control is the best way to get on top of potential infections.

“Fungicides are more effective as protectants than eradicants, so it’s best to apply them before powdery mildew becomes a problem.”

However, caution is required. Fungicide resistance in powdery mildew is a major issue in Western Australia from overuse of the older triazole fungicides. Flutriafol is one of these fungicides and must be used responsibly.

In 2012, an isolate of powdery mildew from the northern region was shown to have reduced sensitivity to DMI fungicides such as triadimefon, flutriafol and tebuconazole. This signals the first step in the development of fungicide resistance.

Growers should avoid using repeat applications of the same DMI fungicide in one season. Any evidence of failure of the product to control powdery mildew should be reported immediately.

What to look for

Infections appear as white fluffy patches on the leaf surface and, where the disease is severe, whole leaves may be covered. Initial infection occurs during early crop growth in autumn and early winter, increasing through tillering, then waning as crops mature and temperatures rise.

Good nitrogen nutrition, low light intensity and dense crop canopies favour the development of  powdery mildew.

Growers are urged to consult the National Variety Trials website (www.nvtonline.com.au) for up-to-date varietal resistance ratings as these can change from one season to the next. ShepherdPBR logo is a good example, going from MR in 2012, MR-S in 2013 and MS-S in 2014.

Mr Platz suggests that growers can assess the risk of powdery mildew in their paddocks by determining:

  • the field’s recent crop history;
  • powdery mildew in recent barley crops;
  • infected crop residues in the field; and
  • infection in nearby, early-sown crops.

Monitor and report

Growers who have sown varieties rated MS or MS-S should monitor their crops for powdery mildew at the vegetative stages of growth, from early tillering (about a month after sowing). Monitoring would be roughly every 10 days.

If powdery mildew is detected, growers should consider applying an appropriate fungicide immediately, based on varietal resistance level, growth stage, severity of infection and likely future weather conditions.

Growers and agronomists are urged to report high levels of infection to their relevant department of agriculture if it occurs in ShepherdPBR logo, GroutPBR logo or NavigatorPBR logo varieties.

Researchers from Queensland DAFF and the New South Wales Department of Primary Industries are monitoring the spread of mildew strains that attack these varieties.

Awareness is key

Although powdery mildew on barley can cause significant yield losses in susceptible varieties, growers making smart variety choices, monitoring consistently and reacting quickly to infection can largely avoid these losses.

More information:

Greg Platz
07 4660 3633
0408 733 055
greg.platz@daff.qld.gov.au

Jeff Stone
0429 931 376
j.stone@prural.com.au

Steven Simpfendorfer, NSW DPI
0439 581 672
steven.simpfendorfer@dpi.nsw.gov.au

The NSW DPI Winter crop variety sowing guide is available at: www.tinyurl.com/NSWDPIwintersowing

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