Timing is key to controlling powdery mildew in wheat

(left to right) DAFWA plant pathologist Ciara Beard, and CCDM research associate Madeline Tucker at the 2016 GRDC Grains Research Update, Perth, where latest wheat powdery mildew research results were presented.

Photo: Cox Inall Communications

An early season foliar fungicide application could be vital this year to limit potential yield losses from wheat powdery mildew in susceptible areas of the WA grainbelt.

That is the message from Department of Agriculture and Food WA (DAFWA) plant pathologist Ciara Beard, who is conducting GRDC-funded research into management of this costly fungal leaf disease.

Her trials in 2015 highlighted that timing of a single fungicide application as soon as possible after disease is seen moving up the crop canopy is more important than product choice.

She found that a range of fungicide actives, when applied at registered rates on susceptible wheat varieties, boosted yields at four out of six WA field trial sites.

There was an average 8 per cent yield increase (with a range of 0-26 per cent) across all sites from applying fungicides between wheat growth stage GS32 (flag leaf emergence) and GS60 (end of ear emergence), compared to untreated plots.

Ciara says the key to success was controlling powdery mildew before it became too severe and before it reached the flag leaf and wheat heads. Once in the heads, the disease not only reduced yield, but also adversely affected grain quality.

She warns that significant levels of wheat powdery mildew inoculum has been carried over to this season from 2015’s high levels on infested stubble - and potentially could be multiplied if a green bridge of wheat volunteers develops this autumn.

Cause and incidence of wheat powdery mildew in the western region

Powdery mildew inoculum can be air and stubble-borne and can carry-over to crops on volunteer wheat regrowth (green bridge) between seasons.

Until 2015, it had not caused significant damage to WA wheat crops since the late 1990s/early 2000s.

But high levels of summer and autumn rain last year, followed by a humid and moist growing season, resulted in powdery mildew persisting and spreading through wheat crops - especially in susceptible varieties and particularly in the central, northern and Esperance grainbelt regions.

The level of disease in commercial wheat crops across northern agricultural regions, in particular, has been unprecedented in WA in recent years and has prompted GRDC-funded researchers to investigate the effectiveness and profitability of applying fungicides, optimum timing and products, the value of multiple fungicide applications and the potential risks of fungicide resistance.

What to look for in 2016

Ciara says monitoring wheat crops from the start of tillering is essential to detect early symptoms of powdery mildew infection.

She says particularly vulnerable are susceptible varieties, early sown wheat and crops with dense canopies, high nitrogen (N) status and full soil moisture profiles.

Powdery mildew first appears as fluffy, white powdery growths of fungal spores on the leaf surface. Symptoms progress from lower to upper leaves.

Under severe disease pressure as the season progresses, the fluffy, white powdery growth can also affect stems and heads.

Fungal colonies eventually enlarge and merge together and the area surrounding the lesion and on the reverse side of the leaf turns from yellow to brown.

Older infections on leaves and heads turn grey and can develop black fruiting bodies, called cleistothecia, which appear as black specks.

A powdery mildew infected crop can appear yellow from a distance, similar to a crop suffering from water logging or nutrient deficiency. Therefore, close examination is needed.

Weather conditions are key to the progression of this disease, as it prefers temperatures of 15-22°C and high humidity.

Disease progress will slow as humidity drops and when the outlook is for daytime temperatures above 25°C.

Details and photographs of the symptoms of powdery mildew are available on the DAFWA website and on the MyCrop website.

Foliar fungicide strategies for wheat powdery mildew

There is no up-front, preventative protection registered for wheat crops against powdery mildew.

Several seed dressing and in-furrow fungicides are registered for use in wheat, but these do not have registration specifically for powdery mildew in wheat.

A wide range of fungicides are registered as in-season foliar sprays for wheat and the actives include: 
  • Triadimefon
  • Propiconazole
  • Propiconazole plus tebuconazole or cyproconazole
  • Azoxystrobin plus epoxiconazole
  • Epoxiconazole
  • Epoxiconazole and pyraclostrobin
  • Azoxystrobin plus cyproconazole
  • Tebuconazole plus prothioconazole
It is recommended these are used at full label rates and rotated between years.
More information about registered fungicides and in-season wheat powdery mildew management is available from the DAFWA website crop disease information hub.

Fungicide trial results from 2015

GRDC-funded, in-season fungicide application trials were set up in 2015 at Geraldton, Moonyoonooka, Gibson, Munglinup, Sandy Gully and Buntine and carried out by DAFWA - in conjunction with local grower groups, advisers and industry stakeholders.
 
Key yield response and grain quality findings from these 2015 trials included: 
  • Most fungicide products provided significant disease control compared to untreated plots.
  • There was no significant difference between fungicide actives for yield response.
  • Average yields increased 8 per cent, across all sites, when fungicides were applied between GS32 and GS60.
  • Yields increased up to 17 per cent from a single fungicide application between flag leaf and head emergence.
  • Fungicides did not improve yield where disease severity dropped (due to hot dry weather conditions in spring).
  • Crops that had other diseases present, such as yellow spot and/or septoria nodorum, in addition to powdery mildew had a greater yield response to the foliar fungicide applications.
  • Three out of six trials showed grain quality improved with use of a fungicide in the optimum window, with 19 per cent less screenings (on average) in one trial at Munglinup (compared to untreated plots).
  • At the Gibson site, fungicide use increased grain hectolitre weight by 2 per cent.
  • Warm and dry spring conditions at most sites reduced the impact of head infection on grain quality.


Key findings from timing of fungicide applications in the 2015 trials included: 

  • A single fungicide application was effective when applied to crops between GS32 to GS60, prior to development of severe infection.
  • When fungicide was applied prior to flag leaf emergence, infection of newly emerged unprotected leaves was observed and these became a source of re-infection to the crop. These crops might need a second spray (cost effectiveness would need to be considered).
  • Powdery mildew severity on wheat heads was controlled more effectively by fungicide application to reduce canopy infection prior to head emergence than through direct application to heads already exposed to infection.
  • A second fungicide application was trialled at the Geraldton site but did not increase yields significantly, compared to using a single spray, which was likely due to the dry spring in 2015.
  • At the Munglinup site, a second application did increase yields for one fungicide treatment only (out of nine treatments).
  • Key findings from fungicide product comparisons and economics in the 2015 trials included: 
  • All registered fungicide actives for powdery mildew in wheat that were trialled provided good disease control at all sites.
  • Fungicides provided at least four weeks of disease control at most sites in 2015.
  • A single fungicide application was the most profitable option in 2015.
  • For example, profits increased by $33-$115/hectare compared to untreated plots at the West Buntine and Munglinup sites.
  • Profits were optimised when fungicide was applied before disease became severe and before flag leaves and heads were infected, or where disease onset was later in the season and application was made when all leaves had emerged (after GS39) - so the canopy area was fully protected.


Key findings from 2015 trials in terms of crop re-infection included: 

  • Re-infection at some time after fungicide application resulted from emergence of new unprotected tissue, high levels of air-borne spore burden and rapid infection cycle (as few as seven days in optimum disease conditions).
  • Surveys by the GRDC-supported Centre for Crop and Disease Management (CCDM) did not identify any evidence of fungicide resistance in WA wheat powdery mildew at this time. Re-infection was not due to fungicide resistance or failure.
  • Fungicide application at registered rates should protect treated crops and slow disease progression/spread in the crop for up to four weeks.
  • But fungicides are unlikely to totally eradicate disease.
  • If there is high disease incidence and severity in a region, masses of air-borne inoculum have potential to infect newly-emerged, untreated foliage.
  • Later germinating leaves and tillers can also be a source of re-infection for crops if not controlled by a second fungicide application (best when all leaves have emerged).

Ciara says the 2015 trials highlighted that use of foliar fungicides can reduce the incidence of powdery mildew in wheat and potentially increase yield and grain quality, but this is not guaranteed. She says success is most likely to be driven by management strategies that target: 

  • Use of wheat varieties with greater than moderate susceptibility.
  • Disease control in the canopy before it affects the heads.
  • Applying fungicide before disease is too severe.
  • Using a fungicide rate that is sufficient to provide longer protection and reduces the need for a follow-up treatment.


Difficulties in controlling powdery mildew in wheat in 2016 are most likely to arise from: 

  • High inoculum pressure.
  • Favourable weather conditions.
  • Variety susceptibility.
  • Poor canopy penetration of fungicides.
  • Inadequate fungicide rate.
  • Use of outdated fungicide product.
Ciara’s WA wheat powdery mildew 2015 trial results, graphs and tables were presented at this year’s GRDC Grains Research Update, Perth and the full Update paper is available here (see session seven).

Upfront and longer-term protection - 2016 research and beyond

DAFWA is undertaking GRDC-funded trials in 2016 to compare active fungicide Ingredients for use as seed treatments and in-furrow (currently none of the seed dressing and in-furrow fungicides registered for wheat are registered to control powdery mildew in wheat).

In the longer term, the CCDM, based at Curtin University, is continuing vital research into monitoring and treating fungal crop diseases.

To date, there have been no incidences of fungicide resistance in wheat powdery mildew in WA.

But the CCDM has recently observed initial genetic changes (or ‘gateway’ mutations) in wheat powdery mildew samples from New South Wales and Tasmania. This could lead to fungicide resistance issues in this disease in Australia.

CCDM research associate Madeline Tucker says new precision diagnostic technologies are being used by CCDM researchers to track low levels (of as little as 0.1-1 per cent) of fungal diseases in the paddock much earlier in the growing season, ultimately helping in the development of faster and more effective treatment options for grain growers.

She says this has been made possible with advances in digital polymerase chain reaction (dPCR) techniques that are fast-tracking fungal pathogen sampling.

Ms Tucker says the development of early detection screening tests for common cereal diseases in Australia would not be possible without understanding the underlying mechanisms of fungicide resistance, which is a key function of the CCDM.

Please note, this information can be reproduced in newsletters.

More information

Ciara Beard, DAFWA
08 9956 8504
ciara.beard@agric.wa.gov.au

Contact

Melissa Williams, Cox Inall Communications
0428 884 414
melissaw@coxinall.com.au

Useful resources

GRDC Grains Research Updates registration and papers
www.giwa.org.au/2016researchupdates
 
GRDC Fact Sheet ‘Powdery mildew in barley and wheat’
www.grdc.com.au/GRDC-FS-PowderyMildewBarleyWheat
 
Ground Cover Issue 102 - Emerging Issues with Diseases Weeds and Pests Supplement
www.grdc.com.au/GCS102
 
GRDC Wheat GrowNotes
www.grdc.com.au/GrowNotes
 
DAFWA crop disease information hub
www.agric.wa.gov.au/cropdisease
 
DAFWA ‘Managing powdery mildew in wheat’ hub
www.agric.wa.gov.au/spring/managing-powdery-mildew-wheat
 
DAFWA registered foliar fungicide guidelines
www.agric.wa.gov.au/barley/registered-foliar-fungicides-cereals-western-australia-wa
 
Centre for Crop and Disease Management
www.ccdm.com.au

GRDC Project Code DAW00229, CUR00022

Region West