Protecting cereal crops from powdery mildew

Date: 21 Nov 2018

Key Points

  • Cereal powdery mildew can significantly reduce crop yields and grain quality in WA
  • The disease is hard to control when established
  • Monitor crops for symptoms and treat early if a late winter/spring foliar fungicide is needed.

Late cereal plantings and variable seasonal conditions across the Western Australian grain belt this year are making disease predictions difficult for researchers. But the advice is to be vigilant and continue monitoring crops for signs of diseases, such as powdery mildew, that might occur later in the season than usual.

Caused by the fungal pathogens, Blumeria graminis f. hordei in barley and Blumeria graminis f. tritici in wheat, powdery mildew has become more prevalent with the shift to more susceptible crop varieties. On-farm management difficulties for barley powdery mildew are compounded by evolving issues of fungicide resistance.

Department of Primary Industries and Regional Development (DPIRD) research officer Geoff Thomas says powdery mildew might be seen during August and September in 2018 - especially in high rainfall areas - and it is important to control the disease at flag leaf stage, before crops start going into head.

He says fungicide decisions for barley crops will need to consider the decline in control from some triazole based (DeMethylation inhibitor, or DMI) fungicides and resistance to tebuconazole-based products.

Conducive conditions

DPIRD advises that powdery mildew is typically favoured by:

  • A susceptible host
  • Mild temperatures (15-22°C) and high humidity (above 70 per cent)
  • Dense crop canopies, which are correlated to high seeding rate and/or high nitrogen nutrition
  • A good soil moisture profile that promotes canopy humidity
  • Extended periods of humid and damp canopy.

Dry and warm weather conditions that result in periods of low canopy humidity, and temperatures above 25°C, will typically reduce disease prevalence. This means powdery mildew can disappear rapidly in spring months.

Monitoring and what to look for in the paddock

Growers and advisers are encouraged to monitor cereal crops regularly from early tillering, particularly in susceptible varieties, to detect powdery mildew symptoms.

Across a cereal paddock, a powdery mildew-infected crop can appear yellow from a distance. It may look similar to a crop suffering from water logging or nutrient deficiency.
Signs of powdery mildew on cereal plants include:

  • Fluffy, white powdery growths of fungal spores on the leaf surface
  • Spread of white powdery growths from lower to upper leaves, then stems and heads
  • Fungal colonies that enlarge and merge together
  • Yellow to brown areas around the leaf lesion
  • Lesions on leaves and heads that turn grey with age and can develop black fruiting bodies, called cleistothecia, that appear as black specks
  • Leaf death.

More details and images of powdery mildew symptoms can be seen on the DPIRD website.

Reports of outbreaks can be made through the DPIRD-GRDC PestFax service and MyCrop apps.

Potential crop losses

Past research with GRDC investment has found infection from powdery mildew early in the season can reduce cereal yields by up to 25 per cent by reducing photosynthetic leaf area and causing abortion of young tillers.

Infection later in the season, such as between stem elongation and flowering stages, can reduce photosynthetic leaf area and lead to smaller grain size, lower yields and grain quality downgrades.

Severe infection can also cause crop lodging through weakened stems.

The earlier the infection and the higher up the plant it spreads, the bigger the potential yield loss.

DPIRD trials have found severe infection at later crop stages (after Zadoks Growth Scale Z39) can cause 5-25 percent yield loss.

Value of foliar fungicides

DPIRD research, with GRDC investment, has shown yields can increase by 5 to 17 per cent when fungicide is applied from flag leaf to head emergence.

In 2015 trials, an average yield response of 10 per cent (in a range of 3-26 per cent) was achieved from a single fungicide spray in four out of six trials conducted by DPIRD and industry partners Landmark, Imtrade, Liebe Group and Northampton Agri Services.

Geoff Thomas says higher yield responses can occur if powdery mildew is present with other diseases and these are also effectively controlled.

He says in trials where disease established at an earlier growth stage (stem extension) and two sprays were applied, yield responses of up to 20-25 per cent were recorded in high yield-potential areas.

Geoff says it is important to note, however, that where powdery mildew is the dominant disease, yield responses are not guaranteed.

For example, in 2015, two out of six trials across the grainbelt had no significant response to foliar fungicide application.

At one site, where the crop was late-sown and experienced hot dry spring conditions, disease pressure fell quickly and reduced fungicide impact (and therefore the need for application).
Geoff says in seasonal conditions such as this, if disease is not severe - or levels fall naturally in with late warm weather - then fungicide application is unlikely to provide significant yield benefits.

For more information and powdery mildew trial results read the 'Fungicides for managing powdery mildew in wheat historical trial report' on the DPIRD website.

Foliar fungicide options

Barley

For barley powdery mildew, CCDM Fungicide Resistance Group leader Fran Lopez-Ruiz says the compromised triazole ingredients tebuconazole and triadimefon will have reduced efficacy and are not recommended for use in WA.

He says use of tebuconazole will increase selection pressure on the fungicide resistant strains of powdery mildew (and other fungal diseases - with emerging fungicide resistance issues for diseases such as the net blotches).

But products containing other triazole fungicide-active ingredients should be effective and include:

  • Epoxiconazole (for example, Opus® 125)
  • Prothioconazole + tebuconazole (for example, Prosaro®)
  • Propiconazole (for example, Tilt®).

Other options for barley powdery mildew control that should be used as a single application in one season include formulations that combine active ingredients such as:

  • Cyproconazole + azoxystrobin (for example, Amistar Xtra®).

It is advised that strobilurin products are protectant and should not be applied to heavily infected crops.

In multiple application programs, Dr Lopez-Ruiz recommends use of strobilurins and SDHI products in rotation with uncompromised triazole options. This can help to minimise potential pressure on this chemistry.

Caption: Powdery Mildew has been seen in pockets of WA on wheat, with leaf symptoms pictured. PHOTO: CCDM

Wheat

Dr Lopez-Ruiz says there is confirmed resistance to the highly effective strobilurin (QoI fungicides) in some eastern states and advises minimal use in this State where possible.

He says this would involve only a single spray per season and, if wheat did require a further treatment, then using an alternate fungicide group and active ingredient. This could be a DMI formulation that was not in the previous fungicide application, even if that DMI was a joint DMI and QoI mixture.

Best practice fungicide uses

Product choice, rate and water volume used should reflect the range and time of onset of the diseases to be managed, and the density of the crop canopy.

It is important to read the product label directions and restrictions. Other recommendations include:

  • Use appropriate fungicides to manage the extent of the disease risk
  • If repeated applications of fungicide are needed in a season, use uncompromised fungicides in a rotation
  • Apply fungicides according to label recommendations.

Tackling powdery mildew fungicide resistance

To prolong the use of effective fungicides it is recommended to use integrated disease management strategies that include:

  • Appropriate cereal variety selection for environment and seasonal risks
  • Using seed and fertiliser fungicide treatments at seeding
  • Employing foliar control measures at the start of disease onset (if needed).

More research with GRDC investment is being carried out into barley and wheat fungicide resistance at the CCDM and DPIRD at a genetics and agronomic level.

Researchers will use 2018 crop samples to assist with this effort, so if growers suspect they have fungicide resistance in their paddocks, they are encouraged to contact the CCDM’s Fungicide Resistance Group at frg@curtin.edu.au.

Contact

Geoff Thomas
08 9368 3262, 
geoff.j.thomas@dpird.wa.gov.au

Carole Kerr, CCDM 
08 9690 2160, 
carole.kerr@curtin.edu.au

More information

GRDC Research Codes: DAW00229, CUR00023

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