Wheat powdery mildew
Powdery mildew, caused by the fungus Blumeria graminis f.sp. tritici, is an important disease of wheat worldwide and has become important in Western Australian wheat crops in recent seasons. Over the past few years the Department of Agriculture and Food, WA (DAFWA) plant pathology group have been working on many aspects of the biology and management of this disease including variety resistance screening, use of fungicides at seeding or during the season and how variations in the virulence of the pathogen might change variety responses.
Ongoing research will look at regional variations in disease virulence, understanding how seasonal conditions influence disease development, interactions of crop nutrition with disease severity and variety by fungicide interactions. The Centre for Crop and Disease Management (CCDM) at Curtin University is monitoring wheat powdery mildew for any evidence of fungicide resistance.
Powdery mildew produces masses of spores which appear as white powdery pustules on leaves, stems and heads of wheat, it is easily spread by wind. It has a short latent period (around 7 days) so it can develop rapidly in crops when conditions are favourable. Coupled with this, a high proportion of our popular wheat varieties are moderately susceptible-susceptible (MSS) or worse, making the majority of the area sown to wheat in WA vulnerable to this disease. Humid mild weather and a thick lush damp canopy favour the development and spread of the disease. This can be winter in the northern wheatbelt and perfect spring weather in the southern wheatbelt.
A few of the more recent research outcomes, which will help empower growers to be better equipped to fight this disease, are outlined in this edition of Protecting WA Crops:
WA’s wheat powdery mildew history
Powdery mildew has always been present in the WA wheatbelt but has not been a widespread disease of significance for many years.
It has become more prevalent in WA wheat crops over the past few years and this can be attributed to the following factors;
- Susceptible varieties making up a large proportion of recent wheat crops
- Gradual increase in inoculum pressure
- Changes in mildew virulence
- Conducive seasons being experienced.
In 2013 and 2014, powdery mildew was evident in several crops requiring fungicide intervention. In 2015, it was widespread and damaged wheat crops in the northern and central wheatbelt and in the Esperance region. The disease persisted through winter and into spring and proved difficult to control in many crops. Prominent questions facing growers and advisers during 2015 centred around potential yield losses, profitability of applying fungicides, best timing and product, value of multiple sprays and potential risk of fungicide resistance.
The disease was again problematic in 2016, particularly in regions north of the great eastern highway, with in-crop fungicide intervention again required.
How do we test wheat varieties for resistance?
GRDC-supported Project: DAW00235 - NVT Disease Screening for Wheat and Barley. Manisha Shankar and Hossein Golzar.
Varieties from the National Variety Testing (NVT) set are screened annually in DAFWA controlled environment glasshouse nurseries against a ‘pool’ of powdery mildew isolates which have been collected from across the WA wheatbelt over the last five years.
In the most recent DAFWA wheat variety guides, 24 of 39 rated varieties were rated susceptible (S) or very susceptible (VS) with three varieties (Grenade, Scout, Tungsten) being rated moderately susceptible (MS) and six varieties (Arrino, Eradu, Fortune, Frame, Magenta, Yitpi) being rated moderately resistant to moderately susceptible (MRMS) or moderately resistant (MR).
In associated testing of this isolate collection against a group of wheat lines with known resistance genes, it is evident that many of these resistance genes are ineffective against WA isolates of powdery mildew. However, several postulated genes are still effective and contribute to response in varieties ranked MS or better. In four years of assessment of virulence, responses for most resistance genes has been stable, however loss of effectiveness for four resistance genes was observed between isolates, indicating potential for some variability in resistance response (Golzar et al., 2016).
Further research into the distribution of any powdery mildew pathotype variability is underway.
How effective are fungicide treatments for managing powdery mildew
GRDC-supported Project: DAW00229 - Improving grower surveillance, management, epidemiology knowledge and tools to manage crop disease. Geoff Thomas, Ciara Beard, Andrea Hills, Kithsiri Jayasena ( plus industry partners*)
Given the susceptibility of most popular varieties, managing this disease in recent seasons has required fungicide intervention. The rapid disease cycle, massive production of windborne spores and timing of outbreaks occurring prior to flag leaf emergence in many crops means that this intervention needs to provide economic and timely protection of crops.
Trials by DAFWA and other industry groups have shown that fungicides applied at seeding, as seed dressings or in-furrow, or in-crop by foliar application, can provide significant reduction in disease progress, however yield responses can be variable from no response to around 25%.
Does foliar fungicide application increase yields?
DAFWA trial results from trials between 1999-2002 suggest that yield responses to foliar applied fungicide varied between 4-17% in crops yielding 2-3t/ha. Subsequent trials in 2015 and 2016 (GRDC-supported Project DAW00229) have supported these earlier results with yield responses ranging from 0-26% with an average of ~8%. Yield response is greatest when crop infection occurs early (such as from stem extension), and favourable weather conditions allow the disease to persist into spring.
What about prophylactic fungicide treatments at seeding?
A range of seed dressing and in-furrow products are registered in wheat for diseases including smuts and bunts, rhizoctonia root rot, wheat rusts, yellow spot and septoria tritici but none are currently registered for powdery mildew. The benefit of systemic fungicides applied at seeding as a method of foliar disease control is through the fungicide being available in newly emerging leaves, providing a continuous source of protection. The length of protection varies between products, rates and disease pressure.
DAFWA field trials (GRDC-supported Project DAW00229) established at four sites in 2016 were designed to evaluate the efficacy of seed, fertiliser and foliar applied fungicide on the severity and yield impact of wheat foliar diseases including powdery mildew. Sites were established with susceptible varieties at Geraldton, Dalwallinu, Moora and Tambellup. The trial results are presented in detail in a 2017 GRDC Research Updates paper by Geoff Thomas and others.
All products tested in these trials are registered for use in wheat for other diseases and we were interested to test how they influence powdery mildew development. These products are currently used prophylactically to address a wide range of disease risks. When disease occurs they can delay disease onset and reduce or eliminate the need for foliar applications but with low disease severity they are unlikely to provide yield benefit.
The following fungicide products were tested against powdery mildew; seed dressings fluxapyroxad (Systiva® 150mL/100kg seed), triadimenol (Baytan T® 150mL/100kg seed), and fluquinconazole (Jockey Stayer® 450mL/100kg seed); in-furrow coated on granular fertiliser flutriafol (Flutriafol 250® 400mL/ha), azoxystrobin + metalaxyl-M (Uniform® 400mL/ha), and triadimefon (Triadimefon 500 Dry® 200g/ha), and foliar propiconazole (Tilt® 250EC 250-500mL/ha) and prothioconazole + tebuconazole (Prosaro® 420SC 150-300mL/ha) applied at Z30-31, Z39-45 or Z55, with appropriate water rates and adjuvants as per label recommendations.
At the two sites (Geraldton and Dalwallinu) where powdery mildew infection commenced during stem elongation, all tested seed dressing and in-furrow fungicides had significant impact on reducing disease severity and incidence. Product differences were evident between sites.
The most severe disease occurred at the Geraldton site, all products tested reduced incidence and severity of mildew resulting in at least 0.6t/ha yield response. At this site with disease occurring during stem extension and conditions favouring disease development, the at-seeding treatments provided similar benefit as foliar applications at Z31 or Z39 (Figure 1). Greater response may have been evident if at-seeding treatments had been followed by foliar applications.
Some care and awareness of risks associated with fungicide resistance should be employed in utilisation of these products, particularly for powdery mildew. Despite being applied prior to crop growth, seed dressing and in-furrow fungicides are the first application in a seasonal fungicide program and therefore subsequent foliar fungicide choice and fungicide group rotation needs to account for the active ingredient used.
In-crop foliar applied fungicides?
In 2015, several trials were conducted by DAFWA and other agri-industry companies* exploring the efficacy and impact of foliar applied fungicides on powdery mildew development and subsequent yield responses. The results are presented in detail in a 2016 GRDC Research Updates paper by Ciara Beard and others.
Application of a single foliar fungicide spray at a registered rate to a susceptible variety gave a significant yield response in four out of six trials located across the wheatbelt when powdery mildew infection occurred from stem extension onwards. Average yield response across all trials to a single fungicide spray was 8%. Timing of application (as soon as possible after disease observed moving up canopy) was more important than product choice. In 6 trials, the difference between untreated versus treated was more significant than differences between product or active ingredient used.
The value of applying a second fungicide was variable and in the two trials where it was tested in 2015 was not justified by a significant yield response, possibly due to the dry hot spring in most locations that year.
Wheat powdery mildew fungicide resistance testing
GRDC-supported projects: CUR00022-Fungicide resistance management strategy and communications and CUR00023-Centre for Crop and Disease Management. Fran Lopez-Ruiz and Alex Kay
The Fungicide Resistance Group at the Curtin University Centre for Crop and Disease Management (CCDM) test samples of crop disease collected from growing regions across Australia every year in an effort to understand where, when and how fungicide resistance is arising in the field.
During the 2015 and 2016 growing season, DAFWA researchers, growers, agronomists and industry assisted the Fungicide Resistance Group in collecting more than 75 samples of wheat powdery mildew from high disease pressure areas in Western Australia.
Samples were tested by quickly screening the material for known genetic mutations associated with fungicide resistance. The process works by crushing the diseased leaf samples into a liquid solution and then using a highly sensitive instrument (digital PCR) for the detection of mutations associated with fungicide resistance. The methodology is also very sensitive and can detect fungicide resistance mutations at very low frequencies (0.1%), which makes it capable of finding ‘a needle in a hay stack’.
Disease testing from the 2015 and 2016 growing season gave reassuring results for WA growers by confirming that wheat powdery mildew in Western Australia still has no detectable cases of fungicide resistance. However, the group have detected two mutations in wheat powdery mildew in Tasmania and Victoria and this find should serve as a future warning for WA growers.
The first is the G143A mutation which gives rise to strobilurin (Group 11 fungicides) resistance in wheat powdery mildew. The second is the Y136F mutation which does not confer resistance but is a gateway mutation for triazole (Group 3 DMI fungicides) resistance in wheat powdery mildew.
Despite not identifying resistance mutation in powdery mildew in our wheatbelt to date, Western Australia is by no means safe from a resistance outbreak and growers need to be vigilant. This resistance find means that fungicide control of wheat powdery mildew is very fragile and highlights the need for integrated disease management measures to help reduce our reliance on fungicides for disease control.
What is the best strategy for managing wheat powdery mildew?
Choose the most resistant agronomically suitable variety. Varieties range in susceptibility from VS to R (Mace is MSS and Scepter is provisionally rated as SVS). Variety screening trials show that varieties of MS or better resistance ranking provide significant reduction in disease severity. For MSS or worse varieties, prepare a disease management strategy relevant to the resistance ranking.
Fungicide in-furrow or as seed dressing
Several seed dressing and in-furrow fungicides registered in wheat, but not for powdery mildew, can delay onset and slow development of powdery mildew, providing early season protection and helping reduce inoculum build up in the canopy. A list of registered foliar fungicides is available on the DAFWA website.
Applying foliar fungicides can reduce the impact of disease, however yield responses and positive economic benefits from fungicide application are never guaranteed.
For susceptible varieties, budget for a foliar fungicide application for powdery mildew control. Application of fungicide is recommended when the disease is present and moving up the canopy and the outlook is for continuing moist/humid conditions. It is crucial to target management strategies to protect leaves from infection and delay the progress of infection before it infects the flag leaves and heads, particularly in regions with high yield or quality expectations. Fungicides are more efficient as protectants than eradicants, they can struggle to manage disease if it has become rampant in a crop; once heads are infected control is extremely difficult.
Use a registered fungicide at a rate sufficient to provide longer protection and this will also reduce the need for follow-up treatment, differences between products may occur but a general recommendation is to use a registered product at full label rate.
A list of registered foliar fungicides is available on the DAFWA website.
Time of sowing
Powdery mildew risk is greatest for early sown or short season varieties where upper canopy and heads are exposed to disease for longer in favourable environments. Early sown susceptible varieties should be monitored closely to avoid significant damage occurring before management is instigated.
Powdery mildew is carried between seasons on infested stubble and multiplied by the presence of a green bridge. The presence of susceptible regrowth at the start of the season will multiply this risk and likely necessitate disease management in crops and potentially increase benefits from any at-seeding fungicide responses.
Crop rotation and health
Rotate wheat crops with non-host crops such as canola, barley or legumes; keep crops healthy, but avoid excess use of nitrogen as that may increase powdery mildew risk. Further information on this and fungicide resistance testing is availableon the CCDM website
To maximise disease control and reduce risk of fungicide resistance developing
- Select wheat varieties that have resistance to wheat powdery mildew.
- Minimise disease carry-over by removing the green bridge.
- Avoid seeding wheat into infected wheat stubble or reduce the amount of stubble to reduce the disease load carry over.
- Use recommended fungicide label rates and avoid using more than two sprays of any product per season
- Apply foliar fungicide to control the disease as early as practical and before it becomes severe
- Only 1 application of strobilurin (Group 11) fungicides per season.
- Do not apply the same DMI (Group 3) fungicide twice in a row. Rotate within the Group. Remember seed dressing or in-furrow fungicides count too
Golzar H, Shankar M and D’Antuono M (2016) Responses of commercial wheat varieties and differential lines to western Australian powdery mildew (Blumeria graminis f. Sp. tritici) populations. Australasian Plant Pathology 45:347–355.
Managing Powdery Mildew in Wheat, on the DAFWA website
* Thank you to our industry partners who conducted foliar fungicide trials in 2015 and shared their results with us: Brad Westphal and Phil Smyth of Landmark; Michael Macpherson, Imtrade Australia and the Liebe Group; and Leigh Nairn, Northampton Agri Services.
The research undertaken by DAFWA reported here is made possible by the significant contributions of growers through the support of the GRDC. Thank you also to the growers who hosted the trials.
Copyright © Western Australian Agriculture Authority, 2017.
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