Fungicide resistant wheat powdery mildew – update on management and resistance testing

Take home messages

• Varietal resistance can play an important role in managing wheat powdery mildew. The variety Grenade CL Plus (MS) had less powdery mildew infection in the untreated than Chief CL Plus and Scepter (SVS) treated with a two-spray fungicide strategy. Regardless, Scepter was the highest yielding variety.
• The application of group 11 QoI fungicides increased the frequency of resistance mutation G143A at the QoI target at three trial sites where resistance was present at low levels initially.
• Multiple diseases were present at trial sites in the 2022 season. Fungicides providing broad-spectrum disease control, particularly for stripe rust, were the highest yielding treatments.
• APVMA permits have been issued for the use of Legend® (quinoxyfen) (PER93197), Vivando® (metrafenone) (PER93198) and Talendo® (proquinazid) (PER93216) for the control of wheat powdery mildew. These products have provided good control of powdery mildew in wheat trials from 2019–2022.
• WPM head infection reduced yield at Port Neill when severity exceeded 40% head infection in 2022.
• Fungicide resistance surveys indicate increasing levels of QoI fungicide resistance and saturation of the mutation associated with DMI fungicide reduced sensitivity.

Background

Wheat powdery mildew (WPM) was widespread across south-eastern Australia in the 2022 season, occurring in most wheat growing regions, expanding its area of incidence compared with historical occurrence. There are a range of interacting factors that have caused this, including the predominance of SVS varieties grown in most regions over a long period of time, early crop establishment in many regions in 2022, conducive environmental conditions for developing large crop canopies and for disease development, and inoculum source carrying over from previous seasons. Difficulty achieving high levels of disease control with what were considered robust and well-timed fungicides were reported in many regions. Increasing prevalence of resistance and reduced sensitivity to group 11 QoI and group 3 DMI fungicides has been implicated in these control failures. Following recent SAGIT project (TC120) findings, investment by GRDC (TRE2204-001RTX) is seeking to quantify the extent of resistance development across the regions and identify management strategies for WPM given resistance development.

Method

Small plot trials were established at four locations in 2022, at Port Neill, Bute and Malinong, SA and Katamatite, Vic. In a range of WPM resistance populations, these trials investigated post-emergent fungicide efficacy, pre-emergent fungicide efficacy, fungicide timing and varietal resistance interactions. Season 2022 was conducive to development of a range of diseases, including Septoria, stripe rust and leaf rust. Three of the four locations were impacted by moderate to high levels of stripe rust. Assessments endeavoured to account for these other diseases and quantify their impacts in addition to WPM. WPM samples were collected from the trials to measure the frequencies of mutations G143A at CytB and Y136F at Cyp51, that are associated with resistance to QoI and reduced sensitivity to DMI fungicides, respectively.

Variety trial

A variety trial, located at Bute, compared six varieties: Chief CL Plus (SVS), Scepter (SVS), Mace (MSS), Grenade CL Plus (MS), Calibre (S) and Brumby (R). Four fungicide strategies were applied to Chief CL Plus , Scepter, Mace and Grenade CL Plus:

• Nil = no fungicide applied
• Strategy 1 = Amistar® Xtra @ 400mL GS39
• Strategy 2 = Epoxiconazole 125h @ 500mL/ha GS31 fb Amistar Xtra @ 400mL/ha GS39
• Complete = complete control of powdery mildew.

h Epoxiconazole 125 label rate for powdery mildew is 250mL/ha, 500 mL/ha is maximum label rate for wheat for control of leaf rust

Fungicide efficacy trials

Four product trials were implemented as small plot trials located at Bute, Port Neill, Malinong and Katamatite. Bute and Port Neill trials are discussed in this paper, Bute treatments are shown in Table 3. Product rates at Bute were the high label rate, unless specified otherwise in Table 3.

Table 1: Site details for fungicide efficacy trials at Bute and Port Neill.

Fungicide resistance survey

A field survey was conducted of WPM collected in 145 commercial paddocks in September and October. The frequencies of the mutations G143A at CytB and Y136F at Cyp51 in regions across SA and Vic, including the Eyre Peninsula, SA Mallee and Upper SE of SA, were determined. These add to the database of 73 paddocks sampled from the Yorke Peninsula and Mid North SA in 2021 and NE Vic and southern NSW in 2020.

Results and discussion

Varietal resistance to wheat powdery mildew

The benefit of varietal resistance in limiting WPM build up is clear in untreated plots, where WPM pustule number typically followed the variety resistance rating (Figure 1). This is consistent with findings in both 2020 and 2021 (Trengove et al. 2021, Trengove et al. 2022). In the Bute region, Calibre performed better than its S rating in both 2022 and 2021 (Trengove at al. 2022), being more closely aligned with Mace (MSS) and Grenade CL Plus (MS) in those seasons, respectively. WPM is a highly variable pathogen, and this deviation from expected performance based on resistance rating may reflect the local pathotype that is present. Brumby all but eliminated WPM development, highlighting its R status. Brumby’s high level of resistance is derived from a major gene and supported by alternate minor genes that confer a lower level of resistance. Due to the high genetic variability in WPM, pathotypes may already exist that can overcome this major gene resistance and have virulence on this variety, where virulence will then depend on the performance of the minor genes. This was observed in an isolated hot spot in Brumby in 2021 in a WPM variety trial at Bute, SA (Trengove et al. 2022). Brumby is expected to provide excellent resistance when first grown in a region. However, there is a risk it will be overcome by more virulent pathotypes if they are selected across a wide area on a repeated basis. The timeframe over which this may occur will depend on the frequency and regional extent of virulent pathotypes, and the area of varietal selection. This makes rotating varieties an important strategy in managing WPM.

In an SVS variety like Chief CL Plus, a robust fungicide program, like strategy 2, was required to reduce WPM levels significantly, but still had more WPM than Grenade CL Plus (MS) with no fungicide treatment. Untreated plots were severely affected by stripe rust and leaf rust late in the season, being the main influence on yield in those plots (data not shown). With the nil plots excluded due to stripe rust, within variety, there was no grain yield difference between fungicide programs, except for the variety Chief CL Plus (Figure 2). WPM continued to develop late in the season in Chief CL Plus , resulting in a 0.67t/ha difference between strategy 2 and complete WPM control. Responses of similar magnitude were recorded in SVS varieties in 2020 and 2021 to WPM control (Trengove et al. 2021, Trengove et al. 2022).

Figure 1. Variety by fungicide trial at Bute 2022.WPM pustules on the stem and leaf sheath assessed 27 September 2022 (Pr(>F) = <0.001).

Wheat powdery mildew fungicide resistance and post-emergent fungicide performance

Mutation frequency for Y136F at Cyp51 was high at all trial sites, averaging over 99%, regardless of treatment. This indicates that the gateway mutation associated with reduced sensitivity to group 3 fungicides is saturated at all trial site locations. This is likely due to the strong selection pressure that WPM populations are under because of the reliance on DMI fungicides, and this includes DMI fungicides applied to volunteer wheat in other crops. Trial sites at Bute in 2020 and 2021 had 70% and 87% frequency of Y136F mutation and this was consistent with survey data indicating this reached saturation in a relatively short time period.

Figure 2. Variety by fungicide trial at Bute 2022.WPM pustules (grey bars) on the Flag minus 1, assessed 4 November 2022 (Pr(>F) = <0.001) and final grain yield (orange dots) (Pr(>F) = <0.001).

Mutation frequency for G143A at CytB that confers resistance to group 11 QoI fungicides ranged from 1.2% to 24% across sites in the untreated control (Table 2). There is a trend for treatments containing the group 11 fungicide azoxystrobin to increase this frequency across the sites. This is expected, where the continual use of group 11 QoI fungicides maintains selection pressure on the population. This is consistent with 2021 results from Bute, where treatments including azoxystrobin increased mutation frequency from 19% to 48.5% (Trengove et al. 2022).

Table 2: Fungicide treatment effect in four product efficacy trials on per cent frequency of G143A mutation at CytB, conferring resistance to group 11 QoI fungicides. Letters denote treatments that are significantly different.

c Azoxystrobin (Mirador 625) is registered in wheat only when mixed with a DMI mix partner. It has been applied standalone in this trial for research and demonstration purposes.

g tebuzonazole applied alone is not registered for the control of wheat powdery mildew

f prothioconazole is not registered in wheat when applied stand alone

WPM control at Bute was poor, with single active DMI products being no better than the untreated (Table 3). Dual active DMI Prosaro® provided some control. Azoxystrobin reduced WPM infection, both standalone and in the dual active group 3 and 11 mixtures. Given low levels of QoI resistance at the site, this is not unexpected, however it is not likely to be a long-term solution given ongoing selection for resistant individuals (Table 2). Aviator Xpro is a DMI plus SDHI mix but is no better than the standalone prothioconazole DMI component (Proviso®), which is consistent with previous results

The APVMA has issued permits for three fungicides for the control of powdery mildew in wheat. Legend® and other registered products with 250g/L quinoxyfen (PER93197), Talendo (PER93216) and Vivando (PER93198) are now able to be used for WPM control until 31 July 2024.

Critical use comments that are common to all three of these permits include:

• Apply as a protectant only
• Do not apply more than 2 applications per crop
• Apply in accordance with the current CropLife Fungicide Resistance Management Strategy.

Critical use comments specific to each permitted fungicide are detailed in Table 3.

Table 3: Critical comments for permitted products Legend (PER93197), Talendo (PER93216) and Vivando (PER93198) for control of powdery mildew in wheat.

The products Legend and Talendo were included in the efficacy trials in 2022 and provided high levels of WPM control at Bute in 2022 (Table 4), and this result is consistent with trial results in 2020 and 2021. Vivando was included in the 2020 trials alongside Legend and Talendoand they all performed similarly (data not presented).

It is important to note that diseases do not occur in isolation though, and broad-spectrum fungicides were required to control all diseases present at the site including stripe rust, Septoria tritici blotch, Wirrega blotch and WPM. Stripe rust infection and its control was the biggest determinant of grain yield and products that controlled stripe rust were the highest yielding, where the untreated control yielded 20% of the best treatments. Legend and Talendoprovided no stripe rust control and were only marginally better than untreated control for grain yield. Mildew specific fungicides will need to be applied with an appropriate mix partner to provide broad spectrum disease control. Where Talendo was applied with Telbek or Telbek + Proviso, the stripe rust control from those products was not adequate to maximise grain yield.

When QoI group 11 fungicides are rendered ineffective due to resistance, and control from SDHI group 7 fungicides is typically low, the DMI group 3 fungicides have been the remaining fungicidal control option, albeit at reduced levels due to reduced sensitivity. A trial at Bute investigated the effect of applying DMI actives at full label rates, standalone or in two-way and three-way mixes, to try and optimise control. Active ingredients included tebuconazole, epoxiconazole and prothioconazole. Results indicate that increasing the load of DMI by applying active ingredients in combination provided better control than applying the actives as standalone treatments (Figure 3).

Table 4: Fungicide effect on wheat powdery mildew, Wirrega blotch and Septoria tritici, stripe rust and grain yield in Chief CL PlusA wheat at Bute, SA, 2022.

a data has been transformed to log10(1 + pustule count).

b blotch score is the leaf area percent of the flag minus 1, 2 and 3 affected by necrosis caused by Wirrega blotch and Septoria tritici combined.

d Legend is available for use under PER93917. Talendo is available for use under PER93216.

e Telbek Adavelt is only registered in wheat for septotia control

I Veritas Opti is not registered for wheat powdery mildew control but is registered in wheat for the control of other diseases

NOTE: Wheat foliar diseases commonly occur in disease complex’s. Fungicides listed above have differing label claims for the various diseases. Data is presented to indicate the relative activity of each product in terms of wheat powdery mildew control.

Efficacy is not assured for any product that does not have a stated label (permit) claim for the targeted disease and the user assumes all risk for any poor performance.

Figure 3. TotalWPM pustule number assessed on the Flag minus 1, 2 and 3 and the lower stem on 29 September 2022 for Chief CL Plus treated with group 3 DMI fungicide combinations.

Fungicide applied at head emergence at Port Neill resulted in different levels of WPM head infection, with treatment scores ranging from 2.4 to 5.2 in the untreated (Figure 4). The relationship between WPM head infection and grain yield indicates when the head score was less than 4, there was little difference in grain yield, but it declined when the head score exceeded 4, where the untreated control yielded 3.2t/ha. A head score of 4 indicates approximately 40% of the head has visible mildew growth.

Figure 4: WPM head score in Vixen wheat at Port Neill on 3 November and grain yield response (Y = -0.232x2 + 1.494x +1.637, R2 = 0.689).

Wheat powdery mildew fungicide resistance survey

The mutation frequency for Y136F at Cyp51 is a gateway mutation that indicates reduced sensitivity to group 3 fungicides may exist. It does not infer that the DMI fungicides will not work, just that the pathogen may be less sensitive. From the paddock survey of 145 samples collected in 2022 from the Eyre Peninsula, SA Mallee, Upper South East and Victoria, the incidence of this mutation averaged 98.6%, with a minimum value of 57% at a site near Nundroo, however a paddock in close proximity had a value of 98%. This survey indicates that this mutation is near saturation within the pathogen population and there is little geographical trend identified from the survey.

Mutation frequency for G143A at CytB confers resistance to group 11 QoI fungicides such as azoxystrobin. For the samples collected in 2022, there is a geographical trend for the mutation to increase in frequency from the west to the east, with the highest values occurring in the higher rainfall areas in Victoria (Figure 5). This trend is likely a response to increasing QoI fungicide use according to yield potential. This trend was similar in 2021, apart from the Upper Yorke area around Alford to Port Broughton where wheat powdery mildew has been a problem for several years, and demonstrates that increasing frequency of use is associated with increased levels of resistance in the population.

Figure 5: Frequency of wheat powdery mildew G143A mutation at CytB for survey paddocks sampled 2022 (circles) and 2021 (triangles).

Acknowledgements

The research undertaken as part of this project is made possible by the significant contributions of growers through both trial cooperation and the support of the GRDC, the author would like to thank them for their continued support. PER93917 is a result of SAGIT and GRDC investments, field studies and regulatory, and the support of Grain Producers Australia (GPA) as the permit holder. The input during this project from Tara Garrard is gratefully acknowledged.

References

Trengove S, Sherriff S, Bruce J Lopez Ruiz F (2021) Management of powdery mildew on fungicide resistant wheat. 2021 GRDC Adelaide Grains Research Update.

Trengove S, Sherriff S, Bruce J, Lopez Ruiz F, Dodhia K (2022) Fungicide resistant wheat powdery mildew – management and resistance testing. 2022 GRDC Online Grains Research Update.

Contact details

Stuart Sherriff
Trengove Consulting
Bute SA 5560
0408 282 202
sherriff.stuart@gmail.com