Management of powdery mildew on fungicide resistant wheat

Take home message

  • Improving varietal resistance from susceptible to very susceptible (SVS) to moderately susceptible (MS) had a greater impact on reducing powdery mildew infection than use of any registered fungicide.
  • In furrow and seed treatments provided early powdery mildew control, however this effect dissipated as the growing season progressed.
  • Demethylation inhibitors (DMI) and quinone outside inhibitors (QoI) fungicides provided powdery mildew control, despite reduced sensitivity and resistance being detected within the population to these modes of action.
  • Yield loss related to powdery mildew stem infection assessed at mid booting (GS45) ranged from 4-9.4kg grain/ha/stem pustule across three trials, resulting in yield loss of up to 17%.
  • Cost of control and likely return on investment (ROI) should take into consideration the spatially variable nature of powdery mildew.

Background

Wheat powdery mildew (WPM) has been documented to cause up to 25% yield loss in Australia. Common wheat varieties that are currently being grown have poor varietal resistance, with many having ratings of susceptible to very susceptible (SVS), and only a few varieties rated as moderately susceptible to susceptible (MSS) or moderately susceptible (MS). There has been a large shift in area sown to SVS varieties in the last four years, with the dominant variety Mace that is MSS being superseded by Scepter that is SVS. Consequently, there is a heavy reliance on fungicides for WPM control. Fungicide options have relied heavily on the DMI group 3 (triazole) products such as tebuconazole and epoxiconazole, and these are the basis of many fungicide mixes. In more recent years, there has also been increasing use of the QoI group 11 (strobilurin) actives in mixtures with DMI fungicides in products such as Amistar Xtra® (azoxystrobin + cyproconazole). Often the fungicide strategy has been targeting other pathogens, such as stripe rust and Septoria, with application timing and product selection targeted to these pathogens. The use of these products has also been providing some control of susceptible populations of WPM provided suitable application coverage is achieved. However, by its nature, WPM often infects low down in the canopy covering lower leaves, leaf sheathes and stem where good spray coverage can be difficult with late application timings.

Fungicide resistance in WPM was identified in the northern Yorke Peninsula region in 2019, with testing performed by the Centre for Crop and Disease Management (CCDM). DMI and QoI groups were both implicated. With reduced efficacy expected from these modes of action, currently the registered alternative mode of action options are limited to the succinate dehydrogenase inhibitors (SDHI) (group 7) products such as Aviator® Xpro® (bixafen + prothioconazole) or ElatusTM Ace (benzovindiflupyr + propiconazole), which are mixtures of group 7 (SDHI) and group 3 (DMI) active ingredients.

Trials were initiated in 2020 as part of a SAGIT project to better understand the best practice management of WPM given emerging fungicide resistance issues.

Method

In 2020 five trials were implemented near Bute, northern Yorke Peninsula. Each of the five trials had a particular focus, these were:

  1. Varietal resistance and post emergent fungicides (Sown 11 May).
    • Four varieties with disease ratings for WPM ranging from MS to SVS and four fungicide strategies.
  2. Pre-emergent fungicides (sown to Chief CL Plus, 11 May).
    • Seven pre-emergent fungicides +/– post emergent fungicide.
  3. Post emergent fungicides (sown to Scepter, 4 May).
    • A range of post emergent fungicide treatments applied twice at GS32 and GS45.
  4. Fungicide timing (sown to Scepter, 4 May).
    • Fungicide applied at four timings (GS14, 32, 45 and/or 65) in 10 timing combinations.
  5. Fungicide sequencing (sown to Chief CL Plus, 11 May).
    • A trial focused on controlling resistant powdery mildew using 15 combinations of pre-emergent and post emergent fungicides from a range of fungicide groups.

The trials were located at a site where fungicide resistant WPM was detected in a survey during 2019. At this site in 2019, 64% of the powdery mildew population had reduced sensitivity to the DMI (Group 3) fungicides and 1.5% of the population was resistant to the strobilurin (Group 11) fungicides.

Powdery mildew assessments were made on three occasions for each trial targeting early infection, mid-season infection and late infection in the head. For the first two assessment timings, individual pustules were counted on the stem and each leaf. Where pustules merged, an individual pustule was counted as an area of 2mm2. For the head infection a 0 – 9 score was used where 0 = no powdery mildew, 5 = 50% coverage of powdery mildew, 9 = 90% coverage, etc.

Wheat powdery mildew was first identified at the site on 22 June 2020 at GS14, with a single pustule being observed. Rainfall at Bute in 2020 was characterised by periods of wet and dry throughout the growing season, with a dry early mid-winter and dry early spring (Figure 1). In particular, the trials appeared moisture stressed in mid-September, losing significant green leaf area as a result, and potentially limiting disease progression. These results should be interpreted in that context.

Figure 1. Column bar graph showing the weekly rainfall at Bute in 2020. April to October rainfall 301 millimetres, 2020 annual rainfall 390 millimetres.

Figure 1. Weekly rainfall at Bute in 2020. April to October rainfall 301mm, 2020 annual rainfall 390mm.

Results and discussion

Varietal resistance to wheat powdery mildew

Four varieties were selected for this trial with a range of resistance levels to determine the benefit of varietal resistance and its interaction with fungicide use. The varieties were Kord CL Plus (MS), Mace (MSS), Scepter (SVS) and Chief CL Plus (SVS). Scepter and Chief CL Plus were both chosen as they have commonly been grown in the area and field observations indicated that Chief CL Plus may be more susceptible, despite both being rated SVS in 2019. Kord CL Plus was chosen despite being lower yielding, as it is one of the only MS rated main season varieties available for this area.

Wheat powdery mildew pustule counts showed that varietal resistance had a much greater impact on infection level than the application of epoxiconazole at GS32 (Figure 2). Epoxiconazole use reduced pustule number by 22-51%, whereas changing from an SVS variety (Chief CL Plus or Scepter) to an MS variety (Kord CL Plus) reduced pustule number by 74-82%.

Comparing yields of untreated with yields of best performing fungicide treatment for each variety indicates yield gain from fungicide use of up to 17%, 9%, 8% and 1% for Chief CL Plus, Scepter, Mace and Kord CL Plus, respectively (Figure 3). The scale of yield increase in response to fungicide treatment is reasonably consistent with varietal resistance rating, where the yield response to fungicide declines with improved varietal resistance.

Figure 2. Stacked column bar graph showing the variety and epoxiconazole fungicide effect on pustule number at growth stage 45 (August 25) on the stem and leaves flag – 3 and flag – 2. Epoxiconazole (125 grams per Litre) applied at 500 millilitres per hectare at growth stage 32 (July 16). Pr (>F) value = 0.027. Letters denote significant differences between pustule totals.

Figure 2. Variety and epoxiconazole fungicide effect on pustule number at GS45 (August 25) on the stem and leaves flag – 3 and flag – 2. Epoxiconazole (125g/L) applied at 500ml/ha at GS32 (July 16). Pr (>F) value = 0.027. Letters denote significant differences between pustule totals.

Figure 3. Column bar graphs showing variety and fungicide effect on grain yield. Epoxiconazole (125 gram per Litre) applied at 500 millilitres per hectare at growth stage 32 (July 16). Amistar Xtra® (azoxystrobin + cyproconazole) applied at 400 millilitres per hectare at growth stage 45 (August 25). Variety*fungicide Pr (>F) = 0.057, LSD (0.1) = 0.41t/ha, variety Pr (>F) = <0.001, LSD (0.05) = 0.19 t/ha, fungicide Pr (>F) = 0.004, LSD (0.05) = 0.19.

Figure 3. Variety and fungicide effect on grain yield. Epoxiconazole (125g/L) applied at 500ml/ha at GS32 (July 16). Amistar Xtra® (azoxystrobin + cyproconazole) applied at 400ml/ha at GS45 (August 25). Variety*fungicide Pr (>F) = 0.057, LSD (0.1) = 0.41t/ha, variety Pr (>F) = <0.001, LSD (0.05) = 0.19 t/ha, fungicide Pr (>F) = 0.004, LSD (0.05) = 0.19.

Pre-emergent fungicides

Several in furrow fungicide treatments reduced WPM infection early in the season (Figure 4a). Flutriafol treatments appeared to be the best of these, reducing pustule number by up to 76%, whereas Uniform® treatments were similar to the untreated control. However, all treatments were ineffective as the season progressed and were not able to provide a long-term benefit in this instance (Figure 4b), despite the application of benzovindiflupyr + propiconazole (Elatus Ace) at 500ml/ha at GS32 and azoxystrobin + cyproconazole (Amistar Xtra) at 400ml/ha at GS45. The change in pustule number from 22 July (Figure 4a) to 24 August (Figure 4b) indicates the large increase in disease pressure over that time frame. No yield benefit was observed in response to in furrow fungicide treatment, however a 6.7% (0.26t/ha) increase was observed in response to post emergent fungicide application of Elatus Ace followed by Amistar Xtra (data not shown).

It is important to put these results in the context of a small-scale plot trial where airborne spores from adjacent plots with poorer control will increase infection load on treatments with better control that were kept clean early. In the case of a highly effective pre-emergent fungicide in a broad scale situation, such as a large paddock, the response to the pre-emergent fungicide may last longer than reported here, as there will be less inoculum load present as a result of good early control. This impediment is a common problem in plot trials where different treatment times are being compared and airborne pathogens are involved.

Figure 4.Two stacked column bar graphs showing on the left hand side the number of pustules on the stem, 4th leaf and 5th leaf on 22 July, Pr (>F) = <0.001, LSD (0.05) = 0.11, and on the right hand side the number of pustules on the stem, flag -2 and flag -1 on 24 August, Pr (>F) = 0.002, LSD (0.05) = 0.13, averaged across plus and minus post emergent ElatusTM Ace fungicide treatments. Letters denote significant differences between pustule totals.

Figure 4. a) Number of pustules on the stem, 4th leaf and 5th leaf on 22 July, Pr (>F) = <0.001, LSD (0.05) = 0.11, b) Number of pustules on the stem, flag -2 and flag -1 on 24 August, Pr (>F) = 0.002, LSD (0.05) = 0.13, averaged across plus and minus post emergent ElatusTM Ace fungicide treatments. Letters denote significant differences between pustule totals.

Post emergent fungicides

Demethylation inhibitors (Group 3) and strobilurin (Group 11) fungicides applied individually provided 56-76% pustule reduction at 13 August and 28-76% pustule reduction at 11 September (Figure 5 and 6), despite reduced sensitivity (64%) and fungicide resistance (1.5%) being detected in WPM at this site in 2019, respectively. There was a difference within the group 3 fungicides, with the commonly used fungicide, epoxiconazole performing poorer than both the tebuconazole and tebuconazole + prothioconazole (Prosaro®) fungicides. This could suggest a better performance of particular DMI fungicides when reduced sensitivity has developed due to the emergence of specific mutations at the DMI target site. The combination of group 3 and group 11 fungicides was better than either applied alone at the earlier assessment and that continued at the 11 September assessment, where epoxiconazole + azoxystrobin (Tazer XpertTM) was one of the best treatments. However, cyproconazole + azoxystrobin (Amistar Xtra) did not provide a significant advantage over the better DMIs such as tebuconazole or Prosaro. While the QoI treatments worked reasonably well, the continuous use of this group of fungicides will inevitably lead to the accumulation of resistant individuals and disease control problems in the future.

New SDHI plus DMI fungicide mixtures, bixafen + prothioconazole (Aviator Xpro) and benzovindiflupyr + propiconazole (Elatus Ace) provided 58 and 67% pustule reduction, respectively at the earlier assessment and 59 and 84% reduction at the later assessment (Figure 5 and 6). These provide little or no improvement compared with the better DMI actives, posing the question ‘do the SDHI actives provide much WPM control, or is it the DMI mix partner doing most of the work?’ A standalone SDHI fungicide (A (7)), not registered in wheat, performed poorly, suggesting that the group 3 DMI mix partner in the SDHI products were providing a significant level of the control.

Figure 5. Stacked bar column graph showing the number of powdery mildew pustules for selected treatments on the stem and flag leaf minus 3 and 2 counted 13 August 2020. Post emergent fungicides applied 16 July. Letters denote significant differences between log transformed totals Pr (>F) = <0.001.

Figure 5. Number of powdery mildew pustules for selected treatments on the stem and flag leaf minus 3 and 2 counted 13 August 2020. Post emergent fungicides applied 16 July. Letters denote significant differences between log transformed totals Pr (>F) = <0.001.

Figure 6. Stacked column bar graph showing number of powdery mildew pustules for selected treatments on flag leaf minus 2 and 1 counted 11 September 2020. Post emergent fungicides applied 16 July and 25 August. Letters denote significant differences between log transformed totals Pr (>F) = <0.001.

Figure 6. Number of powdery mildew pustules for selected treatments on flag leaf minus 2 and 1 counted 11 September 2020. Post emergent fungicides applied 16 July and 25 August. Letters denote significant differences between log transformed totals Pr (>F) = <0.001.

Linear regression between stem pustules assessed at mid booting (GS45) and grain yield indicates yield loss in the range of 4-9.4kg/ha/stem pustule across three trials (Figure 7). This may provide a guide to predicting yield loss in season but requires further validation across multiple sites and seasons.

Figure 7. Line graphs showing linear regression of stem pustule numbers in August and wheat grain yield (tonne per hectare) for the variety * fungicide (y = -0.0057x + 4.8967, R2 = 0.2572), pre-emergent fungicide (y = -0.004 + 4.6494, R2 = 0.1239) and sequencing fungicide trials (y = -0.0094x + 4.4692, R2 = 0.4938).

Figure 7. Linear regression of stem pustule numbers in August and wheat grain yield (t/ha) for the variety * fungicide (y = -0.0057x + 4.8967, R2 = 0.2572), pre-emergent fungicide (y = -0.004 + 4.6494, R2 = 0.1239) and sequencing fungicide trials (y = -0.0094x + 4.4692, R2 = 0.4938).

Sensitivity analysis of fungicide costs versus potential yield loss

Wheat powdery mildew incidence and severity has been observed to be highly variable spatially, particularly in the northern Yorke Peninsula region where this work has been undertaken. Typically, the disease is worst where the crop is growing on the lighter textured sands and dunes, whereas on the heavier textured soils in the swales, disease incidence tends to be much lower. The basis for these differences is not well understood. The area at high risk of severe WPM infection will have an impact on the likely returns from investment in fungicide.

Partial gross margin (PGM) sensitivity analysis indicates that with a $23/ha fungicide cost, 14% of the area or greater would need to be at high risk from severe WPM infection to generate a positive return on fungicide investment across the whole paddock, given the assumptions listed in Table 1. Where fungicide resistance increases and alternative modes of action are required, fungicide price is likely to increase. In this example if the fungicide cost increases to $37/ha then 22% of the area or greater would need to be at high risk of severe WPM infection to generate a positive return on fungicide investment, given the assumptions listed in Table 2.

This is a simplistic model of spatial distribution of WPM. It is likely that there will be some benefit of additional fungicides on other areas of the paddock that do not have the high WPM pressure seen on sandy rises. Therefore, these PGMs may underestimate the returns, and therefore, a smaller area of high WPM pressure will be required to cover fungicide costs.

Table 1. Partial gross margin sensitivity analysis for varying areas of soil type more susceptible to wheat powdery mildew (WPM). Analysis uses the following assumptions: farm gate wheat price $250/t, additional fungicide cost including application $23/ha, potential wheat yield 4t/ha, yield loss associated with WPM in more susceptible areas 17%.

Paddock area with high WPM

Paddock area with low WPM

Paddock average yield with no additional fungicide (t/ha)

Farm gate gross income without additional fungicide ($/ha)

Farm gate gross income with additional fungicide ($/ha)

Partial gross margin
(PGM) ($/ha)

0%

100%

4.00

1000

977

-23

10%

90%

3.93

983

977

-6

20%

80%

3.86

966

977

11

30%

70%

3.80

949

977

28

40%

60%

3.73

932

977

45

Table 2. Partial gross margin sensitivity analysis for varying areas of soil type more susceptible to wheat powdery mildew (WPM). Analysis uses the following assumptions: farm gate wheat price $250/t, additional fungicide cost including application $37/ha, potential wheat yield 4t/ha, yield loss associated with WPM in more susceptible areas 17%.

Paddock area with high WPM

Paddock area with low WPM

Paddock average yield with no additional fungicide (t/ha)

Farm gate gross income without additional fungicide ($/ha)

Farm gate gross income with additional fungicide ($/ha)

Partial gross margin
(PGM) ($/ha)

0%

100%

4.00

1000

963

-37

10%

90%

3.93

983

963

-20

20%

80%

3.86

966

963

-3

30%

70%

3.80

949

963

14

40%

60%

3.73

932

963

31

Conclusion

Shifts in fungicide sensitivity and resistance to both DMI group 3 and QoI group 11 fungicides have been detected in WPM. Despite this, fungicides from these groups are currently still providing reasonable control. Selection of varieties with improved WPM resistance can reduce disease pressure more than any currently registered fungicide and negates the need for fungicide use for this pathogen.

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 SAGIT, the authors would like to thank them for their continued support. The input during this project from Michael Brougham, Hugh Wallwork, Tara Garrard, Kejal Dodhia and Nick Poole is gratefully acknowledged.

Contact details

Sam Trengove
Trengove Consulting, Bute SA
Phone 0428262057
samtrenny34@hotmail.com
@TrengoveSam

Varieties displaying this symbol beside them are protected under the Plant Breeders Rights Act 1994