Fungicide resistance update – Southern region
Fungicide resistance update – Southern region
Author: Nick Poole, Tracey Wylie, Kat Fuhrmann, Ben Morris (FAR Australia) Kejal Dodhia, Steven Chang, Fran Lopez-Ruiz (Centre for Crops and Disease Management), Steven Simpfendorfer (NSW DPI Tamworth), Sam Trengove (Trengove Consulting), AFREN Team | Date: 18 Jul 2024
Take home messages
- Monitoring and analysis of pathogen populations by CCDM in 2021 revealed new resistance mutations affecting fungicide performance for the first time in Australia and, in other cases, existing mutations being more widespread and affecting more states.
- In a field trial in NE Victoria, which combined field efficacy with laboratory analysis, testing has revealed significant differences in DMI (Group 3, triazole) performance for control of wheat powdery mildew (WPM),
- The results illustrated that the weaker compounds (triadimefon, epoxiconazole, tebuconazole, cyproconazole and propiconazole) provided less than 50% control of WPM.
- Fungicide resistance and reduced sensitivity can be slowed down by using integrated disease management (IDM) approaches that reduce the number of fungicide applications required.
- To ‘slow the train that’s heading to fungicide resistance’, growers and advisers need to adopt fungicide resistance management strategies that avoid repeated applications of the same modes of action and active ingredients.
- IDM strategies can include crop rotation, stubble management, green bridge control, sowing more disease resistant (avoid susceptible) cultivars, nutrition and canopy management (for example, grazing) to minimise disease pressure.
Background
Fungicide resistance is a major concern for Australian growers, as it potentially reduces the efficacy of fungicides and their ability to protect grain yield and profit potential. To minimise the yield gap on cropping farms, it is essential to maintain impact of these agrichemicals through fungicide resistance management strategies. The first step in recognising the importance of this problem is to understand which pathogens are developing issues and to which fungicide actives.
The research reported in this paper includes fungicide actives epoxiconazole, propiconazole, prothioconazole and tebuconazole that have been applied standalone (un-registered) for the research purpose of determining the resistance profile for specific mode of action groups and actives. Only products that are registered for use in Australia should be used and in accordance with directions for use on their respective labels.
What is the current status of fungicide resistance and reduced sensitivity in Australia
Over the last decade, the Fungicide Resistance Group (FRG) at the Centre for Crop and Disease Management, (CCDM at Curtin University) has been working with industry and other researchers to establish a fast and cost-effective monitoring system for fungicide resistance of common fungal pathogens of broadacre grain crops. Current cases of fungicide resistance and reduced sensitivity in Australian broadacre grain crops are outlined in Table 1.
Table 1: Fungicide resistance and reduced sensitivity cases identified in Australian broadacre grains crops.
Disease | Pathogen | Fungicide group | Compounds affected | Region (status*) | Industry implications |
---|---|---|---|---|---|
Barley powdery mildew | Blumeria graminis f.sp. hordei | 3 (DMI) | Tebuconazole | WA (R), Qld, NSW, Vic, Tas, (L) | Field resistance and reduced sensitivity to some actives |
Wheat powdery mildew | Blumeria graminis f.sp. tritici | 3 (DMI) | Propiconazole | NSW, Vic (R), Tas, SA (L) | Field resistance to some actives in NSW and Vic. The gateway mutation is the first step towards resistance. This mutation does not seem to reduce efficacy in the field but, combined with other mutations, can affect DMI efficacy |
11 (QoI) | Azoxystrobin | Vic, Tas, SA, NSW (R) | Field resistance to all Group 11 fungicides | ||
Barley net-form of net blotch | Pyrenophora teres f.sp. teres | 3 (DMI) | Tebuconazole Epoxiconazole | WA (R), | Field resistance and reduced sensitivity to some actives |
7 (SDHI) | Fluxapyroxad | SA (R, RS), VIC (L) | Reduced sensitivity or resistance depending on the frequency of resistant population | ||
Barley spot-form of net blotch | Pyrenophora teres f.sp. maculata | 3 (DMI) | Tebuconazole Epoxiconazole | WA (R, RS) | Field resistance to some actives |
7 (SDHI) | Fluxapyroxad | WA (R, RS) | Field resistance and reduced sensitivity | ||
Wheat septoria | Zymoseptoria tritici | 3 (DMI) | Tebuconazole | NSW, Vic, SA, Tas (RS) | Reduced sensitivity |
11 (QoI) | Azoxystrobin | SA, (Millicent region) (R) | Frequency of A143 mutation in Millicent region unknown. 32 STB samples collected from 29 locations across Victoria, South Australia and NSW in 2021 did not detect the mutation associated with resistance to QoI fungicides | ||
Canola blackleg disease | Leptosphaeria maculans | 3 (DMI) | Tebuconazole Flutriafol | VIC, NSW, SA, WA (RS) | Reduced sensitivity |
*Reduced sensitivity (RS): Fungi are considered as having reduced sensitivity to a fungicide when a fungicide application does not work optimally but does not completely fail. In most cases, this would be related to small reductions in product performance which may not be noticeable at the field level. In some cases, growers may find that they need to use increased rates of the fungicide to obtain the previous level of control. Reduced sensitivity needs to be confirmed through specialised laboratory testing. Note that mutations that cause field failure (full resistance) present at lower frequencies in a pathogen population would give similar field symptoms to mutations that cause small reductions in field performance but which do not cause field failure.
Resistant (R): Resistance occurs when the fungicide fails to provide an acceptable level of control of the target pathogen in the field at full label rates. Resistance needs to be confirmed with laboratory testing and be clearly linked with an unacceptable loss of disease control when using the fungicide in the field at full label rates.
Laboratory detection (L): Measurable differences in sensitivity of the pathogen to the fungicide when tested in the laboratory. Detection of resistance in the lab can often be made before the fungicide’s performance is impacted in the field.
Fungicide reduced sensitivity and resistance in NSW/SA/Victoria
Wheat powdery mildew (WPM) was particularly problematic in 2020 but was less damaging in 2021. Steven Simpfendorfer (NSW DPI) co-ordinated 22 samples of WPM for testing with CCDM in 2020–2021 and the results revealed widespread fungicide reduced sensitivity in the DMIs and resistance in the QoIs. The F136 mutation in WPM is a gateway mutation that doesn’t confer field resistance but, in combinations with other mutations (which are still being characterised) in the same gene, does confer reduced sensitivity in the field.
Further samples collected across South Australia, Southern NSW and Victoria have confirmed the G143A mutation, which is associated with Group 11 (QoI) resistance. The frequency of this mutation increases from West to East (Figure 1.)
Figure 1. Frequency (%) of the G143A mutation. Triangles show 2020/21 samples and circles show 2022 samples. Source: Trengove Consutling.
FAR, working in collaboration with CCDM and NSW DPI, ran an irrigated trial at Katamatite in NE Victoria in 2021 to determine the field performance of different modes of action and DMI active ingredients for control of WPM. The results illustrated some interesting differences in field performance which, whilst not all statistically significant, illustrated that the weaker compounds of epoxiconazole (Opus®) and propiconazole (Bumper®) (both group 3 DMIs) were giving less than 50% control (Figure 2). Isolates from this trial were taken in October (post-application) and the samples sent to CCDM for fungicide resistance testing. Analysis for the presence of the A143 mutation that affects WPM control globally when using group 11 QoIs (strobilurins) was present in all treatments (Figure 3). This fungicide is not generally applied alone in Australia but in mixtures with DMIs, however, it demonstrates the selection pressure that can occur in a season when we use fungicide actives that are at higher risk of resistance development in the pathogen. Significant differences were noted to the untreated in the level of the QoI mutation in plots treated with DMIs and the Group 5 fungicide spiroxamine (Prosper®).
Figure 2. Influence of two spray fungicide application (GS37/39 and GS59) on wheat powdery mildew (WPM) infection on different components of upper canopy of wheat cv ScepterA, Katamatite, Vic 2021. Data labels based on total WPM infection of all plant components listed. These treatments were included to test the full range of available individual fungicide actives, some of which are only approved in mixtures. Always read and follow product labels.
Figure 3. Fractional abundance of the A143 mutation in the different fungicide treatments applied for WPM control in wheat cv ScepterA, Katamatite, Vic 2021. (CCDM analysis).When the mutation at G143A occurs, the G amino acid in the wild type is replaced with an A amino acid. Always read and follow product labels.
What does this mean for growers and advisers
Fungicide resistance management strategies which should be used within broader IDM include:
- With wheat and barley crops where two to three fungicide applications occur within a season, avoid repeat applications of the same product/active ingredient and, where possible, also avoid the same mode of action in the same crop. This is particularly important when using Group 11 QoI (strobilurins) and Group 7 SDHIs, which preferably would only be used once in a growing season
- Avoid using the seed treatment fluxapyroxad (Systiva®) year after year in barley without rotating with foliar fungicides of a different mode of action during the season
- Avoid applying the same DMI (triazole) Group 3 fungicide twice in a row, irrespective of whether the DMI is applied alone or as a mixture with another mode of action
- Avoid the use of tebuconazole alone and flutriafol for Septoria tritici blotch (STB) pathogen control in regions where reduced sensitivity is problematic, as these Group 3 DMIs are more affected by reduced sensitivity strains than other DMIs
- Group 3 DMIs, such as epoxiconazole (Opus®) or triazole mixtures such as prothioconazole and tebuconazole (Prosaro®), when used alone, are best reserved for less important spray timings or, in situations where disease pressure is low, in higher yielding scenarios.
- With SDHI seed treatments, such as fluxapyroxad (Systiva®), or QoI fungicides used in-furrow, such as azoxystrobin (Uniform®), consider using a subsequent foliar fungicide with a different mode of action, and therefore avoiding, if possible, a second application of SDHI or QoI fungicide active.
Clearly, the best way to avoid fungicide resistance is not to use fungicides! However, in high disease pressure regions, this would be an unprofitable decision. When a cultivar’s genetic resistance breaks down or is incomplete, it is imperative that growers and advisers have access to a diverse range of effective fungicides (in terms of mode of action) for controlling leaf disease. Hence, we need to protect their longevity. In order to protect fungicides, one of the most effective measures is to minimise the number of fungicide applications during the season. Therefore, consider all aspects of an Integrated Disease Management (IDM) strategy when putting your cropping plans together at the start of the season, since this will help reduce our overall fungicide dependency.
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 authors would like to thank them for their continued support. FAR Australia would like to acknowledge the assistance of Andrew McPherson and James Reilly in sourcing and managing the field trial conducted near Katamatite in 2021.
Contact details
Nick Poole
FAR Australia
Shed 2/63 Holder Rd, Bannockburn VIC 3331
08 5266 1290
0499 888 066
nick.poole@faraustralia.com.au
Fran Lopez-Ruiz
Centre for Crop and Disease Management
Curtin University, Perth WA 6845
08 9266 3061
fran.lopezruiz@curtin.edu.au
GRDC Project Code: CUR2302-002RTX, CUR1403-002BLX, TGC2204-001RTX,