Fungicide resistance – what can we learn from previous experiences

Take home messages

  • Fungicide resistance needs to be minimised through integrated management approaches.
  • The number of fungicide applications over time is a key driver fuelling the shift (the selection of more resistant strains) in pathogen populations towards fungicide resistance.
  • To ‘slow the train’, growers and advisers need to adopt anti resistance measures when using fungicides that avoid repeating the same active ingredients, and wherever possible, in an integrated disease management (IDM) approach.
  • Integrated management strategies include rotating chemistries, using less disease susceptible cultivars and cultural practices to minimise disease.

Background

Fungicide resistance is a major concern for Australian growers, as it potentially reduces the efficacy of fungicides and therefore yields. For fungicides to maintain their efficacy, fungicide resistance management strategies need to be implemented. These strategies are at both a regional level and a paddock level.

  • Regionally, growers should consider local cropping practices, pathogen diversity and environmental conditions to determine disease risk and therefore the requirement for fungicides.
  • At the paddock level, reduce the risk of fungicide resistance by rotating crops i.e., don’t grow barley on barley, use resistant cultivars where possible, and rotating fungicide control measures.

What is the difference between resistance and reduced sensitivity?

When fungal disease pathogen is termed as sensitive to a fungicide it is considered that the fungicide will control the disease at specified label rates.

Reduced sensitivity means that the fungicide does not work optimally to control the fungal pathogen but does not completely fail. In most cases, this is related to small changes in the pathogen at the target site where the fungicide binds to the fungus. It typically results in small reductions in product performance, which may not be noticeable at the field level. In some cases, for example with Group 3 DMIs against STB, growers may find that they need to use increased rates of the fungicide to obtain the previous level of control. Reduced sensitivity must be confirmed by laboratory analysis.

Resistant means the fungicide fails to provide an acceptable level of control of the specified pathogen in the field at full label rates. Resistance must 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 under the correct application conditions.

What is the current status of fungicide resistance and reduced sensitivity in Australia?

Over the last eight years the Fungicide Resistance Group (FRG) (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 diseases 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 broad acre grains crops

Disease

Pathogen

Fungicide Group

Compounds affected

Region

Industry implications

Barley powdery mildew

Blumeria graminis f.sp. hordei

3 (DMI)

Tebuconazole, propiconazole, flutriafol

Qld, NSW, Vic, Tas, WA

Field resistance to some Group 3 DMI fungicides

Wheat powdery mildew

Blumeria graminis f.sp. tritici

11 (QoI)

All group 11

NSW, Vic, Tas, SA

Field resistance to all Group 11 fungicides

Barley net-form of net blotch

Pyrenophora teres f.sp. teres

3 (DMI)

Tebuconazole, propiconazole,

prothioconazole

WA

Reduced sensitivity that does not cause field failure

7 (SDHI)

Fluxapyroxad

Bixafen

SA

(Primarily Yorke Peninsula)

Reduced sensitivity

Barley spot-form of net blotch

Pyrenophora teres f.sp. maculata

3 (DMI)

Tebuconazole, epoxiconazole

Propiconazole

WA

Field resistance to old generation Group 3 fungicides

7 (SDHI)

Fluxapyroxad

Bixafen

WA

(Cunderdin region)

Reduced sensitivity

Wheat Septoria

leaf blotch

Zymoseptoria tritici

3 (DMI)

Tebuconazole, flutriafol,  propiconazole, cyproconazole, triadimenol

NSW, Vic, SA, Tas

Reduced sensitivity that does not cause complete field failure

Canola Blackleg disease

Leptosphaeria maculans

3 (DMI)

Tebuconazole, Flutriafol, prothiconazole, fluquinoconazole

VIC, NSW, SA, WA

Reduced sensitivity that does not cause complete field failure

Anti-resistance measures when using fungicides as part of an Integrated Disease Management (IDM) strategy

  • With wheat and barley crops where two to three applications of fungicide are applied, 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 STB pathogen control, as these Group 3 DMIs are more affected by reduced sensitivity strains than other DMIs.
  • Group 3 DMIs (for example, triazoles e.g. epoxiconazole (Opus®) or triazole mixtures (e.g. prothioconazole and tebuconazole (Prosaro®)) 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 Uniform® containing azoxystrobin, consider foliar fungicide follow ups which have a different mode of action, and therefore, avoiding, if possible, a second application of SDHI or QoI fungicides.

Influence of fungicide rate

Growers and agronomists frequently ask the question whether dose rates have an impact on how likely fungicide resistance is to evolve. Resistance comes in many forms and trying to manipulate fungicide rates should not be seen as the core resistance management strategy. The reality is that using the most appropriate rate for effective disease control is the best strategy for managing resistance. Label rates have been developed to provide robust and reliable control of the target disease.

In many cases the full label rate is the most appropriate rate for control. However, for some diseases, the lower rate from the label range of a fungicide can be used in conjunction with a crop variety that has a good disease resistance rating because disease pressure will be lower. Contrary to what might be the case with other agrichemicals, there is evidence that by using a higher rate than necessary increases the risk of resistance, as removing all the sensitive individuals provides more opportunity for these resistant individuals to dominate the population and hence be the strain colonising the plant. This is particularly the case with Group 11 QoIs and Group 7 SDHIs fungicides.

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 the disease. Hence, we need to protect their longevity. In order to protect them, one of the most effective measures is to minimise the number of fungicide applications applied during the season.

Principle components of IDM

Rotations – where possible avoid high risk crop rotations for disease, for example, avoid growing barley on barley or wheat on wheat.

Seed hygiene – minimise the use of seed from paddocks where there were high levels of disease that could be seedborne (e.g., Ramularia, net form net blotch).

Use less disease susceptible cultivars, particularly when sowing early. Where this is not possible delay the sowing of the most susceptible cultivars to reduce disease pressure where the phenology of the cultivar is adapted to the later development window.

Cultural control such as avoid sowing close to previous years stubble, or destroying/removing stubble where possible.

Grazing early sown cereal crops up to GS30 to reduce disease pressure.

AFREN (Australian Fungicide Resistance Extension Network)

The Australian Fungicide Resistance Extension Network (AFREN) was established to develop and deliver fungicide resistance resources for grains growers and advisers across the country. It brings together regional plant pathologists, fungicide resistance experts and communications and extension specialists.

AFREN equips growers with the knowledge and understanding that they need to reduce the emergence and manage the impacts of fungicide resistance in Australian grains crops.

As members of AFREN, the authors of this paper are keen to hear if you believe you are encountering reduced sensitivity or resistance in your broadacre crops.

Contact details

Angela Van de Wouw
110 Natimuk Road, Horsham, Victoria 3400
0439 900919
apvdw2@unimelb.edu.au

Nick Poole and Tom Price,
FAR Australia,
Shed 2/63 Holder Rd, Bannockburn, Victoria 3331
08 5266 1290
0499 888 066
nick.poole@faraustralia.com.au

GRDC Project Code: CUR1905-001SAX,