27 October 2025
Under conditions favourable to disease development, downy mildew can lead to severe crop losses, especially when fungicide resistance develops. This eBulletin covers the key reasons resistance occurs, how to detect and confirm it, and strategies for effective management.
About downy mildew
When conditions favour disease prevalence, Downy mildew is one of the most important diseases of grapevines in Australia. The disease is caused by the pathogen Plasmopara viticola, which affects green grapevine tissue. When the following set of conditions arise, downy mildew can become a problem:
What is fungicide resistance in downy mildew and why does it matter?
Downy mildew can develop resistance to fungicides when the pathogen Plasmopara viticola changes genetically over time, making fungicides less effective or even ineffective at controlling it.
Fungicide resistance can have serious flow-on effects for winegrape production. As resistance builds, growers face fewer effective fungicide choices, sprays are applied more often, and input costs are higher. Fungicide resistance can also increase environmental concerns, attract negative public attention, reduce yields, and even lead to fruit rejection at the winery.
Read more about how fungicide resistance occurs and how to avoid it here.
How do you know if you have resistance to downy mildew fungicides?
If you are applying fungicides each year and the incidence and severity of downy mildew in your vineyard seems to be getting worse, you may have fungicide resistance or reduced sensitivity.
Downy mildew fungicide resistance detections in Australian vineyards
Recent research led by South Australian Research and Development Institute (SARDI), a research division of the Department of Primary Industries and Regions (PIRSA), has confirmed the detection of downy mildew fungicide resistance to several fungicide groups commonly used in vineyards.
| Active ingredient | Fungicide Activity Group | Resistance status |
| metalaxyl | 4 | Detected |
| pyraclostrobin | 11 | Detected |
| dimethomorph | 40 | Not detected |
| mandipropamid | 40 | Not detected |
Source: South Australian Research and Development Institute (SARDI)
While only one representative formulation per active ingredient was evaluated as part of this research, a range of registered products for each of the above active ingredients can be found in Table 2 of the 2025/26 Agrochemicals registered for use in Australian viticulture, commonly known as the ‘Dog book’. A full list of detections from this research can be found on page 18 in the 2025/26 ‘Dog book’.
How can you test for downy mildew resistance?
If you suspect you may have fungicide resistance in downy mildew, a free national testing service to monitor fungicide resistance in downy mildew, powdery mildew and botrytis in winegrapes is available (until June 2027).
Samples for downy mildew resistance testing can be submitted to the SARDI Horticulture Pathology Laboratory by following the detailed sampling instructions here. Biosecurity considerations apply. Prior to sending samples to the laboratory, please contact ismail.ismail@sa.gov.au.
This testing service is part of a research project led by SARDI in collaboration with Curtin University and the AWRI, and funded by Wine Australia, the Cooperative Research Centre for Solving Antimicrobial Resistance in Agribusiness, Food, and Environments (SAAFE CRC), SARDI and Curtin University.
Importance of spray program management
Overuse of the same fungicide groups as part of a spray program is a key contributor to the development of downy mildew fungicide resistance.
One of the key information sources provided annually in the ‘Dog book’ to assist you in reducing downy mildew fungicide resistance is the CropLife Australia resistance management strategy on page 15.
Before you order chemicals for the new season, review this strategy alongside your proposed spray program. Take note of:
- the fungicide group associated with each product you plan to apply;
- which fungicide groups must be applied as a protectant − Groups 11, 11+3, 21, 40, 45+40, 40+49 and 49, or can be applied after an infection event but before the first sign of oilspots – Group 4;
- which fungicide groups must be applied as part of a co-formulation or tank-mixed with an alternative chemical activity group − Groups 4* and 49;
- the suggested maximum number of times each fungicide group be applied in the season; and
- whether the fungicide group can be applied as a consecutive spray in the program – Groups 4, 21, 40 and 45+40.
Note that some fungicide groups and/or products are restricted or prohibited from use by some wineries or grape suppliers. Refer to Table 2 in the ‘Dog book’ and your winery/grape purchaser’s guidelines before you spray.
In addition to reading the CropLife Australia resistance management strategies, always read chemical labels prior to spray application, to ensure you are applying the correct treatment rate.
*As advised in the AWRI eBulletin on 25 August 2025, the APVMA registration of Group 4+M3 co-formulated downy mildew products Axiom MZ WG (mancozeb + metalaxyl-M) and Axiom MZ 720 (mancozeb + metalaxyl) was stopped on 01/07/2025. These fungicides are being replaced by Group 4 Axiom Flexi (metalaxyl-M) which must be tank-mixed with a product containing a different mode of action to avoid downy mildew fungicide resistance.
Further information
- Agrochemicals registered for use in Australian viticulture, commonly known as the ‘Dog book’
- AWRI helpdesk – call 08 8313 6600, or email helpdesk@awri.com.au.
Acknowledgements
The fungicide resistance testing work has been supported by the SAAFE CRC whose activities are funded by the Australian Government’s Cooperative Research Centre Programme.
The AWRI thanks Mark Sosnowski and Ismail Ismail from SARDI and Lincoln Harper and Fran Lopez Ruiz from Curtin University for their review.
This eBulletin is supported by Wine Australia with levies from Australia’s grapegrowers and winemakers and matching funds from the Australian Government. AWRI is a member of the Wine Innovation Cluster in Adelaide, SA.
