Fungicide resistance management strategy for grapevine powdery mildew (Erysiphe necator) in New Zealand
(Issued 10 August 2015: This strategy supersedes previous resistance management strategies for grapevine powdery mildew)
Disclaimer: The information contained in this document on the prevention and management of fungicide resistance in grapevine powdery mildew (Erysiphe necator) it is accurate as far as the authors are aware; however neither the authors nor any organisation with which they are affiliated warrant that it represents all existing information on Erysiphe necator resistance, or that resistance to the fungicides mentioned, or to any other fungicides, will not develop, or that the degree of resistance will decrease as a result of the proposed resistance management strategy being followed, or that the fungicides mentioned, either singly or in combination, will provide control powdery mildew in every situation.
Introduction
Grapevine powdery mildew (Erysiphe necator) can develop resistance to fungicides with site-specific modes of action and the presence of resistance to some of the fungicides in a spray programme is likely to decrease vineyard control of powdery mildew. New Zealand vineyards have recently experienced widespread difficulty in controlling powdery mildew and survey results have shown that a degree of resistance is present to two fungicide groups, quinone outside inhibitors (QoIs) and demethylation inhibitors (DMIs).
The shift from sensitivity to resistance
Repeated use of a given group of at-risk fungicides may cause a gradual shift in the E. necator population from predominantly sensitive strains, through an increasing proportion of less sensitive strains to an eventual predominance of strains highly resistant to that group. The time period for this shift depends on the mode of action of the fungicide group, the genetics of resistance in the fungus and the frequency with which the fungicide group is used. “Practical resistance” is the term used when the predominant strains are so resistant that they cause a measurable loss of disease control when a product containing the at-risk fungicide group is used on its own. Fungicide resistance in E. necator is specific and is not related to fungicide resistance in other grape pathogens or other powdery mildews.
Rationale for this resistance management strategy
This resistance management strategy aims to maintain effective powdery mildew control as resistance develops to some fungicide groups. It uses local knowledge about the groups affected by resistance and, where local information is not available, it uses international information and expert advice. For each at-risk group the strategy includes recommendations on:
- Discontinued use of active ingredients believed to be affected by practical resistance in E. necator
- Careful management of groups that are known to be at risk from resistance but for which there is no current evidence for a loss of sensitivity in E. necator
- Specified numbers of applications per season for at-risk groups and the need to alternate or mix at-risk fungicides with fungicides that are not at risk from resistance development
- Avoiding use of at-risk products on well established powdery mildew.
E. necator has the potential to develop resistance by separate genetic mutations to six fungicide groups (Groups 3, 5, 7, 11, 13, U6) and because there is not cross-resistance between these groups, group alternation is a sound strategy for preventing or managing resistance. Another strategy is to mix each at-risk product with an effective dose of a protectant fungicide. The only effective protectant currently available is sulphur, which may not be not universally compatible with all other fungicide products. These recommendations therefore stipulate strict alternation of fungicide groups (i.e. avoid consecutive applications of products in the same group) and mixing each at-risk product with sulphur, where this is recommended by product manufacturers. Other protectants (e.g. fatty acids, lime sulphur, mineral oils and potassium bicarbonate) all have relatively low efficacy against powdery mildew and should not be relied on for resistance management where powdery mildew risk is high.
Comments are included in Section 4 on products that are mixtures of at-risk fungicide groups. Further information on fungicide resistance in New Zealand can be found at: http://resistance.nzpps.org/.
Fungicide groups at risk from resistance in E. necator
| Mode of action (MOA) group | FRAC code | Resistance risk | Period of use in NZ (years) |
|---|---|---|---|
| Demethylation inhibitor (DMI) | 3 | Medium | >30 |
| Quinone outside inhibitor (QoI; strobilurin) | 11 | High | ~15 |
| Succinate dehydrogenase inhibitor (SDHI) | 7 | Medium-High | <5 |
| Amine (morpholine) | 5 | Low-Medium | <5 |
| Azanaphthalene (AZN) | 13 | Medium | <5 |
| Phenyl-acetamide | U6 | unknown | <1 |
General resistance management recommendations
- Apply fungicides that are at risk from resistance in E. necator as part of a season long spray programme that alternates fungicides in different mode of action groups
- Preferably, mix each application of an at-risk product with sulphur, but observe manufacturers’ recommendations about sulphur mixing
- Observe product label and wine industry recommendations on application rates and pre-harvest intervals
- Apply at-risk fungicides preventatively, i.e. when disease risk is high, but before disease has become established. Do not use at-risk fungicides to control established disease, as this increases the likelihood of resistance development
- Always use recommended application rates and ensure thorough spray coverage of all bunches and foliage, including the upper leaf canopy
- Do not use an at-risk fungicide in a vineyard where E. necator may have already developed resistance, as indicated by scientific data, previous failure to control disease or previous high use of a particular fungicide group. Unwanted spray shed stock of a product that is affected by resistance should be sprayed onto the vineyard in a tank mix with a high rate of sulphur early in the season
Fungicide group recommendations
Group 3 - DMI fungicides
DMI products
| DMI active ingredient | Example products |
|---|---|
| triforine | Saprol® |
| myclobutanil | Systhane®, ProstarTM, Validus® |
| penconazole | Topas®, Citara® |
| cyproconazole | Alto® |
DMI resistance status
Sensitivity of E. necator to DMIs is decreasing and the decrease is greater in Marlborough than in Hawke's Bay. Other regions have not been surveyed. Different DMI active ingredients have lost sensitivity to different degrees. There has been a marked decrease in sensitivity to myclobutanil, with Marlborough vineyard populations showing low to moderate myclobutanil resistance. This may be affecting disease control in the most resistant vineyards. Sensitivity to penconazole is also decreasing, although penconazole is probably still effective in some vineyards, particularly in Hawke's Bay. Cyproconazole is the least affected by resistance of the DMI fungicides that have been tested. Triforine has not been tested.
DMI use guideline
- Apply a maximum of one DMI (Group 3)-containing product per season, preferably in mixture with an effective dose of sulphur. See manufacturers’ recommendations about mixing with sulphur
- DMIs should not be used in vineyards with suspected DMI resistance (e.g. where disease control has been poor or where DMI’s have been used frequently)
- Avoid the use of myclobutanil except in vineyards where it is known to be providing effective powdery mildew control
- Use DMIs at full label rate before disease has become established and ensure good spray coverage
Group 11 – QoI (strobilurin) fungicides
QoI products
| QoI active ingredient | Example products |
|---|---|
| azoxystrobin | Agpro Azoxystrobin, Amistar®, Atlantis, Avoir, Inspire®, Mirador®, OrbitTM, Roxy®, SaluteTM, Tazer®, |
| pyraclostrobin | Cabrio®, Pristine® (with the SDHI boscalid) |
| trifloxystrobin | Twist®, Protiva® Luna® Sensation (with the SDHI fluopyram) |
QoI resistance status
Surveys in Marlborough and Hawke's Bay indicate that there is almost complete resistance to QoI fungicides in vineyard populations of E. necator. Other regions may be similarly affected.
QoI use guideline
- It is recommended not to use QoI (Group 11) fungicides for powdery mildew control in New Zealand vineyards
- QoIs are believed to still be effective for downy mildew control
- QoIs do not have reliable efficacy against botrytis
Group 7 – Succinate dehydrogenase inhibitor (SDHI) fungicides
SDHI products
| Active ingredient | Example products |
|---|---|
| boscalid | Pristine® (with the QoI pyraclostrobin) |
| fluopyram | Luna Sensation® (with the QoI trifloxystrobin) |
SDHI resistance status
Practical resistance is not reported internationally in E. necator, but risk is considered to be medium to high. No sensitivity information is currently available in New Zealand.
SDHI use guideline
- Apply a maximum of two SDHI (Group 7)-containing products per season in strict alternation with an effective powdery mildew fungicide from a different group
- Mix each application with an effective dose of sulphur, if recommended by the manufacturer
- Available SDHI products are mixed with a QoI fungicide and because QoI resistance is widespread, this combination of active ingredients does not reduce the risk of SDHI resistance development in powdery mildew.
Group 5 – Amine (morpholine) fungicides
Amine Products
| Active ingredient | Example products |
|---|---|
| spiroxamine | Spiral®, Impulse® |
Amine resistance status
Practical resistance is not reported internationally in E. necator, but risk is considered to be low to medium. No sensitivity information is available in New Zealand.
Amine use guideline
- Apply a maximum of two amine (Group5)-containing products per season in strict alternation with an effective powdery mildew fungicide from a different group
- Mix each application with an effective dose of sulphur, if recommended by the manufacturer.
Group 13 – Azanaphthaline (AZN) fungicides
AZN products
| Active ingredient | Example products |
|---|---|
| quinoxyfen | Quintec®, Proxima® |
| proquinazid | Talendo® |
AZN resistance status
Resistance not reported internationally in E. necator, but risk is considered to be medium. No sensitivity information is available in New Zealand.
AZN use guideline
- Apply a maximum of three AZN (Group 13)-containing applications per season in strict alternation with an effective powdery mildew fungicide from a different group
- Mix each application with an effective dose of sulphur, if recommended by the manufacturer
- Product labels may recommend a maximum number of AZN consecutive applications, but it is preferable to alternate each AZN-containing application with an effective dose of a fungicide from a different group.
Group U6 – Phenyl-acetamide fungicides
Phenyl-acetamide products
| Active ingredient | Example products |
|---|---|
| cyflufenamid | Flute® |
Phenyl-acetamide resistance status
Resistance risk not listed by FRAC. No sensitivity information available in New Zealand.
Phenyl-acetamide use guideline
- Apply a maximum of two Phenyl-acetamide (Group U6)-containing applications per season in strict alternation with an effective powdery mildew fungicide from a different group
- Mix each application with an effective dose of sulphur, if recommended by the manufacturer
- Product labels may recommend a maximum number of consecutive applications but it is preferable to alternate each Phenyl-acetamide-containing application with an effective dose of a fungicide from a different group.
Mixtures of at-risk fungicides
Mixing an at-risk fungicide with an effective dose of a fungicide from a different group is an important method to delay resistance development. Some products contain mixtures of two at-risk fungicides but these are undesirable for resistance management because, as has previously happened in New Zealand, resistance can develop to one component without detection, leaving the other component also vulnerable to resistance. The QoI+SDHI mixtures, Pristine® (pyraclostrobin + boscalid) and Luna® Sensation (trifloxystrobin + fluopyram) must be considered as stand-alone SDHI products for resistance management because of widespread QoI resistance in E. necator. If used against powdery mildew, each application should be mixed with sulphur, if the manufacturer recommends it, and must be alternated with a non-SDHI and a non-QoI product.
Partners for mixing and alternation with at-risk fungicides
Use of natural/organic products (lime sulphur, mineral oil, fatty acids and potassium bicarbonate) as alternation partners will assist fungicide resistance management, but they may not provide consistently effective control of powdery mildew unless high dose rates, short spray intervals and good spray coverage are achieved. These products are generally not compatible with at-risk fungicides. Sulphur is the fungicide that is most effective for mixing, but most labels for at-risk products are silent on mixing with sulphur. Always check compatibility when mixing fungicide products by doing a bucket test for physical compatibility, followed by a one bay knapsack test for phytotoxicity. Seek information on sulphur mixing from the product supplier.
Product label resistance statements
Product labels for every at-risk fungicide must include a resistance statement like the example below:
“Resistance prevention or management
ALTO® 100 SL contains the GROUP 3 fungicide, cyproconazole. Resistance to this fungicide could develop from repeated use. To minimise this risk, use strictly in accordance with crop-specific resistance recommendations. Do not use another product containing a GROUP 3 fungicide in conjunction with ALTO® 100 SL. Follow the individual disease resistance management strategies for each crop. [Details of the crop resistance guidelines for this product then follow].”
Acknowledgements
The development of this strategy was assisted by comments received from Adria Crop Protection, BASF New Zealand, Bayer CropScience, Dow AgroSciences, Etec Crop Solutions, Fruitfed Supplies, Syngenta Crop Protection and Zelam.
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