Succinate dehydrogenase inhibitor (SDHI) [Group 7] resistance management strategy
Disclaimer:
The fungicide and bactericide resistance information in the mode of action group table, the mode of action-based strategies and the crop-based strategies is intended to help manage the effects of pesticide resistance on plant disease control by agrochemicals in New Zealand. The New Zealand Committee on Pesticide Resistance (NZCPR) is a subcommittee of the New Zealand Plant Protection Society (NZPPS). The following parties: NZCPR, NZPPS, any people named in the strategies and any organisation the people may be affiliated with, do not guarantee that the resistance information is complete, consistent, accurate or up-to-date, or that resistance will not develop, or that the degree of resistance will decrease, as a result of the resistance information being used, and the parties accept no liability for loss or damage arising from the use of the resistance information contained in these strategies.
Also see the separate crop-based strategies for use of SDHIs against Venturia inaequalis (black spot; scab) in apples, here, against Monilinia spp. in Summerfruit (here) and against Erysiphe necator (grapevine powdery mildew) here.
Product perspective
SDHI fungicides are derived from diverse chemistry and specifically inhibit fungal respiration by blocking the ubiquinone-binding sites in the mitochondrial complex II (Avenot & Michailides 2010). They have been given the FRAC mode of action group code number 7. Depending on the host and pathogen, SDHIs have protectant, translaminar or systemic activity. Table 1 shows the chemical groups and SDHI active ingredients registered in New Zealand. Generation I SDHIs, including carboxin and oxycarboxin, have been used internationally since the late 1960s and are highly effective against basidiomycete pathogens such as rusts and Rhizoctonia sp. The newer generation II SDHIs, such as boscalid, fluxapyroxad, penthiopyrad, isopyrazam and fluopyram, have activity against a wider spectrum of fungal pathogens of various crops including cereals, grapes, stonefruit, strawberries, kiwifruit, apples and cucurbits (Table 2).
| Chemical group | Common name | Trade name |
|---|---|---|
| Oxathiin-carboxamide | carboxin | Vitaflo® 200 |
| Pyridine-carboxamide | boscalid | Pristine®, Collis®, Unistar® |
| Pyrazole-carboxamide | bixafen | Aviator® Xpro |
| fluxapyroxad | Systiva®, Adexar®, Sercadis®, Merivon® Imtrex®, Revystar® | |
| isopyrazam | Seguris Flexi® | |
| penthiopyrad | Fontellis®, Velista® | |
| benzoviniflupyr | Elatus® Plus | |
| isoflucypram | Caley® Iblon®, Vimoy® Iblon® | |
| Phenol-oxo-ethyl thiophene amide | isofetamid | Kenja 400SC |
| N-methoxy-(phenyl-ethyl)-pyrazole-carboxamides | pydiflumetofen | Mirivas®, Prime |
| Pyridinyl-ethyl-benzamide | fluopyram | Luna® Devotion, Luna® Sensation, Luna® Privilege, Raxil® Star, Velum® Prime. |
| Pathogen | Disease | Crop |
|---|---|---|
| Puccinia hordei | Brown rust | Barley |
| Pyrenophora teres | Net blotch | Barley |
| Ramularia collo-cygni | Ramularia leaf and awn spot | Barley |
| Rhynchosporium secalis | Scald | Barley |
| Puccinia recondita | Leaf rust | Wheat |
| Puccinia striiformis | Stripe rust | Wheat |
| Mycosphaerella graminicola | Speckled leaf blotch | Wheat |
| Venturia inaequalis | Black spot (scab) | Apple |
| Venturia pyrina | Black spot (scab) | Pear |
| Podoshaera leucotricha | Powdery mildew | Apple |
| Botrytis cinerea | Dry eye rot | Apple |
| Botrytis cinerea | Botrytis bunch rot (grey mould) | Grapes |
| Puccinia graminis | Stem rust | Ryegrass seed crops |
| Puccinia coronata | Crown rust | Ryegrass seed crops |
| Erysiphe necator | Powdery mildew | Grapes |
| Monilinia fructicola | Blossom blight/brown rot | Stonefruit |
| Botrytis cinerea | Grey mould | Stonefruit |
| Didymella bryoniae | Gummy stem blight | Squash and pumpkin |
| Sphaerotheca fuliginea, syn. Podosphaera xanthii and Erysiphe cichoracearum |
Powdery mildew | Squash and pumpkin |
| Sclerotinia sclerotiorum | Sclerotinia blossom and fruit rot | Kiwifruit |
| Sclerotium cepivorum | White rot | Onion |
| Colletotrichum cereale, Colletotrichum graminicola Rhizoctonia solani Sclerotinia homeocarpa Bipolaris spp., Curvularia spp, Dreschlera spp., Exserohilium laetisaria fuciformis Limonomyces roseipellis Rhizoctonia zeae Waitea circinate var. circinata Ophiosphaerella narmari basidiomycete fungi |
Anthracnose
|
Turf |
Current status of SDHI resistance
The Fungicide Resistance Action committee (FRAC) lists SDHI fungicides as medium to high risk of selecting for resistance in fungal pathogen populations based primarily on single-site mutations of the gene encoding the target succinate dehydrogenase enzyme. Resistance is widespread to the older generation I SDHI carboxin as well as to the generation II SDHI boscalid. Loss of sensitivity is starting to show up for other generation II SDHIs in New Zealand. Sensitivity tests have identified fungal pathogen isolates with various degrees of resistance to SDHIs for Alternaria solani (potato early blight), Botrytis cinerea (grey mould of boysenberry and dry eye rot of apple), Venturia inaequalis (apple black spot) and Stemphylium vesicarium (stemphylium leaf blight of onion).
Positive cross-resistance occurs between all members of the SDHI group, but the degree of resistance in a given pathogen population to individual SDHI active ingredients differs between active ingredients, rather like it does for DMIs.
Resistance management strategy
General guidelines
-
Effective disease management is essential to delay the build-up of resistant pathogen strains and should combine use of appropriate fungicides with cultural control methods.
-
Apply products containing SDHI fungicides at full label rates, intervals and crop stages according to manufacturers’ instructions. The number of applications of products containing SDHI fungicides must be limited according to the guidelines below for specific crops.
-
Maximum numbers of SDHI applications apply to the crop, not to individual target diseases. Where mixtures (tank mix or co-formulation) or alternation are required, the partner fungicide should be capable of providing satisfactory control of the target disease when used alone, and must be from a different activity group (i.e. not Group 7).
-
Refer to manufacturers' recommendations for rates of partner fungicides. SDHI fungicides should be used preventatively and not curatively, when the amount of disease is low, but disease risk is high. Do not use SDHI fungicides when disease is well established.
Cereals
Foliar Applications
-
Use a maximum of two applications of products containing SDHI fungicide per crop.
-
SDHI fungicides must be applied in mixture with a fungicide of a different group, e.g., DMI (triazole) with efficacy against the target disease.
-
Apply SDHI fungicide products at full label rates and intervals according to manufacturers’ recommendations. Do not use SDHI fungicides in repeated applications at reduced rates as this increases the risk of selecting resistance in pathogen populations.
-
Apply SDHI fungicides preventively or as early as possible in the disease cycle. Do not rely only on the curative potential of SDHI fungicides
Seed Treatment Applications
-
Seed treatment products that are selective by rate or efficacy against seed/soil-borne diseases or foliar pathogens at low risk of resistance do not count as one of the two SDHI applications.
-
Seed treatment products which provide control of foliar diseases at high risk of resistance e.g., scald, net blotch, speckled leaf blotch and powdery mildew do count against the total number of SDHI applications.
Ryegrass seed crops
Use a maximum of two applications of products containing SDHI fungicide per crop. SDHI fungicides must be applied either in strict alternation or in mixture.
Squash and pumpkin
Use a maximum of two applications of products containing SDHI fungicide per crop, preferably in mixture. If used alone, apply SDHI fungicides in strict alternation with effective fungicides from a different mode of action group.
Stonefruit
Use a maximum of three applications of products containing SDHI fungicide per season, preferably in mixture. If used alone, apply SDHI fungicides in strict alternation with effective fungicides from a different mode of action group. If used in mixture, do not make more than two consecutive applications of products containing a SDHI fungicide. Use of SDHI fungicides should be restricted to a single stage of crop development. Use during the flowering period and do not use near harvest time.
Grape
Use a maximum of two applications of products containing SDHI fungicide per season, preferably in mixture. If used alone, apply SDHI fungicides in strict alternation with effective fungicides from a different mode of action group. When targeted specifically against botrytis bunch rot (grey mould, B. cinerea), the maximum numbers of applications of SDHI fungicides are given in Table 3. These maxima apply to all diseases of grapes and must not be exceeded when targeting powdery mildew (Erysiphe necator).
| Total no. B. cinerea spray applications per season | 1 | 2 | 3 | 4 | 5 or more |
|---|---|---|---|---|---|
| Maximum number of solo SDHI applications1 | 1 | 1 | 1 | 2 | 2 |
| Maximum number of SDHI applications in mixture | 1 | 1 | 2 | 2 | 2 |
1 Where two applications of SDHI fungicides are made alone, they must not be consecutive and the alternate fungicide application(s) must be capable of providing effective suppression of B. cinerea.
Apple and pear
-
Apply a maximum of four SDHI-containing products per season in strict alternation with an effective fungicide from a different mode of action group.
-
Apply only in mixture with a broad-spectrum protectant fungicide that is not at risk from resistance at full label rate.
-
Use SDHIs before disease has become established. Do not use when severe black spot has developed.
Kiwifruit
Use a maximum of two SDHI containing fungicides per year over all diseases, preferably in mixture.
If used alone, apply SDHI fungicides in strict alternation with fungicides in a different mode of action group. Use of SDHI fungicides should be restricted to a single stage of crop development. Use during the flowering period and do not use near harvest time.
Other crops
Use a maximum of three SDHI containing fungicides per year over all diseases, preferably in mixture. If used alone, apply SDHI fungicides in strict alternation with fungicides in a different mode of action group.
Resistance prevention strategy implementation
These resistance prevention and management guidelines should be implemented for all products containing a SDHI fungicide by including a statement on the product label under the heading "Resistance Management". This should include the statement "This product contains a GROUP 7 (SDHI) fungicide that is at risk from resistance development. Do not use in conjunction with other products containing a GROUP 7 (SDHI) fungicide when adhering to the fungicide mixture and alternation guidelines for the specific crops below." The general guidelines and appropriate crop-specific guidelines for Group 7 fungicide applications should then follow.
Acknowledgements
This strategy includes information from The Fungicide Resistance Action Committee (FRAC) about mode of action groups, the occurrence of resistance worldwide and resistance management recommendations.
References
Avenot HF, Michailides TJ 2010 Progress in understanding molecular mechanisms and evolution of resistance to succinate dehydrogenase inhibiting (SDHI) fungicides in phytopathogenic fungi. Crop Protection 29: 643-651.
© 2026 New Zealand Plant Protection Society (Incorporated). All rights reserved.