Introduction
The succinate dehydrogenase inhibitor (SDHI) fungicide class is the fastest growing in terms of new compounds produced and launched into the fungicide market in turfgrass and in agriculture in general. This fungicide class, which inhibits the complex II of fungal respiration, is not new and was originally called carboxamide fungicides. The earliest compound in this class, carboxin, being first marketed around 1966 and was used mainly as a seed treatment to control basidiomycete pathogens such as smuts. Several active ingredients followed carboxin however, these compounds gave only slightly broader-spectrum control compared with carboxin. The first carboxamide with a bona fide broad-spectrum foliar activity was boscalid, launched in the early 2000’s (in turfgrass in 2003) Recently, several new fungicides inhibiting the succinate dehydrogenase (SDH) enzyme have been collectively named SDHIs and are currently available. The Fungicide Resistance A Committee (FRAC) currently lists twenty-three (23) SDHI compounds (See table 1), belonging to different chemical types. A subset of active ingredients is now labeled for turf (table 2, table 3). Currently the “overall” spectrum of SDHI fungicides is extremely broad, being comparable with the QoI spectrum, with the exception of oomycete (e.i. Pythium) control.
Mode of action and active ingredients
The target of SDHI fungicides is the succinate dehydrogenase (SDH) complex in the mitochondrial respiratory chain also referred to as complex II or succinate:ubiquinone oxidoreductase (SQR), thereby inhibiting fungal respiration. This leads to cells being unable to produce energy which in turn hinders fungal growth. This mode of action is different from the QoI inhibitors (strobilurins) which act to inhibit the respiratory chain at the complex III (coenzyme Q: cytochrome c-oxidoreductase).
Table 1. SDHI fungicides listed by Fungicide Resistance Action Committee (FRAC)
| Chemical group | Common name |
| phenyl-benzamides | benodanil
flutolanil mepronil |
| phenyl-oxo-ethyl thiophene amide | isofetamid |
| pyridinyl-ethyl- benzamides | fluopyram |
| furan- carboxamides | fenfuram |
| oxathiin- carboxamides | carboxin
oxycarboxin |
| thiazole- carboxamides | thifluzamide |
| pyrazole-4- carboxamides | benzovindiflupyr
bixafen fluindapyr fluxapyroxad furametpyr inpyrfluxam isopyrazam penflufen penthiopyrad sedaxane |
| N-cyclopropyl-N- benzyl-pyrazole- carboxamides | isoflucypram |
| N-methoxy-(phenyl- ethyl)-pyrazole- carboxamides | pydiflumetofen |
| pyridine- carboxamides | boscalid |
| pyrazine- carboxamides | pyraziflumid |
Table 2. SDHI active ingredients and commercial products in turfgrass
| Common name | Some trade name | Diseases on label* | Company |
| boscalid | Emerald | Dollar spot, bentgrass dead spot | BASF |
| flutolanil | Prostar | Brown patch, large patch, fairy ring, red thread, pink patch, yellow patch, southern blight, gray snow mold | Bayer |
| fluxapyroxad | Xzemplar | Brown patch, dollar spot, fairy ring, large patch, leaf and sheath spot, leaf spots, snow molds, summer patch, algae | BASF |
| isofetamid | Kabuto | Dollar spot, spring dead spot | PBI-Gordon |
| penthiopyrad | Velista | Dollar spot, brown patch, anthracnose, large patch, leaf spots, red thread, pink patch, powdery mildew, rust, gray snow mold | |
| pydiflumetofen | Posterity | Dollar spot, fairy ring, microdochium patch, Spring dead spot | Syngenta |
| fluopyram | Indemnify | Nematodes | Bayer |
*Does not include any 2(ee) recommendations
Table 3. Prepacked mixtures using SDHI fungicides in turfgrass
| Trade names | Common name | Marketed by |
| Exteris stressgard | fluopyram +trifloxystrobin | Bayer |
| Honor Intrinsec | boscalid + pyraclostrobin | BASF |
| Lexicon Intrinsec | fluxapyroxad + pyraclostrobin | BASF |
| Systar | flutalonil + thiopanate methyl | Regal |
| Tekken | Isofetamid + tebuconazole | PBI Gordon |
Word of caution
SDHI fungicides are classified as medium to high risk of resistant development due to the specificity of their mode of action and widespread use in other crops. Resistance to carboxin, flutalonil and boscalid was reported shortly after their registration. Recently, resistance has been reported in Botritis cinerea (grapes, strawberry, apple and kiwi), Alternaria alternata (almond and potatoes), Didymella brioniae (cucumber), Corynespora cassicola (cucurbits), and Podosphaera xanthii (cucurbits).
Due to their unique mode and site of action, the SDHI fungicides have shown no cross-resistance with other chemical classes such as strobilurins, benzimidazoles or anilinopyrimidines. However, with the increase use of SDHI fungicides in turfgrass, the potential exists for fungicide resistance to develop. Therefore, it is wise to rotate different target site fungicides. A complete list of management and avoidance of SDHI fungicide resistance is located in the FRAC publication https://www.frac.info/docs/default-source/sdhi-wg/sdhi-meeting-minutes/minutes-of-the-2018-sdhi-meeting-11-12th-of-december-2018-with-recommendations-for-2019.pdf?sfvrsn=fa9c4b9a_2
Bibliography
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Avenot, H.F. and T.J. Michailides. 2010. Progress in understanding molecular mechanisms and evolution of resistance to succinate dehydrogenase inhibiting (SDHI) fungicides in phytopathogenic fungi. Crop Protection 29:643-651.
Avenot, H. F., Thomas, A., Gitaitis, R. D., Langston, D. B., Jr., and Stevenson, K. L. 2012. Molecular characterization of boscalid- and penthiopyrad-resistant Didymella bryoniae and assessment of their sensitivity to fluopyram. Pest Manage. Sci. 68:645-651.
Hu, M.-J., Fernandez-Ortuño, D., and Schnabel, G. 2016. Monitoring resistance to SDHI fungicides in Botrytis cinerea from strawberry fields. Plant Dis. 100: 959-965.
Sierotzki, H., and Scalliet, G. 2013. A review of current knowledge of resistance aspects for the next-generation succinate dehydrogenase inhibitor fungicides. Phytopathology 103:880-887.
