The FRAC SDHI Working Group is responsible for common resistance management recommendations for the SDHI fungicides (benodanil, bixafen, boscalid, carboxin, fenfuram, fluopyram, flutolanil, furametpyr, isopyrazam, mepronil, oxycarboxin,penflufen, penthiopyrad, sedaxane and thifluzamid).

The above-mentioned fungicides are in general cross-resistant and have been grouped under the FRAC Code No. 7 in the revised FRAC Code List.

CODE	TARGET SITE OF ACTION	GROUP NAME	CHEMICAL GROUP	COMMON NAME	COMMENTS
7	Complex II; succinate- dehydrogenase		Phenyl-benzamides	Benodanil Flutolanil Mepronil	Resistance known for several fungal species in field populations and lab mutants. Target site mutations in sdh gene, e.g. H/Y (or H/L) at 257, 267, 272 or P225L. Medium-high risk. Resistance management required
			Pyridinyl-ethyl- benzamide	Fluopyram
			Furan-carboxamides	Fenfuram
			Oxathiin- carboxamides	Carboxin Oxycarboxin
			Thiazole- carboxamides	Thifluzamide
			Pyrazole- carboxamides	Bixafen Furametpyr Isopyrazam Penflufen Penthiopyrad Sedaxane
			Pyridine- carboxamides	Boscalid

SDHI fungicides were discovered more than 40 years ago. Due to the limited disease and application spectrum of the “first generation” carboxamides, resistance under commercial conditions remained limited to a few diseases (basidiomycetes) and crops, e.g. Puccinia horiana, chrysanthemum rust, or Ustilago nuda, loose smut in barley.

In addition to these “first generation” molecules, the group of SDH inhibitors was meanwhile increased, with broad spectrum fungicides controlling a variety of diseases in various crops.

The target enzyme of SDH inhibitors is succinate dehydrogenase (SDH, so-called complex II in the mitochondrial respiration chain), which is a functional part of the tricarboxylic cycle and linked to the mitochondrial electron transport chain (Keon et al., 1991). SDH consists of four subunits (A, B, C and D) and the binding site of ubiquinone (and of SDHIs) is formed by the subunits B, C and D.

Baseline studies and monitoring programmes have been initiated and are ongoing. Several target site mutations have been detected both in lab generated mutants (Stammler et al, 2007) and field studies (FRAC).

The currently known facts on target site mutations are summarised in a separate document (Published and newer cases of SDHI resistance in fungi).

List of confirmed cases of field resistance to SDHI.

Crop	Disease	Geography	Source, reference
Pistachio	Alternaria alternata	US, California	Avenot HF, Michailides TJ (2007). Resistance to boscalid in Alternaria alternata isolates from pistachio in California. Plant disease 91 (10), 1345-1350
Cucurbits	Corynespora cassiicola	Japan, Ibaraki	Miyamoto T, Ishii H, Seko T, Tomita Y, Kobori S & Ogawara T (2007). Occurrence of boscalid-resistant isolates of cucumber Corynespora leaf spot fungus (C. cassiicola). Japanese Journal of Phytopathology 74: in press. (Japanese abstr.)
Cucurbits	Podosphaera xanthii	US, NY and GA	M.T. McGrath (2008). Fungicide sensitivity in Podosphaera xanthii and efficacy for cucurbit powdery mildew in NY, USA, in 2003-2006. Journal of Plant Pathology 90 (2, Supplement), S 2.90 M. Miazzi and M.T. McGrath (2008). Sensitivity of Podosphaera xanthii to registered fungicides and experimentals in GA and NY, USA, in 2007. Journal of Plant Pathology (2008), 90 (2, Supplement), S 2.90

A complete overview on resistant plant pathogenic organisms, including published cases of SDHI resistance,
can be viewed in the Publications area of the FRAC web site under (List of Resistant Plant Pathogens).

Keon JPR; White GA; Hargreaves JA (1991). Isolation, characterization and sequence of a gene conferring resistance to the systemic fungicide carboxin from the maize smut pathogen, Ustilago maydis. Current Genetics 19, 475-481

Stammler, G., Brix, H.-D., Glaettli, A., Semar, M., Schoefl, U. (2007). Biological properties of the carboxamide boscalid including recent studies on its mode of action. XVI International Plant Protection Congress 2007