The phenylamides are a highly active class of fungicides specifically controlling plant pathogens of the Oomycetes (the downy mildews of the Peronosporales and Sclerosporales, as well as most members of the Pythiales (e.g. Phytophthora and Pythium spp.) and Saprolegniales; Gisi 2002). They penetrate the plant tissue rapidly and are translocated acropetally within the plant. Phenylamide fungicides have been in commercial use since 1978. The following active ingredients are classified as phenylamides: metalaxyl, metalaxyl-M (=mefenoxam), furalaxyl and oxadixyl, benalaxyl and benalaxyl-M (=kiralaxyl) and ofurace (Gisi and Ziegler 2002; Müller and Gisi 2007).
Despite the presence of Phenylamide-resistant isolates in major target pathogen populations, they continue to be used by growers as effective tools to manage disease when used in accordance with sound resistance management programs (see General Use Recommendations). The use recommendations are well established and do not create significant controversy among member companies, officials and advisors. Current sensitivity monitoring data are produced by only a few research groups in industry and academia. The presence of resistant subpopulations at varying proportions is well documented in several plant pathogen species of Oomycetes on a range of crops worldwide (Gisi and Cohen 1996; Gisi and Sierotzki 2008; FRAC Resistance Survey List, www.frac.info). However, sensitive subpopulations have not disappeared, even though PA-containing products have been used continuously in similar quantities and intensities over the past 30 years. This strongly suggests that the recommended anti-resistance strategies are successful and that biological processes (e.g. sexual reproduction, fitness) of the pathogens may contribute to equilibrate sensitivity in populations. Sampling and testing methods for resistance monitoring have been published through FRAC in 1992 (EPPO Bulletin 22, 297-322) and are still valid.
Site of Action and Mechanisms of Resistance
The PA-fungicides inhibit rRNA biosynthesis (polymerase complex I) in the target pathogens. The mechanism of resistance may involve one (or two) major gene(s) and potentially several minor genes. The target gene and the site of mutation(s) in the genome have not been mapped so far. Therefore, there are no molecular methods available for the detection of resistance.
Gisi U. 1992. FRAC methods for monitoring the sensitivity of fungal pathogens to phenylamide fungicides developed by the Phenylamide Fungicide Resistance Action Committee (PA-FRAC) of GIFAP. EPPO Bulletin 22(2):297-322.
Gisi U. 2002. Chemical control of downy mildews. In: Spencer PTN, Gisi U, Lebeda A, editors. Advances in Downy Mildew Research. Dordrecht (the Netherlands): Kluwer p. 119-159.
Gisi U, Cohen Y. 1996. Resistance to phenylamide fungicides: A case study with Phytophthora infestans involving mating type and race structure. Annual Review of Phytopathology 34:549-572.
Gisi U, Sierotzki H. 2008. Fungicide modes of action and resistance in downy mildews. European Journal of Plant Pathology 122:157-167.
Gisi U, Ziegler H. 2002. Phenylamides (acylalanines and related): Metalaxyl, metalaxyl-M, furalaxyl, benalaxyl, ofurace, oxadixyl. In: Plimmer, JR editor. Encyclopedia of Agrochemical. New York (USA): John Wiley p. 609-616.
Müller U, Gisi U. 2007. Newest aspects of nucleic acid synthesis inhibitors – metalaxyl-M. In: Krämer W, Schirmer U, editors. Modern Crop Protection Compounds., Weinheim (Germany): Wiley-VCH. p. 739-746.
Sozzi D, Schwinn FJ, Gisi U. 1992. Determination of the sensitivity of Phytophthora infestans to phenylamides: a leaf disc method. EPPO Bulletin 22(2):306-309.
Staehle-Csech U, Gisi U. 1992. Determination of the sensitivity of Plasmopara viticola to phenylamides. EPPO Bulletin 22(2):314-316.
Williams R, Gisi U. 1992. Monitoring pathogen sensitivity to phenylamide fungicides: principles and interpretation. EPPO Bulletin 22(2):299-306.