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).
Code | Target site of action | Group name | Chemical group | Common name | Comments |
4 | RNA Polymerase I | PA-fungicides (Phenylamides) | acyl- alanines | benalaxyl benalaxyl-M (=kiralaxyl) furalaxyl metalaxyl metalaxyl-M (=mefenoxam) |
Resistance and cross-resistance well known in various Oomycetes but mechanism unknown. High risk. See FRAC Phenylamide Guidelines for resistance management. |
oxazolidinones | oxadixyl | ||||
butyrolactones | ofurace |
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.