Fungicide modes of action and resistance in downy mildews

Abstract

Among oomycetes, Plasmopara viticola on grape and Phytophthora infestans on potato are agronomically the most important pathogens requiring control measures to avoid crop losses. Several chemical classes of fungicides are available with different properties in systemicity, specificity, duration of activity and risk of resistance. The major site-specific fungicides are the Quinone outside inhibitors (QoIs; e.g. azoxystrobin), phenylamides (e.g. mefenoxam), carboxylic acid amides (CAAs; e.g. dimethomorph, mandipropamid) and cyano-acetamide oximes (cymoxanil). In addition, multi-site fungicides such as mancozeb, folpet, chlorothalonil and copper formulations are important for disease control especially in mixtures or in alternation with site-specific fungicides. QoIs inhibit mitochondrial respiration, phenylamides the polymerization of r-RNA, whereas the mode of action of the other two site-specific classes is unknown but not multi-site. The use of site-specific fungicides has in many cases selected for resistant pathogen populations. QoIs are known to follow maternal, largely monogenic inheritance of resistance; they bear a high resistance risk for many but not all oomycetes. For phenylamides, inheritance of resistance is based on nuclear, probably monogenic mechanisms involving one or two semidominant genes; resistance risk is high for all oomycetes. The molecular mechanism of resistance to QoIs is mostly based on the G143A mutation in the cytochrome b gene; for phenylamides it is largely unknown. Resistance risk for CAA fungicides is considered as low to moderate depending on the pathogen species. Resistance to CAAs is controlled by two nuclear, recessive genes; the molecular mechanism is unknown. For QoIs and CAAs, resistance in field populations of P. viticola may gradually decline when applications are stopped.

Cucurbit downy mildew (Pseudoperonospora cubensis)—biology, ecology, epidemiology, host-pathogen interaction and control

“…Although chemical control by fungicides may have negative environmental effects and limitations (de Waard et al 1993), fungicides still constitute the predominant part of the control measures used against oomycetes (Cohen and Coffey 1986;Lebeda and Schwinn 1994;Gisi 2002;Gisi and Sierotzki 2008). According to Gisi (2002) the sales value of fungicides against downy mildews (Table 6) amounted to 1.2 billion SFr in 1996, of which 10% were used to fight mainly P. cubensis on cucurbit crops .…”

Tolerance and Bioaccumulation of Copper by the Entomopathogen Beauveria bassiana (Bals.-Criv.) Vuill. Exposed to Various Copper-Based Fungicides

“…which acts by direct contact, inducing denaturation of enzymes and proteins of the cell membrane of the spores and mycelium leading to a general disruption of metabolism and cell integrity. It also inhibits spore germination (Gisi and Sierotzki 2008).…”

Diversity of defence mechanisms in plant–oomycete interactions: a case study of Lactuca spp. and Bremia lactucae

“…molecular mapping of genes responsible for resistance/susceptibility and virulence/avirulence both in host plants and the pathogen (Kuang et al 2004(Kuang et al , 2006Michelmore and Wong 2008); (6.) development of new control methods based on screening of various 'natural' compounds for their activity to suppress lettuce downy mildew (Portz et al 2008;Sedlářová et al 2007b), utilization of new generations of fungicides (Cohen et al 2008;Gisi and Sierotzki 2008) and induced resistance.…”