Fungicide resistance of plant pathogenic fungi and their chemosensitization as a tool to increase anti-disease effects of triazoles and strobilurines

The chemical method for plant protection is still the most reliable way to provide the high yield of economically significant crops and ensure its quality. In the world agriculture, at least 150 different fungicidal compounds with different mechanisms of action are now used, and the number of products developed and registered on their basis is much more. Triazoles and strobilurins belong to fungicides, which have expanded the opportunities to control causative agents of the most damaging diseases (D. Fernández-Ortuño et al., 2008). Nevertheless, multiple applications of fungicides during each new growing season are often required to achieve an effective control of fungal and oomycete pathogens. Such extensive applications of fungicides exacerbate negative impact on environment, and promote developing the resistance by these pathogens, representing the most disturbing consequence of fungicidal treatments (J.A. Lucas et al., 2015) that makes them relatively short-lived and eventually uneconomical (K.J. Brent et al., 2007; R.P. Oliver, 2014). Attempts to combat resistant forms of plant pathogenic fungi and oomycetes by increasing the dosage of fungicides and treatment numbers are futile, as they cause accumulation of more and more resistant strains in fungal populations. Therefore, control of these pathogens by minimal effective dosages of fungicides, without any decrease in the fungicidal efficacy, and search for ways to overcome the plant pathogen resistance to fungicides are dominant trends in plant protection for current sustainable agriculture. At the same time, the rejection of modern fungicides with high and medium risk of the resistance, including strobilurins and triazoles, does not seem to be practically rational, since they provide a highly effective control of a wide range of diseases and have several other advantages (A.V. Filippov et al., 2016). Chemosensitization of plant pathogens by natural compounds to increase efficacy of fungicidal treatments is an approach to solving the aforementioned problems. Chemosensitization can be accomplished by combining a commercial fungicide with a certain non- or marginally fungicidal substance at concentrations where, alone, neither compounds would be effective, while after their co-application a synergistic fungicidal effect is achieved, sometimes at a level significantly exceeding that of the fungicide dosages to which resistant strains are insensitive (B.C. Campbell et al., 2012; V.G. Dzhavakhiya et al., 2012). Since biochemical and structural targets of chemosensitizing substances differ from those targeted by fungicides, chemosensitization do not contribute to the selection of resistant pathogenic form, and reduces the toxic impact on the environment by lowering effective dosage levels of toxic fungicides. In this review, the promise of chemosensitization as an antiresistant strategy to improve efficacy of the protective fungicide effect is exemplified by experiments with several economically significant phytopathogenic fungi, which sensitivity to strobilurins and triazoles was demonstrated to enhance significantly by co-application of these fungicides with secondary plant or microbial metabolites and their synthetic analogues. In addition, the problem of the development of resistance in plant pathogenic fungi and the methods for its management are briefly described, information on the types and main mechanisms of resistance, in particular, those responsible for resistance to triazoles and strobilurins as well as data on the mechanisms of action of some chemosensitizers are presented.