Genetics of fungicide resistance

Acquired resistance to fungicides in fungal plant pathogens is a challenge in modern crop protection. Fungi are indeed very able to adapt to changing environmental conditions, such as the introduction of a new fungicide in the agricultural practice. Several genetic mechanisms may underlay fungicide resistance and influence the chance and time of its appearance and spreading in fungal populations. Resistance may be caused by mutations in major genes (monogenic or oligogenic resistance) or in minor genes (polygenic resistance) which may occur in nuclear genes as well as in cytoplasmic genes. They are immediately expressed in haploid fungi, while they may be dominant or recessive in diploid fungi. Allelic variants may cause different levels of resistance and/or different negative pleiotropic effects on the fitness of resistant mutants. The sexual process, where occurring, plays an important role in releasing new recombinant genotypes in fungal populations. Heterokaryosis provides multinucleate fungi with a further mechanism of adaptation. Resistant mutants can be obtained from samples representative of field population of a pathogen or under laboratory conditions through selection of spontaneous mutations or following chemical or physical mutagenesis. Nowadays, molecular tools, such as gene cloning, sequencing, site-directed mutagenesis and gene replacement, make genetic studies on fungicide resistance amenable even in asexual fungi for which classical genetic analysis of meiotic progeny is not feasible.