Cercospora leaf spot (CLS), caused by the fungal pathogen Cercospora beticola, is the most important foliar pathogen of sugar beet worldwide. Extensive reliance on fungicides to manage CLS has resulted in the evolution of fungicide resistance in C. beticola worldwide, including populations in the Czech Republic. One important class of fungicides used to manage CLS is the sterol demethylation inhibitors (DMI). The aim of our study was to assess DMI resistance in C. beticola from the Czech Republic and elucidate the molecular basis of DMI resistance in this population. A total of 50 isolates were collected in 2018 and 2019 from the major sugar beet growing regions of the Czech Republic and assessed for in vitro sensitivity to the DMI fungicides propiconazole, prochloraz, and epoxiconazole. These analyses identified three strains that exhibited 50% effective concentration (EC₅₀) values > 1.0 μg mL^–1 against respective fungicides, which were therefore considered resistant. In contrast, strains that exhibited lowest EC₅₀ values were considered sensitive. To explore the molecular basis of resistance in these three strains, the cytochrome P450-dependent sterol 14α-demethylase (Cyp51) gene was sequenced. Sequence analysis identified a Y464S mutation in all three resistant strains. To assess whether Cyp51 gene expression may play a role in DMI resistance, selected strains were grown in vitro with and without fungicide treatment. These analyses indicated that Cyp51 gene expression was significantly induced after fungicide treatment. Thus, we conclude that Y464S point mutation along with induced Cyp51 gene overexpression is likely responsible for resistance against DMI fungicides in C. beticola from the Czech Republic.

Use of metalaxyl against some soil plant pathogens of the class Peronosporomycetes – A review and two case studies | Matěj Pánek, Štěpán Helmer, Asad Ali | Agricultural Journals

Phytophthora cactorum is considered an important plant pathogen which is causing major damage to strawberry plants worldwide. In the current study, the ability of the active ingredients of seven different fungicides, azoxystrobin, cymoxanil, dimethomorph, fenamidone, fluopicolide, metalaxyl and propamocarb, to suppress the mycelial growth, sporangial formation and zoospore release of P. cactorum isolates, was tested. The variation in resistance against various fungicides was found among the isolates. The active ingredients are also unequally efficient against different life stages of P. cactorum, which is probably associated with their different modes of action. A significant level of resistance was recorded against metalaxyl and dimethomorph; however, these were totally inefficient against the zoospore release, while azoxystrobin did not inhibit mycelial growth. The only fungicide efficient against all three P. cactorum life stages tested was fluopicolide, although the calculated resistance factor gives evidence of the rise of resistance in the majority of isolates even against this fungicide. Significant differences were found between responses to fungicides of isolates from strawberry and from other host species. Based on the Mahalanobis distances calculated in the discriminant analysis comprising all of the assays performed, the similarities among isolates were estimated.