FUNGICIDE EFFECTIVENESS ON CZECH POPULATIONS OF PSEUDOPERONOSPORA CUBENSIS

Application of fungicides is still the principal method for managing harmfulness of Pseudoperonospora cubensis, a causal agent of cucurbit downy mildew, which is one of the most important diseases on cucumber. There are non-systemic (contact) fungicides as well as systemic fungicides (single-site inhibitors). The latter ones are more efficient but, unfortunately, their repeated application highly increases a risk of development of resistant pathogen strains. Therefore, effective chemical control strategies rely mostly upon weekly applications of contact fungicides. Totally 63 isolates of P. cubensis collected in 8 regions of the Czech Republic (25 in 2005, 18 in 2006 and 20 in 2007) were assessed for a degree of their resistance to fungicides. Six fungicide agents were chosen for testing their effectiveness, using five concentrations, in a floating disc bioassay. The active ingredient of two fungicide agents used was metalaxyl. As one of these agents has been so far the most widely used against downy mildew on cucumber in the Czech Republic in the past few years, it enabled us to monitor the distribution and development of resistant strains in Czech P. cubensis populations. Significant differences in effectiveness among fungicides were observed. Metalaxyl, metalaxyl-M, cymoxanil and dimethomorph were found ineffective; nearly all pathogen isolates expressed a resistant reaction to the recommended concentration of these fungicides. In contrast, propamocarb and fosetyl-Al were found effective at the recommended concentrations. Nevertheless, a few strains resistant or tolerant to propamocarb were observed in 2005 and 2006, indicating that selection for resistance strains is in progress in the Czech P. cubensis populations, and thus more screening is necessary. The results indicate that generally, the effectiveness of the studied fungicides against P. cubensis has progressively declined.