Resistance mechanism to carboxylic acid amide fungicides in the cucurbit downy mildew pathogen Pseudoperonospora cubensis.

BACKGROUND: Pseudoperonospora cubensis, the causal oomycete agent of cucurbit downy mildew, is responsible for enormous crop losses in many species of Cucurbitaceae, particularly in cucumber and melon. Disease control is mainly achieved by combinations of host resistance and fungicide applications. However, since 2004, resistance to downy mildew in cucumber has been overcome by the pathogen, thus driving farmers to rely only on fungicide spray applications, including carboxylic acid amide (CAA) fungicides. Recently, CAA-resistant isolates of P. cubensis were recovered, but the underlying mechanism of resistance was not revealed. The purpose of the present study was to identify the molecular mechanism controlling resistance to CAAs in P. cubensis. RESULTS: The four CesA (cellulose synthase) genes responsible for cellulose biosynthesis in P. cubensis were characterised. Resistant strains showed a mutation in the CesA3 gene, at position 1105, leading to an amino acid exchange from glycine to valine or tryptophan. Cross-resistance tests with different CAAs indicated that these mutations lead to resistance against all tested CAAs. CONCLUSION: Point mutations in the CesA3 gene of P. cubensis lead to CAA resistance. Accurate monitoring of these mutations among P. cubensis populations may improve/facilitate adequate recommendation/deployment of fungicides in the field.

Characterization of Colletotrichum graminicola Isolates Resistant to Strobilurin-Related QoI Fungicides.

Isolates of Colletotrichum graminicola were collected from annual bluegrass or bent grass turf in Japan and the United States, and their sensitivities to QoI fungicides (QoIs) as well as their cytochrome b sequences were characterized. Five isolates sampled from turf treated repeatedly with azoxystrobin were highly QoI resistant under both in vivo and in vitro test conditions. The nucleotide sequences of a large cytochrome b gene segment involving the binding site of QoIs were fully homologous for all resistant isolates and contained the G143A target site mutation known to confer QoI resistance in other pathogens. QoI-sensitive isolates collected prior to treatments with QoIs were more diverse with regard to their cytochrome b gene sequences and their phenotype responses to QoIs. All wild-type isolates retained a glycine in position 143 of cytochrome b. Three of the four QoI-sensitive isolates were, in addition, distinguished by leucines in positions 95, 130, and 141, which were exchanged to threonine in all resistant but also in one of the sensitive isolates. In addition to a more pronounced divergence of cytochrome b sequences, the sensitive wild-type isolates also were diverse with regard to the induction of alternative respiration in response to QoI action, as indicated by comparisons of QoI sensitivities displayed in the absence or presence of the alternative oxidase inhibitor salicylhydroxamic acid. These different phenotype responses expressed under in vitro test conditions had no or only a slight impact on anthracnose control in protective applications of azoxystrobin. Isolate responses in vitro were very similar for trifloxystrobin, indicating cross-resistance among the class of QoIs. Our results imply that C. graminicola falls into the class of pathogens with a potential for rapid selection of highly QoI-resistant phenotypes. Frequent monitoring of population sensitivities will be required to determine the status of population responses toward practical QoI resistance.

Baseline Sensitivities of Venturia inaequalis Populations to the Strobilurin Fungicide Kresoxim-methyl.

The efficacies of the new strobilurin fungicide kresoxim-methyl for the protection of apple leaves from infection by baseline populations of Venturia inaequalis were uniform across five major apple growing regions in North America. The mean ED50 value determined for 25 populations was 0.35 µg ml-1, with values ranging from 0.11 µg ml-1 to 0.75 µg ml-1. The mean level of scab control achieved at the kresoxim-methyl dose of 4 µg ml-1 was 93%. For one of the five orchards sampled in each region, kresoxim-methyl sensitivities of germinating conidia were determined. Sensitivities of 250 isolates were broadly distributed, with ED50 values ranging from 0.003 µg ml-1 to 0.14 µg ml-1 and a mean of 0.02 µg ml-1. This broad range of in vitro sensitivities was not reflected for the in vivo efficacy of kresoxim-methyl in the protection of apple leaves from scab infections. The discrepancy between in vivo and in vitro sensitivities implies that in vivo tests are more useful for the monitoring of kresoxim-methyl sensitivities of orchard populations. Because it can be expected that only isolates resistant under both test conditions will be prone to future selection, such isolates will contribute to increased frequencies of the least sensitive isolates described in this baseline study. Testing of in vitro isolate sensitivities will, therefore, provide an additional tool in the monitoring of kresoxim-methyl resistance development.