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 mu g ml(-1), with values ranging from 0.11 mu g ml(-1) to 0.75 mu g ml(-1). The mean level of scab control achieved at the kresoxim-methyl dose of 4 mu g 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 mu g ml(-1) to 0.14 mu g ml(-1) and a mean of 0.02 mu g 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.