The determination of baseline sensitivities of Uncinula necator (Schw.) Burr. to sterol demethylation inhibitors

Abstract

Sterol demethylation inhibitors (DMI's) are important fungicides for the control of grape powdery mildew, Uncinula necator (Schw.) Burr. in all grape growing areas of the world. Reduced efficacy of these fungicides in the control of U. necator has been observed in different countries. The purpose of this study was to determine the baseline sensitivity of three strains of U. necator to four DMI fungicides by different methods.

The first method used was a leaf disc assay. 18mm leaf discs were cut from whole detached grape leaves inoculated with U. necator. After 12 days the level of infection was scored (scores from 0-10, indicating 0-100% infection). Additionally the leaf discs were washed in 1.5% sodium chloride containing 0.15% sodium dodecyl sulphate and the number of spores/disc were determined. EC-50 values based on the score values and the number of spores/ disc varied between 0.029 - 0.128mg/1. Significant differences (p<0.05) between strains in their sensitivity to the different fungicides and between fungicides in their efficacy could be detected. New Zealand strains were found to be more sensitive than strains in France.

The second method used was a germination test of conidia of U. necator on agar amended with different concentrations of the fungicides. The frequency of germination and the elongation of the germ tube were measured. EC-50 values based on these measurements varied between 11.11-87.50mg/1.

The germination test on agar was found to be unsuitable for the determination of baseline sensitivity of U. necator to DMI fungicides.