DOSE-RESPONSE CURVES AND BIOASSAYS USED TO PREDICT THE EFFICACY OF SULFUR SPRAYS FOR POWDERY MILDEW (UNCINULA NECATOR) ON GRAPEVINES
MA WARREN1, KL MURPHY2 and DC GLENN1
1Institute for Horticultural Development, Knoxfield, Private Bag 15, SE Mail Centre, Victoria 3176, Australia; 2Food Science Australia, Private Bag 16, Werribee, Victoria 3030, Australia
Background and objectives
In this study bioassays were developed for powdery mildew (Uncinula necator) to assess the efficacy of pesticide deposits when delivered by commercially available spray equipment. The effect of sulfur on powdery mildew was the first to be investigated as it is one of the most commonly used fungicides.
Materials and methods
The effect of sulfur dose was measured as a reduction in the percentage spore germination relative to rates of germination observed on control leaves. Percentage reduction was plotted against sulfur dose to create a dose-response curve. Probit analysis was used to calculate the effective dose of sulfur per cm2 of leaf area needed to prevent 90% (ED90) of the viable spores from germinating.
The amount of sulfur deposited by typical spray applications in the field was investigated in a series of replicated field trials on northern and southern Victorian vineyards. Vines were sprayed with sulfur (Thiovit) at label rates (2 kg/ha or 200 g/100 l), using two types of spray equipment; an air-shear (low water volume) and an orchard air-blast (high water volume) operated according to manufacturers' recommendations. Applications were made on different canopy types throughout the growing season. The amount of sulfur (per cm2) deposited on inner and outer leaves of the canopy was measured by residue analysis. Field-sprayed leaves were collected and bioassayed in the laboratory.
Results and discussion
There is potential for the use of dose-response curves to determine the bioefficacy of chemical dose deposited in the field for a range of pests and diseases. Bioefficacy data could be used in association with a field-based chemical residue analysis kits to assist growers in determining if they are applying a lethal dose for a given target pest.