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
Optimisation of agrochemical application is an essential part of improved management of pests and diseases in viticulture. Effective disease control needs to be maintained while minimising chemical inputs. This relies on getting the appropriate chemical at the right dose onto the target. Some growers have reported difficulties controlling powdery mildew in their vineyards despite following recommended spray regimes.

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
Dose-response curves were generated to determine how much sulfur needs to be deposited onto a leaf to achieve powdery mildew control. Grapevines (cv. Chardonnay) and powdery mildew inoculum were maintained in heated glasshouses. Leaves free of powdery mildew were detached, placed in vials with water and sprayed using a hand-held atomiser to run-off with various concentrations of sulfur (Thiovit) including the label rate of 200 g per 100 litres. Control leaves were sprayed with water. Leaves were inoculated with powdery mildew spores and placed in a cabinet under conditions favourable for powdery mildew development (24°C, 80-90% RH and 12L:12D). Percentage spore germination was measured 2 days after inoculation.

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
The amount of sulfur deposited per cm² of leaf tissue during the field applications was greatest on outer leaves, with significantly less reaching the innermost leaves of the canopy. The dose-response curve generated for sulfur against powdery mildew predicts that a dose less than the ED₉₀ value of 1.8 pg/cm² would not give adequate control. Many spray applications made during the trials, particularly those on dense canopies later in the season, would not provide adequate control of powdery mildew on the innermost leaves as the sulfur residues on these leaves were below the ED₉₀. Bioassays of field-sprayed leaves confirmed in most cases that the dose delivered to inner leaves by both machines did not give good powdery mildew control. Results from this work show that the problem many growers are having with poor efficacy of sulfur in the control of powdery mildew is due to application of inadequate dose, especially on inner leaves of dense canopies.

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.