Using Point of First Run-off and Spray Volume in Litres per 100 Metres per Metre of Canopy Height for Setting Pesticide Dose

G. O. Furness, A. J. Thompson


In Australia, pesticide labels for fruit trees and grapevines are based on the concentration of chemical (amount per 100 litres) for dilute spraying to the point of first run-off. Chemical rate per hectare is regarded as technically flawed and is therefore no longer provided. A simple and practical way to specify spray volume with this new pesticide label format is litres per 100 metres per metre of canopy height (X L 100 m-1 m-1). This is also a simple and practical parameter to express dose efficiency, with the advantage that it can be directly related to the amount of pesticide deposited per cm2 of foliage. If X L 100 m-1 m-1 is multiplied by 100 the figure obtained is the spray volume applied to one hectare of vertical canopy wall.

In citrus, using a multi-fan sprayer, at a spray volume rate of 25 L 100 m-1 m-1 (1 500 L ha-1), the spray volume deposited per square cm of leaf surface was about 0.7μl cm- 2 , rising to about 0.9 μl cm-2 at 60 L 100 m-1 m-1. Above this spray volume, increases in the spray volume deposited were small. This indicates a point of first run-off in the range 50 to 60 L 100 m-1 m-1 for this sprayer in this orchard.

For concentrate spraying, when a spray volume of 50 L 100 m-1 m-1 was selected as the point of first run-off, the increase in the predicted mean amount of chemical deposited at 25 L 100 m-1 m-1 was small. However, when 100 L 100 m-1 m-1 was selected, the increase was about three times that with dilute spraying at run-off and above. This shows the importance of not using concentrate spraying based on dilute spray volumes that cause excessive run-off, and accurately determining the spray volume at the point of first run-off when attempting concentrate spraying.

There was a small decrease in mean deposition as spraying speed increased from 3 to 6 km h-1, especially from 4 to 6 km h-1.

A preliminary spray coverage trial using a direct blast multi-fan sprayer was also carried out to evaluate the effect of horizontal airstream convergence on spray deposition. Results suggested a slight improvement in spray deposition efficiency and uniformity on citrus trees when compared to that obtained when the same six spray heads were placed in a simple vertical array.

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