Spray deposit cover and fungicide distribution obtained on intensive apple trees by overhead mobile spraying methods

Abstract

AbstractSpray deposit assessments were made on the leaf surfaces in the top, middle and base zones of intensively grown apple trees sprayed by the conventional largevolume automatic method applying 2250 litres/ha and by fast overhead methods with nozzle booms applying 1125 litres/ha and with mist‐blowing equipment applying 562 litres/ha. Qualitative assessment of spray cover, using a 0.05% suspension of Saturn Yellow fluorescent tracer showed that whereas the conventional method gave heavy cover on upper and lower surfaces in all zones, the two overhead methods gave heavier cover on the upper surfaces than on the lower and that an appreciable proportion of the lower surfaces especially in the base zone on trees sprayed by the overhead mist‐blower had no fluorescent deposit. Quantitative assessments of copper spray deposits by colorimetric estimations separately on upper and lower surfaces agreed in general with the fluorescent tracer indications, especially the much greater deposits on the upper surfaces on the trees sprayed from overhead. Monitoring of captan spray deposits before and after each successive fungicide application through the season showed that in the three zones the residual and cumulative deposits from the conventional automatic applications were heavier and more uniform than those from the overhead methods which gave lighter deposits on the middle and base zones. In the intervals during which rain fell, most of the captan deposit levels from all methods of spraying were reduced by >70%, whereas during a rainless interval the reduction was 0–45%. The reduction of captan deposit levels during the rainy intervals was significantly greater on the trees sprayed from overhead than on the conventionally sprayed ones. There was a correlation between the percentage loss of captan and the rainfall in the intervals.