| Abstract: | Petroleum oil sprays are used as pesticides on citrus to control California red scale (Aonidiella aurantii Maskell), but may have phytotoxic effects on trees and fruit. As part of a programme to establish improved specifications for spray oils for South Australia, three oils with different 50 %-distillation temperatures were applied to trees each month from October to May to observe their effects on leaf and fruit drop. The 50%-distillation temperatures of the three oils were 211, 224 and 240°C at 10 mmHg (in order of increasing molecular weight). The oils caused significant leaf drop. The higher the 50%-distillation temperature of the oil, the greater the leaf drop. Most leaf drop was caused when natural leaf drop was greatest. Oils applied from October to February caused severe leaf drop for periods of about 1–2 months, while oils applied from March to May caused severe leaf drop for up to 6 months or more. There was also a tendency for oils with a lower 50%-distillation temperature to cause leaf drop sooner after spraying than oils with a higher 50%-distillation temperature. Significant fruit drop was caused in only a few treatments, but the maximum fruit drop was less than 5% of the total fruit number and of little consequence when compared with the effects of oils on yield through reduced fruit set. Visually, a significant proportion of the total number of leaves present fell in many of the oil spray treatments, and the rate of disappearance of the oil film was directly related to the 50%-distillation temperature of the oil, and to the spraying date. |
