The phytotoxicity of narrow distillation range petroleum spraying oils to valencia orange trees in South Australia. Part I: The influence of distillation temperature and spray timing on yield and alternate cropping

Abstract

Abstract Petroleum oil sprays are used 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 various 50%‐distillation temperatures were applied to trees each month from October to May to observe their effects on crop yield. Some treatments were applied once in each of three consecutive seasons while others were applied only once in the first season. The 50%‐distillation temperatures of the three oils were 211, 224 and 240°C at 10 mmHg. The oils caused significant reductions in yield, and caused or accentuated alternate cropping cycles. The higher the 50%‐distillation temperature of the oil, the greater the effect. The time of application was also important. The greatest yield reductions (expressed as two averages) occurred when the oils were sprayed in November; the tendency to alternate cropping increased with the month of application from December to May. For the oil with the highest 50%‐distillation temperature, the yield reductions were about 30%, and the yields from trees in a severe alternate cropping cycle, in the ‘light crop’ year, were as low as 25% of those on unsprayed trees.