The phytotoxicity of petroleum oil sprays to Vakncia orange (Citrus sinensis (L.) Osbeck) on Poncirus trifoliata (L.) Raf. rootstocks was investigated at Kulnura and at Griffith, New South Wales. Four narrow-range petroleum oils, designated as 206, 212, 214 and 238 oils, were applied as high volume sprays in 3 consecutive years. Annual sprays were applied during the spring, summer and autumn months of November, February and April. Some treatments were sprayed once each year, others twice. The single spray per year treatments involved concentrations of 1.2 or 2% of each oil in February or 2% of the 212, 214 and 238 oils in November or in April. A total concentration of 3.2% oil per year of the 2 12,2 14 and 238 oils was applied in February (2%) and in November of the previous year (1.2%) or in April (1.2%) to the double spray treatments. The effect of the sprays on yield variables, percentage juice, �Brix (w/w sucrose), anhydrous citric acid, rind colour and oil content was studied. Rind colour was not affected by any treatment at either location. Significant (P< 0.05), though inconsistent, effects on other characteristics of fruit quality were found. Increase in 50% distillation temperature and rate of application of oils tended to decrease �Brix. The oils did not significantly affect yield at Kulnura. At Griffith, 2% sprays of 238 oil significantly reduced yield by reducing the number of fruit. At both locations, trees sprayed with 2% oil in April tended to have lower yields than those sprayed in November or February at the same rate. The effect of sprays with total concentrations of 3.2% was variable. The 238 oil was the only oil considered to be potentially phytotoxic. This type of oil should not be used at concentrations > 2% from March to November or in consecutive years at any time. They can be used from late spring to early autumn in consecutive years at 1.2%. Variation in the phytotoxicity of oils between locations in south-eastern Australia appears to be related to variation in night temperatures in summer and early autumn which affect flower initiation.
Variation in Symptom Expression of Exocortis and Gummy Pitting in Citrus Trees on Poncirus trifoliata Rootstock in New South Wales