Evaluation of spray quality by oil- and water-sensitive papers

Nine spray treatments were applied to plots of orange trees, in a randomize d complete block design and four replications. The treatments involved thre e air-carrier sprayers, operating at different volume rates and ground spee ds. Spray mixtures contained a petroleum oil (Sunspray 7E) at a constant ra te of 37.5 L/ha and water rates of 220 to 3750 L/ha. Oil- and water-sensiti ve papers (OSP and WSP) were used as spray targets. The targets were placed at two heights, outside and 0.3-0.4 m inside the canopy. At each location, both OSP and WSP were stapled to the upper and lower leaf surfaces of two neighboring leaves. Additional target pairs were also positioned rigidly on target holders, in the gap between two adjacent trees, at two heights and at horizontal top, horizontal bottom, and vertical positions. Sprayed targe ts were analyzed by an imaging system. The percent area coverage, mean spot diameter; volume mean diameter and spot density were used to evaluate dist ribution patterns. There was a substantial variation in distribution patter ns of different replicates. Both WSP and OSP patterns were significantly af fected by the sprayer treatment and target location. The results were quite variable and the papers may have a limited value for quantitative assessme nt of spray coverage or droplet size measurement of high spray application rates. Among all parameters, the percent area coverage appeared to be the m ost reliable.