Effects of 2,4-D and related substances on fruit-drop, yield, size, and quality of Valencia oranges

During 1947 and 1948, in various districts throughout southern California, 29 comparisons were made between fruit-drop from non-sprayed Valencia orange trees and that from trees sprayed with 2,4-D at concentrations from 4 to 48 p.p.m. Application of 2,4-D at low volumes was made six months before drop, as well as after drop had begun. While the average reduction in fruit-drop from the sprayed trees was 35.1 per cent, in some cases it was as high as 64, and in others, as low as 5 per cent. Studies were also made of the possibility of increasing fruit size of Valencias by use of 2,4-D. In an experiment begun in 1948, 17 different chemicals were applied to Valencia trees at full bloom. Only 2,4-D and 2,4,5-T induced a significant increase in fruit size. Data were obtained from eight additional experiments using actual field-scale applications. Determinations of the effect of the sprays on size and grade of fruit were based on packing house information. As a result of the spray, there was a gain of 28.0 packed boxes of fruit size 220 and larger, and a decrease of 19.8 boxes of fruit size 252 and smaller, per 100 trees sprayed. Periodic measurements showed that larger fruit sizes resulting from use of 2,4-D were due to an increased growth rate. As fruits grew older, they seemed to become less responsive to 2,4-D. In commercial practice, it is suggested that during the 16-week period after flowering, this diminishing response be compensated for by corresponding increases in the concentration of 2,4-D applied.