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

From 1947 to 1950, 34 field experiments were conducted to study the effects of applications of 2,4-D and related substances on fruit-drop, yield, size, and quality of grapefruit in California. It was found that water sprays of 8 ppm 2,4-D reduced mature fruit-drop an average of 44 per cent, as compared with nonsprayed trees, and that this response to 2,4-D occurred under a wide range of climatic and environmental conditions. This spray was more effective in reducing fruit-drop when applied in May than when applied in July, and annual applications of 2,4-D for three years have shown no accumulative detrimental effects. The differences in fruit-drop resulting from the use of various forms of 2,4-D were not great.

Thus far, 2,4-D has not proved incompatible with any agricultural spray chemicals, and has been effective in reducing mature fruit-drop when applied in conjunction with other spray treatments. The 2,4-D sprays had no undesirable effects on the quality of fruit which was mature, or nearly mature, at the time of spraying.

Trees sprayed with 2,400 ppm 2,4-D as a kerosene “aeromist” at less than 10 gallons per acre showed a reduction in mature fruit-drop, as compared with nonsprayed trees, but this method cannot yet be recommended for commercial use. A water spray of 8 ppm 2,4,5-T was as effective as an 8 ppm 2,4-D spray in reducing mature fruit-drop. A spray of 25 ppm naphthalene-acetic acid was not effective, and a spray of 25 ppm naphthoxyacetic acid was only slightly effective.

Application of 8 ppm 2,4-D to trees with young, expanding leaves induced a curling and deformation of the leaves. Subsequent leaf growth was normal. Application of the spray between leaf growth flushes minimized the distortion. No influence on yield or fruit quality was noted as a result of the leaf curl.

There was an increase in fruit size of commercial value if the correct concentration of 2,4-D was sprayed on trees bearing fruit less than 16 weeks old. However, the fruit quality was lowered when trees bearing young fruit (less than 16 weeks) were sprayed with 2,4-D at too high a concentration in relation to the stage of fruit growth. A lowering in quality as a result of an 8 ppm 2,4-D spray was observed in only one experiment, where the application was made on trees in full bloom.