Response of pre-harvest drop of grapefruit on two rootstocks to 2,4-D, MCPA and gibberellic acid

Abstract

Several formulations, concentrations and timings of sprays of 2,4-D (2,4-dichlorophenoxyacetic acid) and MCPA (4-chloro-2-methyl phenoxyacetic acid) were tested for their ability to reduce pre-harvest drop of late hanging Marsh grapefruit growing on rough lemon (Citrus jambheri) and trifoliata (Poncirus trifoliata) rootstocks at Dareton in south-western New South Wales. Gibberellic acid was also tested in several timing combinations with 2,4-D for its ability to improve the quality of late hanging fruit. On rough lemon the established standard treatment of 2,4-D at 20 ppm applied in mid-May was as good or better than all other treatments. On trifoliata this standard spray was again as effective as all others in preventing drop up to mid-November, but a similar spray applied in midJune was more effective from mid-November to mid-December. MCPA applied at 10 or 20 ppm in mid-May was not as effective as the standard spray. The best treatments reduced the average cumulative fruit drop, of 39% on trifoliata and 70% on rough lemon, by at least a half. When gibberellic acid at 10 ppm was added to the standard mid-May spray of 2,4-D, or when it followed it in mid June, it improved the colour and appearance and reduced the rind thickness and puffiness of late hanging fruit.