NAA and 3,5,6-TPA Control Mature Fruit Drop in California Citrus

Abstract

The synthetic auxins NAA and 3,5,6-TPA were investigated for reducing abscission of mature citrus fruit in California (CA). NAA was investigated on navel orange trees in San Joaquin Valley and southern CA locations. Of the seven NAA experiments presented, five had substantial fruit drop. In these five experiments, a treatment of NAA reduced drop by 31% to 88% compared to the untreated control. Although NAA treatments as low as 25 mg·L-1 (acid equivalent) reduced drop, the greatest reductions in drop were obtained by spray concentrations in the 100 to 400 mg·L-1 range. 3,5,6-TPA was investigated for fruit drop control properties on navel orange and grapefruit grown in various CA locations. The untreated control in seven of the ten 3,5,6-TPA experiments had substantial fruit drop. In each of these cases, a treatment of 10, 15 or 20 mg·L-1 (acid equivalent) of 3,5,6-TPA reduced drop 69% to 96% compared to the untreated control. A strong linear response from 3,5,6-TPA in these seven experiments indicates maximum fruit drop reduction from the highest rate investigated. On an acid equivalent basis 3,5,6-TPA seems to be comparable to 2,4-D. Both NAA and 3,5,6-TPA were effective in controlling preharvest fruit drop in citrus under CA conditions. Both materials provided fruit holding late into the harvest season. NAA, and in particular 3,5,6-TPA, offer the potential to provide a substitute for 2,4-D which is commonly used for controlling fruit drop in many countries. Chemical names used: naphthaleneacetic acid (NAA); 3,5,6-trichloro-2-pyridinyloxyacetic acid (3,5,6-TPA, triclopyr); 2,4-dichlorophenoxyacetic acid (2,4-D).