Effect of timing, concentration and stock/scion combination on chemical control of pre-harvest drop of Navel oranges

Abstract

The effects of 4-chloro-2-methyl phenoxyacetic acid (MCPA) and some derivatives of 2,4-dichlorophenoxyacetic acid (2,4-D) applied at various concentrations and application times on preharvest drop from several navel orange clones and rootstocks were investigated. The trees were spray irrigated, on light soils and growing in a semi-arid region. Herps Washington navel strain on Poncirus trifoliata rootstock had a small natural drop and fruit drop from this combination was unaffected by spray treatments. Control trees of Leng navels dropped more fruit than those of GVN or Herps Washington navel strains. There were no differences in fruit drop between MCPA and 2,4-D both applied at 20 p.p.m. in mid May. In one year however, the higher concentration of 2,4-D was superior to MCPA applied at 10 p.p.m. in mid May but was not better than 2,4-D at 10 p.p.m. Two sprays of 2,4-D 10 p.p.m. applied one each in mid May and mid June had no advantage over a single spray of 2,4-D 20 p.p.m. mid May. The four forms of 2,4-D used were the sodium and amine salts (low volatile) and the isopropyl and ethyl esters (high volatile). All the forms sprayed in mid May at 20 p.p.m. proved equally effective. In all cases, fruit quality and yield were unaffected by spray treatments over a number of years.