Improved use of foliar urea on clementine mandarin to manipulate cropping in a sustainable production system

'Cadoux', Clementine mandarin (Citrus reticulata Blanco) is an alternate-bearing cultivar with problems of low fruit set and small fruit size. Properly timed foliar-applied urea has been shown to increase flower intensity, fruit set, fruit size and yield of citrus. In this research, we investigated effect of some spray factors on efficacy of foliar urea on fruit set, fruit size, and yield of 'Cadoux' clementine mandarin. In one trial, urea (containing 46%N) was applied as a foliar spray to fully mature trees during the periods of flower initiation-differentiation, fruit set and 'June drop'. Urea was used at the rate of 1 kg/100 1 and trees were sprayed to the point of run-off. The experiments were run for two consecutive years. Urea increased leaf area, leaf specific dry weight (Mg/cm(2)), leaf N levels and total yield. The yield increase was due to an increase in both fruit number per tree and fruit size. As a consequence, the total number of export grade fruit (diameter > 51 mm) was increased by approximate to 50% regardless of timing of urea application. More fruit were of export grade by the first harvest date indicating an indirect effect of urea on earliness. Incorporation of Mn and Zn in the spray mix with an application at flower initiation-differentiation increased yield to a level similar to that obtained with acidification to pH 6.0. Multiple urea applications during flower initiation-to-bloom did not lead any additional benefit when compared to a single application. In another 2-year trial we investigated the effect of the pH of the spray solution and found that lowering the pH from 7.6 to 6.0 increased leaf N and cumulative yield.