Understanding and Managing Abscission In Subtropical Fruit Crops

For subtropical tree fruit crops, flowering (including pollination and fertilization), fruit set (and its associated early drop period), and June drop [the period June through July when fruit development occurs simultaneously with flushes of vegetative shoot and root growth, creating competition and fruit drop] are phenological stages during which the greatest gains in fruit retention and, hence, final yield can be made. Moreover, events or treatments during these stages of phenology also impact fruit size and quality. Abscission data collected for alternate bearing crops, e.g., 'Hass' avocado (Persea americana Mill.) and seedless Clementine mandarin (Citrus reticulata Blanco) during successive years, provided evidence that periods of flower and fruit drop are caused by endogenous physiological factors. Temperature extremes exacerbate these normal abscission processes. Plant growth regulators (PGRs) are possibly the most powerful tools available for solving production problems in the field. Two factors critical to achieving a desired outcome are properly timing the PGR to tree phenology and understanding the influence of crop load on the tree's response to the PGR applied. In general due to natural fruit thinning, successively later treatments are less effective in increasing total fruit number but of greater benefit in increasing the yield of commercially valuable large-size fruit. Depending on application time, auxins, gibberellic acid, cytokinins and the ethylene biosynthesis inhibitor aminoethoxyvinylglycine have proven effective in increasing total fruit number or yield of large-size fruit. Due to the fact that flowering, fruit set and fruit development are phenological stages of high nutrient demand, a plant growth regulator effect resulting in increased total yield or yield of large-size fruit is obtained from foliar-applied fertilizers, even when the tree is not deficient in the nutrient by standard leaf (or petiole) analyses, by properly timing the fertilizer application to tree phenology. Boron or low-biuret urea applied directly to avocado inflorescences at the cauliflower stage of development increased the number of pollen tubes that penetrated the ovule and increased ovule viability, respectively, resulting in a significant increase in avocado yield. Low-biuret urea applied to the foliage of citrus trees during early drop or at maximum peel thickness, which marks the end of the cell division stage of fruit development, increased yield and yield of commercially valuable large-size fruit, respectively.