December 1, 1996
Citrus fruit drop resulting from glyphosate applications has become more of a concern over the last several years. The objective of this research was to understand the mechanisms behind glyphosate related fruit drop and determine how to reduce these effects in standard grove practices. Specific amounts of glyphosate were applied to individual 'Hamlin' oranges at various stages of maturity and percentage of drop determined. The data indicate 'Hamlin' orange sensitivity in the fall at 0.1 µl per fruit. This rate roughly corresponds to 4 spray droplets of a 2 quart/acre glyphosate application at 20 gallons/acre. Field experiments were conducted comparing herbicide effects from various boom designs and nozzle configurations. The experiments were designed to determine levels of naturally occurring fruit drop, drop occurring from shielded booms without herbicide, as well as herbicide effects. Natural fruit drop ranged from 15 to 33 fruit per tree, mechanically induced fruit drop ranged from 1 to 6 fruit per tree, glyphosate induced fruit drop ranged from 0 to 26 fruit per tree in these experiments. Fully enclosed booms with heavy back covers and plugged off center nozzles had significantly less fruit drop than open booms with no back cover and open off center nozzles.