2017 ASHS Annual Conference

Citrus root systems are very vulnerable to losses when the tree is infected by the bacterial pathogen that causes Huanglongbing (HLB) disease. Gibberellic acid reportedly enhances root growth in tomato. Two tests were conducted with low rates of GA₃ (ProGibb) to determine if it stimulated root growth of field ready citrus nursery trees. The first root development test, conducted in a greenhouse, used disease free trees and GA₃ rates of 0, .00125, .0025, .005, and .0063 g ai/plant as soil drenches. The number of new roots was increased 38 and 31% by the two higher concentrations after 3 weeks. Total and average root length increased by 27 and 28%, respectively, 6 weeks after application and by 21 and 23 % for numbers at the two higher rates after 12 weeks with a second GA₃ application at 6 weeks. A second test of GA₃ applications in two 4-week cycles to roots of nursery trees was done after the trees were infected with HLB. Trees were infected with the HLB disease bacteria by psyllid vectors in a ‘hot psyllid greenhouse’. The GA₃ had less effect on root growth than in the first test on healthy trees. At the end of 8 weeks the healthy control trees has 160 gm of roots while the HLB control plants had 100 demonstrating the reduced root growth of HLB infected plants. The best GA₃ treatment (.0025 gm) had 132 gm versus 100 for the HLB disease control. For dry wt the 0.00125 gm/plants were best and had 22, 18 and 26 % more root, stem and leaf weight, respectively, than the control. Results were not as consistent as the previous test with healthy plants, but variability due to level of HLB infection probably is responsible. It does appear that the GA₃ improved root growth, at least within the first 4 to 8 weeks of treatment for both healthy and HLB infected plants.