Citrus preharvest drop related to HLB disease – Nature and control

Citrus preharvest drop related to HLB disease - Nature and control

Report Date: 02/02/2016
Project: 809   Year: 2015
Category: ACP Vector
Author: L. Gene Albrigo
Sponsor: Citrus Research and Development Foundation

The low concentrations (1/4 rate) of Citrus Fix (2, 4-D) and MaxCel (Cytokinin) with or without ProGibb (GA) every 45 days to Hamlin orange trees in central Florida for the 2015-16 growing season did not reduce fall leaf drop nor preharvest fruit drop of HLB infected trees. Total leaf drop per tree averaged 249 leaves for the Control trees and the treated trees had 3 % more for the MaxCel + 2, 4-D or GA treatments and the combined treatment of all three had 4 % less loss of leaves. Percentage fruit drop was equal to the Control (22.5 %) or higher by one or four %. Two additional tests on Hamlin trees of 2, 4-D + GA compared top Headline, Priaxor or a Control did not affect fruit drop at either site. Average fruit drop for both 1 and 3 (decline) rated trees for all treatments ranged from 27.1 to 29.6 % at one site and 22.5 to 24 % at the other site. Highest rates of fruit loss occurred in October (2 week periods) and decreased to about 1/3rd to 1/4 of these after November 4th at both sites. The test comparing GA rates on root growth of HLB infected Valencia trees was completed and had less effect on root growth than the test on healthy trees. New growth of roots from the bottom third of the pots averaged 25 and 35 cm at 4 and 8 weeks for the highest GA level (.1 gm) while the controls averaged 14 and 20 cm. For the middle of the pot the best growth was for the lower concentration (0.00125) but only at 8 weeks, 18/27 versus 23/23 for the control. For the top third of the pot, there was little differnce between GA concentrations and the control (22/30 versus 20/36). At the end of 8 weeks the healthy trees has 160 gm of roots while the HLB control plants had 100. The best GA treatment (.0025 gm) had 132 gm versus 100 for the control. For dry wt the 0.00125 gm/plant was best and had 22, 18 and 26 % more root, stem and leaf weigth, 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 there was a tendency for GA to improve root growth within the first 4 to 8 weeks of treatment.

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