A Single Application of 2,4-D Can Decrease Preharvest Fruit Drop in HLB-affected ‘Valencia’ Orange Trees

Abstract

The goal of these field trials was to determine if a single spray of 2,4-D in late December could mitigate Citrus sinensis ‘Valencia’ preharvest fruit drop from HLB affected trees. An on-label application of 2,4-D (Citrus Fix) was applied to mature ‘Valencia’ trees at four different sites in the central Florida ridge area in December 2013, and repeated using different trees at the same sites in December 2014. All trees had visible HLB symptoms. At each site, there were four blocks of six trees treated along with four blocks of six untreated control (UTC) trees. Fruit drop counts in 2014 and 2015 came from the four middle trees in each block so there were 16 trees in the treated and UTC at each site. The grove floor under each tree was raked prior to treatment and approximately every two weeks thereafter for fruit drop counts until harvest in March or April. The percentage of fruit dropped was calculated after counting the remaining on-tree fruit at harvest and dividing the number of fruit dropped on each sampling day by the total number of fruit dropped + fruit harvested. At three sites in 2014, average total fruit drop ranged from 24% to 76% and there was no significant effect of the 2,4-D treatment on fruit drop. At a fourth site, fruit drop was significantly decreased by 28% from 47% drop in the UTC to 19% drop in the 2,4-D treated trees. In 2015, at two sites where disease symptoms were visible in 37% to 38% of each tree canopy, the fruit drop was significantly decreased by 5% and 18%, respectively, in the 2,4-D treated trees compared to the UTC. There was no significant effect of the 2,4-D treatment on fruit drop at a third site where symptoms were visible in 55% of the canopy. Combining the percentage of fruit drop results from seven trials at four sites over the two years, the percentage of fruit drop was significantly reduced from 5% to 28% by the single 2, 4-D treatment in three of the seven trials. The site with the highest visible disease index, however, did not have the highest total percentage of fruit dropped, so disease index was not a good predictor of the percentage of fruit drop.