Canopy Density, but Not Bacterial Titers, Predicts Fruit Yield in Huanglongbing-Affected Sweet Orange Trees

Abstract

In Florida, almost all citrus trees are affected with Huanglongbing (HLB), caused by Candidatus Liberibacter asiaticus (CLas). We characterized various parameters of HLB-affected sweet orange trees in response to yield-improving nutritional treatment, including canopy volume, canopy density and CLas Ct values, and found that the treatment improved yield and maintained canopy density for over three years, whereas untreated HLB-affected trees declined in canopy density. The nutritional treatment did not affect CLas titer or the tree canopy volume suggesting that canopy density is a better indicator of fruit yield. To further validate the importance of canopy density, we evaluated three independent orchards (different in tree age or variety) to identify the specific traits that are correlated with fruit yields. We found that canopy density and fruit detachment force (FDF), were positively correlated with fruit yields in independent trials. Canopy density accurately distinguished between mild and severe trees in three field trials. High and low producing HLB trees had the same Ct values. Ct values did not always agree with CLas number in the phloem, as visualized by transmission electron microscopy. Our work identifies canopy density as an efficient trait to predict yields of HLB-affected trees and suggests canopy health is more relevant for yields than the CLas population.

Fruit Size as an Indicator of Fruit Drop in Huanglongbing-affected ‘Valencia’

“…Although multiple HLB-related symptoms (e.g., canopy dieback, increased rates of preharvest fruit drop, poor fruit quality, etc.) contribute to a reduction in marketable yield (Albrigo and Stover 2015;Bassanezi et al 2009;Levy et al 2023;Tang et al 2019, growers are concerned about the increased rates of preharvest fruit rates. The loss of mature fruit in the months leading up to commercial harvest represents a significant visual loss of their revenue.…”

“…The loss of mature fruit in the months leading up to commercial harvest represents a significant visual loss of their revenue. While altered cultural practices, such as altered irrigation schedules (Kadyampakeni and Morgan 2017), nutrient applications (Atta et al 2023), and PGR applications (Albrigo and Stover 2015;Singh et al 2022), can cause a general increase in the health of HLB-affected trees (Levy et al 2023), the effects on preharvest fruit drop are variable.…”