Huanglongbing (HLB), associated with Candidatus Liberibacter asiaticus (Las), was first detected in Florida in late 2005 and is now widely distributed throughout the commercial citrus-growing regions. In recent seasons, concurrent with freeze and drought episodes, symptomatic HLB-infected trees were much more affected by the extremes of temperature and moisture than trees without HLB. Symptoms exhibited by the stressed trees were excessive leaf loss and premature fruit drop even when HLB-infected trees were managed with good nutritional and irrigation practices recommended to support health of HLB-affected trees. This stress intolerance may be due to a loss of fibrous roots. To assess root status of HLB-infected trees on Swingle citrumelo rootstock (Citrus paradisi × Poncirus trifoliata), blocks of 2,307 three-yr-old Hamlin orange trees and 2,693 four-yr-old Valencia orange trees were surveyed visually and with a real time polymerase chain reaction (PCR) assay to determine Las infection status. The incidence of Las-infected trees (pre-symptomatic: Las+, visually negative, and symptomatic: Las+, visually positive) trees was 89% for the Hamlin block and 88% for the Valencia block. Las+ trees had 30 and 37% lower fibrous root mass density for pre-symptomatic and symptomatic trees, respectively, compared to Las- trees. In a second survey, 10- to 25-yr-old Valencia trees on ‘Swingle’ citrumelo or ‘Carrizo’ citrange (C. sinensis (L.) × P. trifoliata) rootstock were sampled within 3-6 months after identification of visual HLB status as symptomatic (Las+, visually positive) or non-symptomatic (Las-, visually negative) in orchards located in the central ridge, south-central and southwest flatwoods. Pairs of HLB symptomatic and non-symptomatic trees were evaluated for PCR status, fibrous root mass density and Phytophthora nicotianae progagules in the rhizosphere soil. Las+ trees had 27-40% lower fibrous root mass density and in one location higher P. nicotianae per root but Phytophthora populations per cm³ soil were high on both Las+ and Las- trees.  Fibrous root loss from HLB damage interacted with P. nicotianae depending on orchard location and time of year.

Florida growers have reported that enhanced nutritional programs (ENPs) maintain productivity of HLB-infected trees.  However, efficacy and sustainability of the nutritional approach for HLB disease management remains uncertain.  Complementary studies of multiple ENPs and their individual components compared to the standard nutritional program (SNP) on nursery and field trees were initiated in 2010.  Two independent nursery trials were initiated with final data collection of the second trial currently underway.  The field site was chosen for its mix of healthy, presymptomatic, and HLB symptomatic trees to determine if observed differences resulted from effects on healthy or infected trees.  We have found no evidence of reduced phloem plugging in ENP treated nursery trees.  Candidatus Liberibacter asiaticus (Las) populations are similar for ENPs and the SNP.  Minor differences in Las movement have been observed.  Las invaded new flush tissue faster in ENP treated trees than SNP trees.  Phosphite treatments have caused Las to favor early invasion of root tissue compared to other treatments.  Preliminary observations of the second nursery trial suggest that foliar symptoms are more apparent on the standard nutrient program compared to ENPs; however, root and canopy decline are unaffected.  Fruit yield and HLB symptoms in field trees treated with ENPs have not differed significantly from the standard nutritional program after two years.  Third year yield data will be presented.

Background

Objective

Design

Measurements

Results

Conclusion

Huanglongbing in grove trees is initially identified by foliar symptoms, most commonly blotchy mottle.  Detection of Candidatus Liberibacter asiaticus (Las) in leaf tissue by qPCR early in disease development is usually limited to symptomatic leaves and proximal young leaves.  Over multiple years, disease symptoms spread to the rest of the canopy.  Although Las has been detected in root tissue, the decline of roots has been assumed to happen later in disease development when photosynthate production and transport have been significantly diminished in the tree canopy.  Observations of initial spread of Las from the bud-inoculation site in the trunk of 1-yr-old potted trees have revealed that Las is frequently detectable in roots months before detection of Las in leaves and foliar symptom development.  Even after symptom development Las is more evenly distributed in root tissue than in the canopy.  Preliminary evidence suggests that Las is also more evenly distributed in roots of grove trees.  Asymptomatic 9 year old grove trees with root Las infection had 26-41% lower root density than asymptomatic trees without detectable root Las.  The loss of root density was independent of Las detection in leaves.  Root loss precedes carbohydrate starvation as evidenced by root starch concentrations, suggesting the bacteria may play a more active role in root loss than phloem plugging.  These results suggest that early invasion of roots by Las leads to root decline before the appearance of foliar symptoms and is likely the cause of larger than expected yield reduction on trees with limited foliar symptoms.