Huanglongbing-related responses of 'Valencia' sweet orange on eight citrus rootstocks during greenhouse trials

Authors: Stover, Eddie; Hall, David; GROSSER, JUDE - University Of Florida; GRUBER, BARRETT - University Of Florida; MOORE, GLORIA - University Of Florida

Submitted to: HortTechnology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/7/2018
Publication Date: 12/2/2018
Citation: Stover, E.W., Hall, D.G., Grosser, J., Gruber, B., Moore, G. 2018. Huanglongbing-related responses of 'Valencia' sweet orange on eight citrus rootstocks during greenhouse trials. HortTechnology. 28(6):776-782. https://doi.org/10.21273/HORTTECH04137-18.
DOI: https://doi.org/10.21273/HORTTECH04137-18

Interpretive Summary: The primary purpose of this experiment was to see if selection of rootstock could enhance development of huanglongbing (HLB)-related decline. If so, then it may permit more rapid identification of huanglongbing -resistant scion types. ‘Valencia’ was budded onto 40+ rootstocks each of Carrizo, Cleopatra, Green-7 from UF, UFR-2, UFR-4, rough lemon, sour orange and US-897. Half of the trees on each rootstock were infected with the huanglongbing pathogen by grafting with infected budwood and half were infected using the insect that transfers the pathogen in the field. After three years, trees on UFR-4 displayed greater overall health than trees on Carrizo (and Green-7, sour orange and US-897). Percentage monthly increase in trunk diameter over the entire experiments was significantly greater in UFR-2 alone in the bud-inoculated trial, and UFR-2 joined only by UFR-4 in the Asian citrus psyllid (ACP) trial. There was no evidence that trees on any other rootstock enhanced pathogen titers compared to Carrizo, which has been our standard rootstock for similar experiments. The industry would greatly benefit from rootstocks that confer HLB-tolerance to susceptible scions, and it is uncertain whether the modest differences in health and growth observed in these greenhouse trials translate into marked economic benefits in the field.

Technical Abstract: The primary purpose of this experiment was to see if selection of rootstock could enhance development of huanglongbing (HLB)-related decline. If so, then it may permit more rapid identification of HLB -resistant scion types. It is well established that trees of some non-grafted/budded rootstock types are HLB-tolerant, but less certain that rootstocks markedly effect susceptible scion tolerance. ‘Valencia’ sweet orange was budded February 2013 onto 40+ rootstocks each of Carrizo, Cleopatra, Green-7 from UF, UFR-2, UFR-4, rough lemon, sour orange and US-897. In Sept. 2013 twenty+ ‘Valencia’ trees on each rootstock were bud inoculated with Candidatus Liberibacter asiaticus (CLas). In a separate experiment similar numbers of trees were inoculated with the CLas vector, the Asian citrus psyllid (ACP, Diaphorina citri) in no-choice inoculations. Trees were maintained in an ACP-free greenhouse using fertilization and irrigation typical for nursery production. Data were collected periodically on trunk diameter and tree height, with tree health assessed on a five point subjective scale. Leaf and root samples were collected annually for quantification of CLas using polymerase chain reaction (PCR) but only samples from a few dates were analyzed. Leaf blotchy mottle symptoms were very slow to develop in both experiments. For the bud-inoculated trees: only rough lemon and sour orange consistently displayed diameters greater than Carrizo (and Green-7); after three years, trees only UFR-4 displayed greater overall health than trees on Carrizo (and Green-7, sour orange and US-897); trees on UFR-4 displayed a higher percentage of plants with leaf Ct for CLas greater than 36 compared to trees on Carrizo (82% and 50% respectively). For the ACP-inoculated trees: only rough lemon, sour orange, and US-897 consistently displayed diameters greater than Carrizo (and Green-7, and UFR-2); after three years, trees on UFR-4 displayed greater overall health than trees on other rootstocks except UFR-2 and sour orange; trees on UFR-2 and Carrizo displayed a higher percentage of plants with leaf Ct values for CLas greater than 36 compared to trees on all other rootstocks except Green-7 and UFR-4. Percentage trunk diameter increase per month, over each entire experiment, was significantly greater in UFR-2 alone in the bud-inoculated trial, and UFR-2 joined only by UFR-4 in the ACP trial. With assessment of CLas titer at only two years (leaves on Dec 2014 and both leaves and roots on Dec 2015), there was no evidence that trees on any other rootstock suppresses titers compared to Carrizo. Less than 9% of plants showed CLas Ct <36, indicating a high percentage of successful infection. The industry would greatly benefit from rootstocks that confer HLB-tolerance to susceptible scions, and it is uncertain whether the modest differences in health and growth observed in these greenhouse trials translate into marked economic benefits in the field.