Huanglongbing-related Responses of 'Valencia' Sweet Orange on Eight Citrus Rootstocks during Greenhouse Trials

The primary objective of this experiment was to determine if the selection of rootstock (Citrus and hybrids) could enhance the development of huanglongbing (HLB)-related symptoms associated with the pathogen Candidatus Liberibacter asiaticus (CLas) in sweet orange (Citrus sinensis). If so, then it may permit more rapid identification of HLB-susceptible compared to HLB-resistant scion types. The secondary objective was to assess the impact of different rootstocks on plant growth parameters and health to determine if trees on any rootstocks displayed reduced sensitivity to HLB-influenced growth restriction. 'Valencia' sweet orange was budded on each of the following eight genotypes: Carrizo (C. sinensis x Poncirus trifoliata); Cleopatra (C. reshni); Green-7 {a complex allotetraploid from somatic hybrids [C. clementina x (C. paradisi x C. reticulata) D C. grandis] x [(C. aurantium D (C. sinensis x P. trifoliata)]}; UFR-2 (a complex allotetraploid from somatic hybrids {[C. clementina x (C. paradisi x C. reticulata)] DC. grandis} x (C. reticulata + P. trifoliata)); UFR-4 (same pedigree as UFR-2); rough lemon (C. jambhiri); sour orange (C. aurantium); and US-897 (C. reticulata x P. trifoliata). Half of the trees on each rootstock were bud-inoculated with CLas and half were inoculated with the asian citrus psyllid [ACP (Diaphorina citri)], which is the CLas vector. During both experiments, no rootstock conferred significantly greater HLB symptom severity compared to trees on Carrizo; however, trees on several rootstocks had reduced HLB severity compared to those on Carrizo. Regarding the bud-inoculated trees after 3 years, trees on UFR-4 displayed greater overall health than trees on Carrizo, Green-7, sour orange, and US897, and trees on UFR-4 had a higher percentage of plants with leaf cycle threshold (Ct) values >36 compared with trees on Cleopatra and rough lemon (62 vs. 26-29 respectively). Regarding the ACP-inoculated trees after 3 years, trees on UFR-4 had better overall health than trees on Carrizo, rough lemon, and US-897, and trees on sour orange had a higher percentage of plants with leaf Ct values greater than 36 only compared to Cleopatra and US-897. The percentage increase in the trunk diameter per month over the course of each entire experiment was significantly greater for UFR-2 in both trials than all rootstocks except UFR-4. Only root CLas titers were sometimes significantly higher for trees on other rootstocks compared to those on Carrizo. Although no rootstock provided acceleration of HLB symptom development compared with Carrizo, some rootstocks conferred significantly greater health compared to Carrizo. However, it is uncertain whether the modest differences in health and growth observed in these greenhouse trials would translate to economic benefits in the field.