Evaluation of existing cultivar/rootstock combinations for HLB resistance/tolerance has revealed potentially valuable tolerance but indicates that early HLB symptoms and earlier CLas titer are unrelated to growth and cropping. This suggests that tolerance verification requires 5+ years of field evaluation. Discovery and utilization of molecular markers may help to accelerate the selection of resistance/tolerance materials by screening young plants and avoiding pathogen inoculation. Plant defense elicited by pathogen-associated molecular patterns (PAMPs) is an important component of disease resistance. In previous studies, we have shown that canker resistance in citrus correlates with responsiveness to Xcc-flg22, the 22 amino acid active region from the flagellin of Xanthomonas citri ssp. citri, the causal agent of citrus canker (Shi, Febres et al. 2015). To study the association between HLB resistance/tolerance and citrus response to flg22 of HLB causal bacterium Candidatus Liberibacter asiaticus (CLas), we designed an RNA-seq experiment comparing transcriptome responses in HLB moderately tolerant �Sun Chu Sha� mandarin and susceptible �Duncan� grapefruit, to Xcc-flg22 and CLas-flg22 (project initiated with Gloria Moore at University of Florida). Recently data analysis revealed that a group of 86 genes were differentially regulated by CLas-flg22 in �Sun Chu Sha� mandarin but not by �Duncan� grapefruit and not associated with differential expression from Xcc-flg22, suggesting they may have roles in HLB tolerance. The 16 genes with highest differential expression were selected for RT-qPCR validation, and 10 genes were consistent with the RNA-seq results. To evaluate if these genes were associated with HLB tolerance, �Cleopatra� mandarin (similar to �Sun Chu Sha�) and �Duncan� grapefruit plants were inoculated with CLas using psyllid infestation. CLas titer and gene expression were monitored biweekly for 10 weeks after inoculation. High bacterial titer (Ct<30) was observed at 2 weeks in �Duncan� but not until 6 weeks in �Cleopatra�. RT-qPCR results indicated that 5 of the studied genes were differentially expressed between the �Cleopatra� HLB-infected and the uninfected control plants, but not in �Duncan�. It is worth noting that the induction of these genes was detected before bacterial infection was detected. Although not fully annotated in the citrus genomic databases, the function of some of these genes include a peroxidase, gibberellin 2-beta-dioxygenase, glucan endo-1,3-beta-D-glucosidase and an F-box domain containing protein. We will continue to characterize the expression of these genes and their association to HLB tolerance in other citrus genotypes, and determine if they may serve as marker genes for selection of tolerant citrus material. Trees of seemingly HLB resistant/tolerant sweet orange-like hybrids and mandarin -types were propagated and replicated trials with standards and have been established in growers' fields, in cooperation with G. McCollum. A mapping population of Fortune x Fairchild has been planted (collaborating Roose and Gmitter) along with related material, in an effort to identify genes associated with tolerance in the mandarin phenotypic group. Seedlings with a range of pedigree contributions from Microcitrus have been received in a collaboration with M. Smith, Queensland Aus. citrus breeder, are being grown, and will be planted in the spring for field testing of HLB resistance. In October 2013, 34 unique genotypes (USDA hybrids) some of which appear to have tolerance to HLB, and 16 standard commercial varieties were exposed to an ACP no-choice feeding trial and transferred to the field at Ft. Pierce FL. Standard growth measurements and disease ratings were initiated in July 2014 and will continue on a quarterly basis. HLB is now widespread and trees of more vigorous scion types are generally the healthiest at this point in time. At three years after planting, there continues to be great variation between selections and it may take 2-3 more years to clearly distinguish tolerant material.