Citrus Huanglongbing (HLB) poses the greatest threat to the survival of the Florida and US citrus industry. Research to incorporate HLB resistance/tolerance into citrus has been recommended by the National Research Council as one of the top priority topics for addressing the HLB threat. A number of Poncirus and Citrus cultivars have been recently found to be tolerant to HLB. Identification, characterization and validation of candidate genes responsible for HLB-resistance are of the remarkable value to develop new HLR-resistant/tolerant citrus varieties. More than 70 different accessions of Citrus species and relatives have been observed in a field planting in Florida (with cooperation of Dr. E. Stover, USDA). Leaf samples were collected from about 450 trees. The titer of CLas in the HLB-infected plants was determined by TaqMan probe-based qPCR, and the HLB symptom severity on the plants was evaluated by experienced researchers. These accessions showed diverse responses (susceptible, tolerant and resistant) against HLB. Many HLB-tolerant citrus relatives were revealed, including Citrus latipes, Poncirus trifoliata, Severinia buxifolia and Microcitrus australis. The genomes of 15 citrus accessions have been sequenced to about 25x coverage (9 to 11 Gb/genome) and the genome of one Poncirus trifoliata accession has been sequenced to about 50x coverage (~21 Gb). Initial analysis indicated good quality of these sequence reads. We have begun a more detailed analysis of sequence polymorphisms and structural variations within candidate genes. We are also validating the expression of our shortlisted candidate genes by real�time PCR among susceptible and resistant citrus accessions.