Huanglongbing (HLB) or “greening” disease of citrus is caused by phloem-limited, uncultured bacteria in the genus “Candidatus Liberibacter”. HLB is one of the most destructive diseases of citrus worldwide and is considered so dangerous to a U.S. citrus production that the USDA has listed “Ca. Liberibacter species” as a Select Agent. HLB is spread by the Asian citrus psyllid, Diaphorina citri, which was intercepted 40 times by APHIS/PPQ at U.S. ports between 1985 and 1998, became established in Florida by 1998, and more recently in Texas (1). HLB was first detected in the United States near Miami, FL during August 2005, and to date has been confirmed to have spread to 12 Florida counties. In addition to citrus, Murraya paniculata (orange jasmine) is a preferred host of D. citri, and retail trade in this ornamental shrub is strongly implicated in the distribution of D. citri (1). M. paniculata is reported to be a cryptic or largely asymptomatic host of “Ca. Liberibacter” (4), but another report concludes that the bacteria cannot replicate in M. paniculata (2). The epidemiological significance of murraya as a host for the HLB pathogen is therefore unclear. We report here the transmission of “Ca. Liberibacter asiaticus” from M. paniculata to citrus. Two M. paniculata plants, suspected of harboring “Ca. Liberibacter” because of their proximity to HLB-infected citrus and infested with D. citri, were removed from the field, treated with insecticide, and transferred to a quarantine facility. Both plants tested positive for “Ca. Liberibacter” by nested PCR using primers OI1 and OI2 (3) as the first set and primers CGO3F (RGG GAA AGA TTT TAT TGG AG) and CGO5R (GAA AAT AYC ATC TCT GAT ATC GT) as the second set. Two, young, sweet orange plants (Citrus sinensis) grown and maintained in psyllid-free greenhouses in Gainesville, FL were infected by dodder (Cuscuta pentagona) grown from seed. After the dodder had become well established on the orange plants, the orange plants were moved adjacent to the two murraya plants and the dodder from the citrus was draped over the murraya. Coinfection of murraya by dodder occurred within a few days. Sixty days later, both murraya plants, both sweet orange plants, and the connecting dodder all repeatedly tested positive for “Ca. Liberibacter” by nested PCR. Beginning 2 weeks later, the orange plants tested positive by standard PCR using primer set OI1 and OI2 or CGO3F and CGO5R, but remained without typical greening symptoms. Sequencing of the PCR products confirmed amplification of “Ca. L. asiaticus” DNA. We conclude that M. paniculata can serve as an infection source of a Select Agent since it can host the HLB pathogen for at least 2 months and the HLB pathogen can be transmitted to sweet orange during this time.
References: (1) S. E. Halbert and K. L. Manjunath. Florida Entomol. 87:330, 2004. (2) T. H. Hung et al. J. Phytopathol. 148:321, 2000. (3) S. Jagoueix et al. Mol. Cell Probes 10:43, 1996. (4) T. Li and C. Ke. Acta Phytophylacica Sin. 29:31, 2002.