First occurrence of Diaphorina citri in East Africa, characterization of the Ca. Liberibacter species causing huanglongbing (HLB) in Tanzania, and potential further spread of D. citri and HLB in Africa and Europe

Greening is a severe and widespread disease of citrus in two main citrus growing areas of the world, Asia and Africa. It is caused by an uncultured phloem restricted bacterium that we have recently characterized from the sequence of its 16S ribosomal DNA. The bacterium is a new "Candidatus' genus, Liberobacter, in the alpha subdivision of the proteobacteria, and two 'Candidatus species' have been recognized: Liberobacter asiaticum and Liberobacter africanum. In this paper we describe a PCR method to detect the two liberobacter species in citrus trees by amplification of a 1160 bp fragment of their 16S rDNA. Distinction between the two species has also been achieved by Xbal digestion of the amplicons.

Diaphorina citri Kuwayama (Hemiptera: Psyllidae) carries Candidatus liberibacter spp., the putative causal agents of Huanglongbing. D. citri reproduces and develops only on the flushing shoots of its rutaceous host plants. Here we examined whether D. citri is attracted to host plant odors and a mixture of synthetic terpenes. Tests conducted in a vertically oriented Y-tube olfactometer showed that both males and females preferentially entered the Y-tube arm containing the odor from the young shoots of Murraya paniculata (L.) Jack and Citrus limon L. Burm. f. cultivar Eureka. Only males exhibited a preference for the odor of C. sinensis L., whereas the odor of C. x paradisi MacFadyen cultivar Rio Red was not attractive to both sexes. The volatiles emitted by young shoots of grapefruit cultivar Rio Red, Meyer lemon (Citrus x limon L. Burm.f.), and M. paniculata were analyzed by gas chromatograph-mass spectrometry. The samples were comprised of monoterpenes, monoterpene esters, and sesquiterpenes. The number of compounds present varied from 2 to 17, whereas the total amount of sample collected over 6 h ranged from 5.6 to 119.8 ng. The quantitatively dominant constituents were (E)-beta-ocimene, linalool, linalyl acetate, and beta-caryophyllene. The attractiveness of a mixture of synthetic terpenes, modeled on the volatiles collected from M. paniculata, was evaluated in screened cages in a no-choice test. At three observation intervals, significantly more individuals were trapped on white targets scented with the mixture than on unscented targets. These results indicate the feasibility of developing D. citri attractants patterned on actual host plant volatiles.