AN EVALUATION OF THE GENETIC DIVERSITY OF XYLELLA FASTIDIOSA ISOLATED FROM DISEASED CITRUS AND COFFEE

Authors: QIN, XIAOTING - VISITING SCIENTIST; MIRANDA, VICENTE - FUNDE CITRUS - BRAZIL; MACHADO, MARCOS - CENTRO DE CITRICULTURA; LEMOS, ELIANA - SAO PAULO STATE UNIV.; Hartung, John

Submitted to: Phytopathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/12/2001
Publication Date: 6/1/2001
Citation: Qin, X., Miranda, V., Machado, M., Lemos, E., Hartung, J.S. 2001. An evaluation of the genetic diversity of xylella fastidiosa isolated from diseased citrus and coffee. Phytopathology.

Interpretive Summary: Citrus variegated chlorosis (CVC) disease currently affects 38% of the orange trees in Brazil, which has the largest citrus industry in the world. CVC is caused by the bacterium Xylella fastidiosa, and is spread by a large number of insects that feed on the sap from infected plants. Although CVC does not occur in the United States, other strains of the same bacteria cause serious diseases of stone fruits and grapevines in the U.S., and the introduction of CVC into the U.S. would be devastating. The grapevine disease, called Pierce's Disease, prevents the widespread cultivation of wine grapes in the Southeastern U.S. and is currently an increasing concern in California. We studied strains of the bacteria from citrus and coffee in Brazil, and from grapevine and other hosts in the U.S., in order to determine how closely related the strains from different hosts are, and whether or not there are different strains in Brazil infecting citrus and coffee. We found that there are four groups of X. fastidiosa strains: citrus/coffee; elm/oak/plum; mulberry; grapevine/almond. We also determined that the bacterial strains infecting coffee and citrus in Brazil, although closely related, are distinct. This suggests that individual strains are adapted to different hosts, rather than a single strain infecting multiple hosts, as is known to occur with this pathogen. In addition to improved diagnostic methods for the citrus and coffee strains of the pathogen, we now have data that suggests that there may in fact be multiple species or pathovars within X. fastidiosa. We may now study how the bacterium spreads in nature, and whether certain species of insect are better able to transmit specific strains of the pathogen.

Technical Abstract: Strains of Xylella fastidiosa Wells et al., isolated from sweet orange trees (C. sinensis Osb.) and coffee trees (Coffea arabica) with symptoms of citrus variegated chlorosis and Requeima do Caf¿, respectively, were indistinguishable based on REP-PCR and ERIC-PCR assays. These strains were also indistinguishable using a previously described PCR assay that distinguished the citrus strains from all other strains of X. fastidiosa. When comparisons were made to reference strains of X. fastidiosa isolated from other hosts using these methods, four groups were consistently identified consistent with the hosts from which the strains originated: citrus/coffee, grapevine/almond, mulberry, and elm/plum/oak. Independent results from RAPD-PCR assays were also consistent with these results. Sequence comparisons of a PCR product amplified from all strains of Xylella fastidiosa confirmed the presence of a Cfo I polymorphism that can be used to distinguish the citrus strains from all others. No nucleotide sequence differences were present in this product amplified from X. fastidiosa from any other host. Thus the amplification product of the coffee strains from Brazil shared the nucleotide sequence of the reference strains from the United States rather than that of the citrus strains from Brazil in spite of the fact that REP- and ERIC-, and RAPD-PCR assays clearly showed the basic genomic organization of the coffee and citrus strains to be very similar to each other and quite different to that of strains from the United States. The ability to distinguish X. fastidiosa strains from citrus and coffee with a PCR-based assay will be useful in epidemiological and etiological studies in Brazil.