Year of Publication

2005

Document Type

Thesis

College

Agriculture

Department

Plant Pathology

First Advisor

Christopher L. Schardl

Abstract

Phytopathogenic bacterium, Xylella fastidiosa, causes a number of economically important diseases, including Pierces disease (PD) of grape and bacterial leaf scorch (BLS) of a number of landscape trees. In Kentucky (KY), BLS affects a number of shade trees including many oak and maple species. In 2001, PD was diagnosed in grapevines in western KY. Xylella fastidiosa is also detected in many asymptomatic landscape plants and grasses. It was the goal of this research to identify hosts of X. fastidiosa around KY and use phylogenetic analysis to compare sequences of the 16S rDNA and gyrase B (gyrB) genes between samples. This research tests the hypothesis that sequence comparison can identify asymptomatic hosts and vectors that serve as a source of inoculum for pathogenic strains of X. fastidiosa. Plant collections were done in urban areas of KY between 2002 and 2004 and samples were tested for the presence of X. fastidiosa by ELISA and PCR. A number of symptomatic and asymptomatic plants were found to be hosts. Primer sets specifically developed for X. fastidiosa were used to amplify part of the 16S rDNA and the gyrB gene from DNA extracted directly from plant tissue. Sequence data from these specifically amplified products were assembled using Phrap, aligned with ClustalW, then phylogenetic analysis was done with Paup 4.0b10 beta. Comparisons with strains outside of Kentucky were also done using X. fastidiosa sequence obtained from NCBI. Maximum parsimony (MP) trees from the 16S rDNA showed a clade of sequence from oak and grass samples that is an outgroup to sequence from NCBI and other samples in this study. According to BLAST, sequences in this outgroup clade seem to be more closely related to the genera Xanthomonas or Stenotrophomonas than Xylella. However, the gyrB gene MP tree showed sequence from three of the samples that were part of this outgroup clade as being closely related to those X. fastidiosa sequences that are part of the ingroup of both 16S rDNA and gyrB trees. The topology difference between the 16S rDNA and gyrB trees suggest there may have been recombination in the genomic region containing one of these genes.

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