2008 Annual Report 1a.Objectives (from AD-416) To develop a microarray-based Xylella fastidiosa detection and identification system. 1b.Approach (from AD-416) A PCR-microarray-based system that combines the sensitivity of PCR and specificity of microarray DNA hybridization will be developed and rapid identification of X. fastidiosa strains. Strain specific DNA sequence information will be identified through genome sequence analyses of multiple X. fastidiosa strains. The PCR-microarray system will be extensively evaluated using X. fastidiosa strains collected from different genotypes/pathotypes from different geographical regions. Documents SCA with UC Davis. Formerly 5302-22000-005-26S (10/05) & 5302-22000-007-14S (5/07). 3.Progress Report This study focuses on the genomic variations between Xf strains causing almond leaf scorch disease (ALSD) and grape Pierce’s disease (PD). The two diseases are commonly found in California. There have been two 16S rDNA genotypes described in California, A-genotype strains cause ALSD only and G-genotype strains cause both PD and ALSD. While G-genotype strains cause two different diseases, little is known about their genetic variation. In this study, we identified a putative protease encoding gene, PD0218 (pspB), in the genome of X. fastidiosa and evaluated the variation at this genomic location in X. fastidiosa populations. The target gene (PD0218) contains tandem repeats of ACDCCA, translated to threonine and proline (TP), within the putative protease functional region. Among 116 X. fastdiosa ALSD and PD strains isolated from seven locations in California, tandem repeat numbers (TRNs) varied from 9 to 47 with a total of 30 TRN genotypes. This indicates that X. fastidiosa possesses an active mechanism of contracting and expanding tandem repeats. A significant TRN variation was found among PD strains (Mean=29.9±14.0, n=45), which could be further divided into two TRN groups: PD-Gsmall (Mean=17.3±4.1, n=24) and PD-Glarge (Mean=44.3±1.2, n=21). Less variation was found in ALSD strains (Mean=21.7±7.5, n=71). Variation was even smaller after partitioning to 16S rDNA genotype (Mean=13.3±2.0, n=43, for G-genotype; Mean=27.1±3.8, n=28, for A-genotype). Genetic variation at PD0218 region is potentially useful for sensitive differentiation of X. fastidiosa strains. However, TRN stability, variation range, and correlation to the target phenotype should be evaluated in epidemiological applications such as pathotype identification and delineation of pathogen origin. Research progress was monitored by regular conference calls, email exchanges, phone calls, reports, and writing. Research findings were presented in the annual meetings of the American Phytopathological Society and in California almond board annual meeting with posters and oral presentations. A manuscript has been published in a refereed journal.