Submitted to: Phytopathology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 2/16/2005
Publication Date: 5/15/2005
Citation: Chen, J., Groves, R.L., Civerolo, E.L., Viveros, M., Freeman, M., Zheng, Y. 2005. Two Xylella fastidiosa genotypes associated with almond leaf scorch disease on the same location in California. Phytopathology. 95(6):708-714. Interpretive Summary: Almond leaf scorch disease (ALSD) has recently reemerged in the San Joaquin Valley of California threatening almond production. ALSD is caused by a bacterial pathogen, Xylella fastidiosa. Although two types or genotypes of X. fastidiosa have been identified to cause ALSD, previous research has been performed based on the assumption that only one of the two genotypes was responsible for ALSD. Utilizing currently available genomic sequence information and SNP technology, this study developed two unique multiplex PCR techniques and detected the simultaneous occurrence of at least two X. fastidiosa genotypes in the same almond orchard as well as a single tree. This was substantiated by RFLP analysis of a different genetic locus, called RST31-RST33, that has been widely accepted for species and strain identification. Furthermore, unique colony morphology was also observed to be associated with these X. fastidiosa genotypes. This is the first report of a mixed genotype infection of X. fastidiosa disease present under natural environmental conditions. The concept of mixed genotype infection could affect the epidemiology X. fastidiosa and the resulting disease management.
Technical Abstract: Almond leaf scorch disease (ALSD) has recently reemerged in the San Joaquin Valley of California threatening almond production. ALSD is caused by Xylella fastidiosa, a nutritionally fastidious bacterium. Single nucleotide polymorhpisms (SNPs) in the 16S rRNA gene of X. fastidiosa strains were identified to characterize bacterial populations in infected trees. Strain specific SNPs were used to design primers for multiplex PCR assays of early passage cultures. At least two genotypically distinct X. fastidiosa strains, G-type and A-type, coexist simultaneously in the same infected almond orchard as well as in single infected leaves. This was substaintiated by RFLP analysis of a different genetic locus that has been widely accepted for species and strain identification. Futhermore, variations of bacterial colony morphology were consistently assoiciated with these X. fastidiosa genotypes. This is the first report of a mixed strain infection of X. fastidiosa disease present under natural environmental conditions. The concept of mixed strain infection could affect the epidemiology of X. fastidiosa and resulting disease management.