Submitted to: Canadian Journal of Plant Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/2007
Publication Date: 12/4/2007
Citation: Chen, J., Groves, R.L., Zheng, Y., Civerolo, E.L., Viveros, M., Freeman, M. 2007. Colony morphology of almond leaf scorch strains of xylella fastidiosa and its epidemiological application. Canadian Journal of Plant Pathology. 29:225-231. Interpretive Summary: Almond leaf scorch disease is caused by the plant pathogenic bacterium Xylella fastidiosa. This disease is re-emerging as an economically serious disease of almonds in California. However, information about the characteristics of strains of this pathogen, as well as the nature and basis of the pathogenicity of these strains in almond for developing effective disease management strategies is limited or non-existent. In this study, two distinct types (i.e., colony morphotypes) of strains with unique cultural characteristics on artificial nutrient media were demonstrated, for the first time, to be associated with diseased almond trees. Bacterial colony morphotyping is a simple tool that can be used to assist in determining the distribution of specific strains in almond leaf scorch-affected trees, and identifying the relative roles of specific strains of the pathogen in disease severity.
Technical Abstract: Xylella fastidiosa is the causal agent of almond leaf scorch disease (ALSD) that is currently re-emerging in California as a potential threat to almond production. In addition to the large amount of genome sequence data accumulated recently, phenotypic descriptions of X. fastidiosa and their biological nature, molecular basis, and genetic mechanism(s) are equally important for understanding this fastidious prokaryote and the development of disease control strategies. In this study, the colony morphologies of X. fastidiosa strains with two genotypes isolated from ALSD-affected trees in California were examined. Although significant variations occurred during the 14 sub-culturing passages, smooth colony morphotype was mostly associated with G-genotype strains. Rough colony morphotype was associated with A-genotype strains. Smooth morphotype was observed to be descended from rough morphotype of A-genotype, indicating the presence of colony phase variation in X. fastidiosa. Both A-genotype and G-genotype strains produced colony imprints or surface etchings on PWG and PW agar media, indicating enzymatic depolymerization of polysaccharides. In silico analyses identified four possible genes related to the imprint phenotype. Colony morphotyping in conjunction with PCR genotyping was further used to assist a survey of genotype distribution among X. fastidiosa ALSD strains collected in the central and southern San Joaquin Valley of California.