Population Structure of Xylella fastidiosa Associated with Almond Leaf Scorch Disease in the San Joaquin Valley of California.

Xylella fastidiosa causes disease in many commercial crops, including almond leaf scorch (ALS) disease in susceptible almond (Prunus dulcis). In this study, genetic diversity and population structure of X. fastidiosa associated with ALS disease were evaluated. Isolates obtained from two almond orchards in Fresno and Kern County in the San Joaquin Valley of California were analyzed for two successive years. Multilocus simple-sequence repeat (SSR) analysis revealed two major genetic clusters that were associated with two host cultivars, 'Sonora' and 'Nonpareil', respectively, regardless of the year of study or location of the orchard. These relationships suggest that host cultivar selection and adaptation are major driving forces shaping ALS X. fastidiosa population structure in the San Joaquin Valley. This finding will provide insight into understanding pathogen adaptation and host selection in the context of ALS disease dynamics.