Distribution of Xylella fastidiosa in Sycamore Associated with Low Temperature and Host Resistance

Henneberger, T. S. M., Stevenson, K. L., Britton, K. O., and Chang, C. J. 2004. Distribution of s Xj)lelEu fastidiosn in sycamore associated with low temperature and host resistance. Plant Dis. 88:95 1-958. Experiments were conducted in the field and laboratory to determine effects of low temperatures 4% on Xq'lella fnstidiosa populations in American sycamore. Roots and shoots from naturally infected trees at two locations were collected monthly. Sap extracted from the samples was tested by enzyme-linked immunosorbent assay for presence of X jastidosa and was diluted and plated on periwinkle wilt medium to determine populations of viable bacteria. Cumulative rainfall and hours below temperature thresholds (-5 to 10°C) were recorded at each site. Bacterial populations in shoots were negatively correlated with cumulative hours below -5°C air temperature ( r = -0.96). In roots, bacterial populations were only weakly correlated with cumulative hours below soil temperature thresholds (-0.61 < r< -0.25). Bacterial populations were not correlated with monthly rainfall. In the laboratory, resistant and susceptible sycamore trees were inoculated with X. fnstidiosa and held in the dark at 5°C or 22°C. After 12 weeks, inoculated stem sections were collected and sap was extracted and tested as described previously. Stems that tested positive for X. fnstidiosa were divided into additional samples and tested as described above. Results of the laboratory study indicated no significant effects of low-temperature treatment (5°C) or host resistance on viable bacteria. Bacterial detection frequency and population size were greatest near the inoculation point and the primary direction of early bacterial spread was acropetal. Additional keywords: mechanical inoculation, sycamore decline The xylem-limited, endophytic bacterium, Xylella fastidiosa Wells et al. (28) has been reported to infect hosts from more than 30 families of monocots and dicots. The pathogen has been implicated as the cause of leaf scorch, decline, and dieback diseases of an increasing number of forest and ornamental trees, including elm (Ulrnus spp.), maple (Acer spp.), oak (Quercus spp.), and sycamore lPlatarzus spp.) (4,11,17,26). Diseases caused by X fastidiosa in landscape and forest trees are characterized by development of foliar marginal necrosis in late summer and, in subsequent growing seasons in milder climates, by progressive basipetal decline