Almond leaf scorch (ALS) is caused by the pathogenic bacterium Xylella fastidiosa and poses a threat to the California almond industry. Almond cultivars are differentially resistant or susceptible to ALS. X. fastidiosa can infect but does not overwinter in resistant cultivars in sufficient numbers to cause symptoms or be detected by polymerase chain reaction. To better understand the biochemical or morphological factors mediating resistance, we extracted and analyzed almond xylem fluid from four almond cultivars differing in ALS susceptibility, including Butte and Carmel cultivars that are field resistant and Peerless and Sonora that are ALS susceptible. Xylem fluid was collected over winter months in 2007 to 2009, as well as July 2008 and April 2009, and analyzed for the following: pH, osmolarity, concentrations of sugars, calcium, magnesium, organic acids, and total phenolics. For most of these analyses, we found no clear differences in xylem fluid from resistant and susceptible almond cultivars. However, during the winter months, resistant cultivars tended to have higher concentrations of total phenolic compounds compared with susceptible cultivars (P = 0.05). In February 2009, Carmel had the highest total phenolic concentration measured, 233 μg/ml of gallic acid equivalents. The lowest phenolic concentrations occurred in April 2009. The cross-sectional areas of xylem vessels in Butte (resistant) and Peerless (susceptible) trees were not significantly different between cultivars.
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