RESUMO
Field grown 2-year-old almond trees (Prunus dulcis cvs. Butte, Carmel, Mission, Ne Plus Ultra, Padre, Peerless, Price, Solano, Sonora, and Thompson) were mechanically inoculated with Xylella fastidiosa in the growing seasons of 2002 and 2003 to study the effect of inoculation date on the movement and colonization of X. fastidiosa and the overwintering persistence of almond leaf scorch disease (ALS) in these cultivars. X. fastidiosa was inoculated into the base of current-season growing shoots in April, May, June, July, August, September, and October. Almond trees inoculated in spring months developed more ALS-symptomatic leaves and more extensive within-plant spread of X. fastidiosa by the end of the current growing season compared with trees inoculated in July, August, September, and October. Trees inoculated in June developed the most severe ALS symptoms during the season in which they were inoculated. Trees inoculated in June and July 2002 had significantly higher disease ratings in 2003 than inoculations made in August and October 2002. Based on disease ratings observed in 2003, 1 year after inoculation, Sonora and Solano were the most ALS susceptible, Mission and Price intermediate, and Carmel, Padre, Ne Plus Ultra, Butte, Peerless, and Thompson were the least susceptible cultivars for allowing X. fastidiosa to overwinter and cause disease the following year. Assessment of all trees in August 2004 indicated that trees inoculated in June and July 2002 had a significantly higher amount of ALS-infected branches than trees inoculated in other months. Butte, Carmel, Padre, and Thompson cultivars had no symptomatic branches, while X. fastidiosa infections persisted or colonized new branches in Sonora, Solano, Peerless, Price, Mission, and Ne Plus Ultra. Based on the 2004 assessment, Sonora was the most susceptible cultivar. Surveys of a diseased orchard in Chico, CA showed large differences in ALS incidence in four almond cultivars. Nonpareil and Peerless had significantly greater incidence of disease than Butte and Carmel over the 2 years surveyed. These data suggest that cultivar susceptibility and the time of X. fastidiosa infection are important factors in determining the persistence of ALS in almond trees.
RESUMO
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.
