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United States Department of Agriculture

Agricultural Research Service

Title: Spatial Analysis of Almond Leaf Scorch Disease in the San Joaquin Valley of California: Factors Affecting Pathogen Distribution and Spread

Authors
item Groves, Russell
item Chen, Jianchi
item Civerolo, Edwin
item Freeman, Mark - UCCE-FRESNO, CA
item Viveros, Mario - UCCE-BAKERSFIELD, CA

Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 2005
Publication Date: June 1, 2005
Citation: Groves, R.L., Chen, J., Civerolo, E.L., Freeman, M., Viveros, M. 2005. Spatial analysis of almond leaf scorch disease in the San Joaquin Valley of California: Factors affecting pathogen distribution and spread. Plant Disease. 86(6):581-589.

Interpretive Summary: Almond leaf scorch disease (ALSD) has recently reemerged as a serious disease threat to almond production areas throughout California's San Joaquin Valley. This disease is caused by the bacterium Xylella fastidiosa (Xf) and is transmitted by sharpshooter leafhoppers and spittlebugs. The overall goal of this research was to identify the distribution and spread patterns of (Xf) among susceptible almond in California's Central Valley. Among the four orchards surveyed, ELISA and bacterial isolation followed by PCR were equally effective in detecting Xf from symptomatic trees. Disease incidence varied among almond cultivars in each of four surveyed orchards with the highest mean infection rates and most severe symptoms present in the cultivar 'Sonora'. Xylella fastidiosa isolates consisted of mixtures of strains present in surveyed orchards. The X. fastidiosa grape strains characterized from each orchard were consistently associated with the most severely affected 'Sonora' trees in three of the four orchards. Spatial analyses revealed aggregations of diseased trees in three of the four orchards with many clusters located along the outermost rows of susceptible cultivars 'Sonora' and 'Carmel'. Geostatistical analyses of disease incidence similarly illustrated spatial differences in ALS among cultivars with significant aggregations adjacent to habitats known to support vector populations. These results illustrate how cultivar susceptibility influences the distribution patterns of ALSD incidence. These analyses provide new information regarding the relative importance of primary inoculum sources and the patterns of Xf movement into and among susceptible almond cultivars.

Technical Abstract: Almond leaf scorch disease (ALSD) has recently reemerged as a serious disease threat to almond production areas throughout California's San Joaquin Valley. This disease, caused by the xylem-limited bacterium Xylella fastidiosa, is transmitted by xylophagous insects including sharpshooter leafhoppers (Hemiptera: Cicadellidae) and spittlebugs (Hemiptera: Cercopidae). The overall goal of this research was to identify factors that influence the distribution and movement of the pathogen. Among the four orchards surveyed, ELISA and bacterial isolation followed by PCR were equally effective in detecting X. fastidiosa from ALSD-symptomatic trees. Disease incidence varied among almond cultivar in each of four surveyed orchards with the highest mean infection rates and most severe symptoms present in the cultivar 'Sonora'. Xylella fastidiosa isolates consisted of mixtures of grape or 'G-genotype' and almond or 'A-genotype' strains present in surveyed orchards. The X. fastidiosa G-genotypes characterized from each orchard were consistently associated with the most severely affected 'Sonora' trees in three of the four orchards. Both ordinary runs and simple randomization analyses revealed aggregations of ALSD in three of the four orchards with a high frequency disease clusters within the outermost rows of susceptible cultivars 'Sonora' and 'Carmel'. Plots of semivariance in ALSD incidence over distance varied in shape and magnitude among cultivars. Semivariance increased over distance in cultivars 'Sonora' and 'Carmel' indicating spatial dependence or aggregations of incidence best fit by a combination of spherical and linear models. These results document both random and aggregate patterns of ALSD spatial distribution in selected orchards and further illustrate how cultivar susceptibility influences the distribution patterns of ALSD incidence and the impact of X. fastidiosa genotype on symptom severity. These analyses provide new information regarding the relative importance of primary inoculum sources and the patterns of X. fastidiosa movement into and among susceptible almond cultivars. Following the recent introduction and establishment of the glassy-winged sharpshooter, Homalodisca coagulata, the future impact upon the epidemiology and spread of ALSD is unknown.

Last Modified: 9/20/2014
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