EPIDEMIOLOGY OF ALMOND LEAF SCORCH DISEASE IN THE SAN JOAQUIN VALLEY OF CALIFORNIA: FACTORS AFFECTING PATHOGEN DISTRIBUTION AND MOVEMENT

Authors: GROVES, RUSSELL - UNIV. OF WISCONSIN, MADIS; Sisterson, Mark; Chen, Jianchi; Lin, Hong

Submitted to: Almond Industry Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: 10/16/2006
Publication Date: 12/6/2007
Citation: Groves, R., Sisterson, M.S., Chen, J., Lin, H. 2007. Epidemiology of Almond Leaf Scorch Disease in the San Joaquin Valley of California: factors affecting pathogen distribution and movement. In: Proceedings of the 34th Almond Industry Conference, December 6-7, 2006, Modesto, California. p. 176-183.

Interpretive Summary: Almond leaf scorch disease (ALSD) has recently reemerged as a serious threat to almond production in California’s San Joaquin Valley. This disease, which is caused by the xylem-limited bacterium Xylella fastidiosa, is vectored by sharpshooters and spittlebugs. Studies found that green sharpshooters and unidentified spittle bugs were the only potential vector species moving into orchards. Green sharpshooters were found in high abundance in pastures and field crops surrounding almond orchards, but in low numbers in the orchards. ALSD incidence varied among almond cultivars and isolates consisted of a mix of ALSD strains. Some strains were consistently associated with the most severely affected ‘Sonora’ trees in three of the four orchards. Diseased trees were aggregated and often in the outermost orchard rows, although random patterns of disease incidence in orchards were also observed. This research provides important information on pathogen vectors and crop susceptibility which will aid in developing management plans for this disease.

Technical Abstract: Almond leaf scorch disease (ALSD) has recently reemerged as a serious threat to almond production areas throughout California’s San Joaquin Valley. This disease, which is caused by the xylem-limited bacterium Xylella fastidiousa, is vectored by xylophagous insects including sharpshooter leafhoppers (Hemiptera: Cicadellidae) and spittlebugs (Hemiptera: Cercopidae). The seasonal population dynamics of potential Xf vectors within and surrounding selected almond orchards was monitored in 2004. Dispersing populations of green sharpshooters and unidentified spittle bugs were the only potential ALSD vector species found to be immigrating into survey orchards. Larger numbers of potential ALSD vector species were collected from surrounding pasture and forage crops with very few potential vectors collected from almond foliage or from vegetation on the orchard floor. ALSD incidence varied among almond cultivars in each orchard 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. 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. Ordinary runs and simple randomization analyses revealed aggregations of ALSD in three of the four orchards with a high frequency of disease clusters present in the outermost orchard rows. 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.