RE-EMERGENCE OF ALMOND LEAF SCORCH DISEASE IN CALIFORNIA, IDENTIFICATION OF INOCULUM SOURCES AND INSECT VECTORS.

Authors: Groves, Russell; Chen, Jianchi; CABRERA, J - UNIV OF CALIF-RIVERSIDE; JOHNSON, MARSHALL - UNIV OF CALIF-RIVERSIDE; DAANE, K - UNIV OF CALIF-BERKELEY

Submitted to: Entomology Society of America Pacific Branch Meeting
Publication Type: Abstract Only
Publication Acceptance Date: 1/13/2006
Publication Date: 3/4/2006
Citation: Groves, R.L., Chen, J., Cabrera, J., Johnson, M., Daane, K. 2006. Re-emergence of almond leaf scorch disease in California, identification of inoculum sources and insect vectors. Entomology Society of America Pacific Branch Meeting. Available: http://pbesa.prosser.wsu.edu/2006abs.pdf pp:36-38

Interpretive Summary:

Technical Abstract: In recent years, almond leaf scorch (ALS) disease has reemerged as a serious disease threat to almond production areas throughout California's San Joaquin Valley (SJV). The disease is caused by the bacterial pathogen, Xylella fastidiosa (Xf) and is reportedly transmitted by xylem feeding sharpshooters (Cicadellidae) and spittlebugs (Cercopidae). In California, there are at least 20 potential vector species capable of transmitting the pathogen; however, the primary vector(s) responsible for movement of ALS strains have not been well documented. The overall goal of this project was to determine the spatial structure of ALS-diseased almond trees, the sources of Xf primary inoculum, and the accurate identification of the insect vector(s) responsible for pathogen persistence and movement. Incidence of ALS was mapped annually in select almond orchards in 2003-2005 to assess/evaluate the relative importance of primary versus secondary Xf spread patterns. Two-dimensional maps of the spatial distribution(s) of diseased trees were generated and disease incidence varied significantly among almond cultivars. Ordinary runs and simple randomization analyses revealed aggregations of ALS-affected trees in four of the five survey orchards with a high frequency of disease clusters present in the outermost orchard rows associated with field borders adjoining forage alfalfa habitats known to support populations of potential vectors. Geo-statistical analyses further revealed spatial dependence in disease incidence within select almond cultivars best fit by a combination of linear and spherical models. Over multiple seasons, successive waves of primary spread may account for the spatial patterns of ALS observed in our study where clusters of infected trees were often associated with adjacent forage crops. The incidence and in-field distribution pattern(s) of Xf infection was determined in forage alfalfa crops adjacent to surveyed almond orchards with a history of ALS disease. Detection frequency of Xf varied among survey locations and ranged between 0.08 and 50.4%. The relative responses of different alfalfa cultivars to infection by genotypically distinct strains of Xf were evaluated in greenhouse inoculation experiments. Inoculated alfalfa cultivars varied in their relative susceptibility to infection by different Xf strains. The seasonal population dynamics of potential Xf vectors collected within forage habitats adjacent to ALS-affected orchards was monitored monthly at 4 field locations in Fresno and Kern Counties through the interval February 2004 to December 2005. The most numerous known Xf-vector species collected from monthly sweep samples was the green sharpshooter (GSS: Draeculacephala minerva) with a total of 1,981 adults collected in 2004 and 4,717 collected in 2005. Less than 1% of the total GSS collected in both years (N=31) were obtained from sweep samples of vegetation on the orchard floor(s) and no adult GSS were collected from sweep samples of almond foliage. The three-cornered alfalfa hopper, Spissistilus festinus, was the most numerous, potentially xylophagous, insect species captured in forage alfalfa habitats in both years of the study with a total of 5,902 and 4,941 adult insects collected in 2004 and 2005, respectively. Genomic DNA of Xf was amplified from adult GSS collected from forage alfalfa crops. Among 1,481 adults GSS assayed for the presence of Xf, approximately 3.0% (N=49) produced a calculated 221 bp amplicon averaging over all sample dates and locations. Further, genomic DNA was amplified from 28 (8.3%) of 233 adult S. festinus collected in forage alfalfa habitats adjacent to ALS-affected almond orchards. In laboratory Xf acquisition bioassays, adult S. festinus acquired both almond and grape strains of the bacteria from an Xf-infected alfalfa cultivar 'WL-625' although not in equal proportions. The epidemiological