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United States Department of Agriculture

Agricultural Research Service

Title: Epidemiology of Almond Leaf Scorch Disease in the San Joaquin Valley of California: Factors Affecting Pathogen Distribution and Movement

Authors
item Groves, Russell
item Chen, Jianchi

Submitted to: Meeting Proceedings
Publication Type: Proceedings
Publication Acceptance Date: November 24, 2004
Publication Date: December 1, 2004
Citation: Groves, R.L., Chen, J. 2004. Epidemiology of almond leaf scorch disease in the San Joaquin Valley of California: Factors affecting pathogen distribution and movement. Proceedings of the 32nd. Annual Almond Conference. p. 159-164

Interpretive Summary: Almond leaf scorch disease (ALSD) has recently reemerged as a serious threat to almond production areas throughout California's San Joaquin Valley. This disease is caused by the xylem-limited bacterium Xylella fastidiosa and this pathogen is transmitted by sharpshooter leafhoppers and spittlebugs. Green sharpshooters and unidentified spittle bugs were the only potential ALSD vector species collected dispersing into orchards. Larger numbers of sharpshooter were collected from surrounding pasture and forage crops with very collected from almond foliage or on the orchard floor. ALSD incidence varied among almond cultivars with the most susceptible being 'Sonora'. Xylella fastidiosa isolates consisted of mixtures of genotypes in the same orchard and within similar trees. Some genotypes were consistently associated with the most severely affected 'Sonora' trees in three of the four orchards. Spatial analyses revealed aggregations of ALSD with disease clusters often in the outermost orchard rows. These results illustrate that ALSD can be vary from random patterns in orchards to clustered in groups of infected trees. Cultivar susceptibility can affect the patterns of ALSD incidence.

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 is caused by the xylem-limited bacterium Xylella fastidiosa and this pathogen is transmitted 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 collected 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 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. 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.

Last Modified: 9/22/2014
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