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Spatial pattern analysis of the incidence of strawberry angular leaf spot under outdoor growing conditions in California
C. GIGOT (1), N. McRoberts (1), W. Turechek (2). (1) Plant Pathology Department, University of California, Davis, CA, U.S.A.; (2) Horticulture Research Laboratory, USDA-ARS, Fort Pierce, FL, U.S.A.

Angular leaf spot (ALS), caused by <i>Xanthomonas fragariae</i>, is a serious disease in California strawberry nursery production because of the quarantine status of the pathogen outside the U.S.A. In addition, substantial yield losses in fruit production fields are possible anywhere strawberry is grown if environmental conditions are conducive. To begin to understand the epidemiology of this disease, we worked in close collaboration with nurseries to characterize the spatial patterns of ALS in different locations in Northern California. At each location, 10 evenly-spaced rows consisting of 20 sampling units spaced 5 m apart were sampled. Within each sampling unit, 9 leaflets were randomly collected within a 1 m<sup>2</sup> quadrat and assessed for disease incidence. The binomial power law regression gave a good description of all 55 data sets collected in 2014 (R<sup>2</sup> = 0.94). The slope of the regression between the log<sub>10</sub> of observed variance and the log<sub>10</sub> of binomial variance was computed for the first time for ALS in nursery field conditions, and was found to have a quite high value of 1.23. This is consistent with what is usually observed for plant pathogens with steep dispersal gradients, typically spread during rainfall or overhead irrigation events. Such quantitative characterizations of ALS epidemiology may us allow to unravel and rank pathogen dispersal mechanisms, with the final aim to lead to optimization of disease control.

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