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Xylella fastidiosa is a xylem-limited bacterium that causes almond leaf scorch (ALS), Pierce's disease of grapevines, and other plant diseases. We surveyed ground vegetation in ALS-infected almond orchards in California's Central Valley for the presence of this bacterium. Plant tissue samples were collected throughout a 2-year period and processed for the presence of X. fastidiosa using restriction enzyme digestion of RST31 and RST33 polymerase chain reaction (PCR) products and bacterial culture on selective media. Overall disease incidence was low in the ground vegetation species; only 63 of 1,369 samples tested positive. Of the 38 species of common ground vegetation tested, 11 tested positive for X. fastidiosa, including such common species as shepherd's purse (Capsella bursa-pastoris), filaree (Erodium spp.), cheeseweed (Malva parvifolia), burclover (Medicago polymorpha), annual bluegrass (Poa annua) London rocket (Sisymbrium irio), and chickweed (Stellaria media). There was a seasonal component to bacterial presence, with positive samples found only between November and March. Both ground vegetation and almond trees were most commonly infected with the almond strain of X. fastidiosa (six of seven surveyed sites). ALS-infected almond samples had an X. fastidiosa concentration within previously reported ranges (1.84 × 106to 2.15 × 107CFU/g); however, we were unable to accurately measure X. fastidiosa titer in sampled ground vegetation for comparison. These results are discussed with respect to ground vegetation management for ALS control.

Groves, R. L., Chen, J., Civerolo, E. L., Freeman, M. W., and Viveros, M. A. 2005. Spatial analysis of almond leaf scorch disease in the San Joaquin Valley of California: Factors affecting pathogen distribution and spread. Plant Dis. 89:581-589. Almond leaf scorch (ALS) disease has emerged as a serious threat to almond (Prunus amygdalus) 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). Among four orchards surveyed, enzyme-linked immunosorbent assay (ELISA) and bacterial isolation followed by polymerase chain reaction (PCR) were equally effective in detecting X. fastidiosa from ALS-symptomatic trees. Disease incidence varied among almond cultivars in each orchard, with the highest mean incidence and most severe symptoms frequently encountered in ‘Sonora’. X. 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 associated with the most severely affected ‘Sonora’ trees in three of the four orchards. Both ordinary runs and simple randomization analyses revealed aggregations of ALS in three of the four orchards. Clusters of ALSaffected trees frequently occurred in the outermost orchard rows. Plots of semivariance in ALS incidence over distance varied in shape and magnitude among cultivars. Semivariance increased over distance in ‘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 ALS spatial distribution in selected orchards and further illustrate how cultivar susceptibility influences the distribution patterns of ALS incidence. Following the recent introduction and establishment of the glassy-winged sharpshooter, Homalodisca coagulata, the impact upon the epidemiology and spread of ALS is unknown. Additional keywords: Pierce’s disease

Almond trees were irrigated from March through November 1994 and 1995 with 70, 85, and 100% of potential evapotranspiration (ETc). Deficit irrigation was accomplished by delivering 70 or 85% of ETc at every irrigation (sustained) or 50% of ETc during 1 June to 31 July (70 regulated) or 1 to 15 July (85 regulated). The natural incidence of dead leaf clusters and dead spurs, twigs, and small branches, measured at harvest, lessened with decreasing amounts of water, and regulated deficits were more effective than sustained deficits in reducing disease. Fruit at early dehiscence on trees in each of the five irrigation treatments were inoculated with 0.1 ml of suspensions of 104 spores per ml of Monilinia fructicola or Rhizopus stolonifer. Monilinia fructicola caused more hull rot than R. stolonifer, and both pathogens responded similarly to the irrigation treatments. The rate of fruit maturation was monitored for approximately 4 weeks before harvest by scoring the percent abscission and dehiscence and measuring the hull moisture content of fruit on trees in each irrigation treatment. Dry kernel weight was determined at harvest. Maturation was slower and kernel weight greater in treatments receiving 85% of ETc than 70% or those under sustained compared with regulated irrigation regimes.

Preharvest termination of weekly irrigation prior to harvest from late June to mid-August 1990 and 1991 was studied for control of almond hull rot disease caused by Rhizopus stolonifer. When trees were irrigated within 2 wk of harvest, there was more infection of hulls by R. stolonifer, more leas strikes per tree, and in 1991, more shoot and branch death per tree than when irrigation was discontinued earlier. Percent hull split incidence and hull moisture content increased as irrigation continued through the season, but these criteria did not explain differences in disease incidence among trees in the first six compared with the last two irrigation termination dates