Spatial Analysis of Almond Leaf Scorch Disease in the San Joaquin Valley of California: Factors Affecting Pathogen Distribution and Spread

Almond leaf scorch (ALS) disease has emerged recently as a serious threat to almond production areas throughout California’s San Joaquin Valley (5,36). This disease is caused by the xylem-limited bacterium Xylella fastidiosa, which also causes several plant diseases in California including Pierce’s disease (PD) of grape, oleander leaf scorch (OLS), and alfalfa dwarf (AD). X. fastidiosa bacterial strains have a diverse host range (5,8,10,30), are genetically diverse, and generally cluster within groups associated with different host species (2,6,7,22,23). Recent studies on the genetic relationships of different X. fastidiosa strains support the hypothesis that the bacterial species consists of more than one subspecies or pathovar (5,8,15). Knowledge of the genetic diversity of X. fastidiosa strains associated with ALS in the central San Joaquin Valley of California, especially as it relates to disease epidemiology, is not well understood. The pathogen is transmitted by xylemfeeding sharpshooters (Cicadellidae) and spittlebugs (Cercopidae) (4,18,28,29,31,33). In California, there are at least 20 species of sharpshooters or spittlebugs capable of transmitting X. fastidiosa (31); however, only four species are considered to be epidemiologically important in transmission of X. fastidiosa to grapes (18,29). The vector(s) of X. fastidiosa associated with ALS, however, has not been well documented. Nevertheless, some leafhopper and spittlebug species have been implicated as possible vectors of X. fastidiosa–ALS strains (28,29). While X. fastidiosa has long been present in the San Joaquin Valley of California (24,32), the incidence of ALS appears to have emerged as a significant threat in numerous locations throughout much of the almond-producing region. This increase in ALS is reported to be widely distributed in the affected areas and often appears to be associated with large acreages of adjoining permanent pasture or irrigated alfalfa forage crops (36). Many vineyards that are recurrently affected by Pierce’s disease in this region of the San Joaquin Valley are associated with similar habitats that harbor X. fastidiosa vectors (17,29). The spatial pattern of PD incidence in susceptible grape in these areas decreases over distance from inoculum sources and often is associated with the edges of vineyards (16,26,27). Tubajika et al. (35) recently demonstrated that infections of PD were not only aggregated in clusters of diseased vines close to field edges, but were also clustered in disease foci over successive years, suggesting that vine-to-vine, or secondary spread of X. fastidiosa by Homalodisca coagulata (Say), was an important mechanism for X. fastidiosa spread. In coastal California, the blue-green sharpshooter, Graphocephala atropunctata (Signoret) (Hemiptera: Cicadellidae), is responsible for primary spread of the pathogen from outside inoculum sources in the early spring (14,27). Limited information is currently available on the distribution and spread of X. fastidiosa infections resulting in ALS in the San Joaquin Valley of California. ALSaffected orchards were previously characterized as having few symptomatic trees, which were randomly and widely distributed throughout affected orchards without any association to known vector dispersal habits (28). In recent years, the area-wide incidence of ALS has increased in portions of the southern San Joaquin Valley of California in regions where it has not previously been documented (25,36). Analyses of the spatial patterns of ALS in these newly affected areas will provide new information regarding the relative importance of primary inoculum sources, patterns of X. fastidiosa movement into and among susceptible almond cultivars, and the necessity for, or epidemiological importance of, roguing ALS-affected trees. The objectives of this study were to determine the spatial pattern of ALS incidence in managed almond orchards naturally infected with X. fastidiosa, to characterize the patterns of disease spread between ALS genotypes, and to describe the differential patterns of susceptibility to disease among affected almond cultivars.