Seasonal Abundance of Draeculacephala minerva and Other Xylella fastidiosa Vectors in California Almond Orchards and Vineyards

Abstract

Almond leaf scorch (ALS) disease is caused by the bacterium Xylella fastidiosa and transmitted by xylem-feeding insects. Reports of increased incidence of ALS-diseased trees in California prompted surveys in three almond [Prunus dulcis (Mill.) D. A. Webb]-growing regions, from June 2003 to September 2005, to determine insect vector species composition and abundance. For comparison, sampling in and near vineyards in the San Joaquin Valley, California, also was completed. Sampling in or near almond orchards collected >42,000 Cicadomorpha of which 4.8% were xylem feeders, including 1912 grass sharpshooter, Draeculacephala minerva Ball; five Xyphon fulgida Nottingham; and a single spittlebug, Philaenus spumarius L. The most abundant vector was D. minerva. Season-long sampling indicated that D. minerva was a year-round resident in and/or near almonds in the Sacramento Valley, but not in the San Joaquin Valley. Similarly, D. minerca was rare in vineyards in the San Joaquin Valley, but was abundant in irrigated pastures near vineyards. D. minerva was most frequently collected along orchard margins, and peak densities were observed in summer, the period of time when bacterial titers are reported to increase in infected trees. Screening of D. minerva for presence of X.fastidiosa found that 1.1% of insects collected near almond orchards and 4.5% of insects collected from pastures tested positive. The X. fastidiosa subspecies and genotype detected in insects collected from orchards matched those collected from ALS-diseased almond trees in the same orchard. Of the few X. fulgida and P. spumarius collected, none tested positive for X. fastidiosa. Results are discussed with respect to X. fastidiosa vector control and detection methods.

Effects of Gender, Origin, and Age on Transmission ofXylella fastidiosato Grapevines byHomalodisca vitripennis(Hemiptera: Cicadellidae)

“…Presence of X. fastidiosa in insect heads was assessed after the IAP by using conventional PCR, similar to methods used in Daane et al (2011). In brief, the anterior portion of the head, the eyes and the insectÕs body were removed, leaving the cibarium, precibarium, and associated mouthparts.…”

Phenology ofXylella fastidiosaand Its Vector Around California Almond Nurseries: An Assessment of Plant Vulnerability to Almond Leaf Scorch Disease

“…D. minerva is well known to be abundant in irrigated pastures, stream banks, and weedy alfalfa fields with perennial grass cover (9,37,49); proximity of such habitats to almond orchards with high incidence of ALSD has been documented (36). However, the role of D. minerva in the primary spread of X. fastidiosa in almond orchards was not well understood until recently.…”

“…However, the role of D. minerva in the primary spread of X. fastidiosa in almond orchards was not well understood until recently. Surveys conducted in orchards in California's Sacramento and San Joaquin valleys showed a genetic link between strains causing ALSD and those carried by D. minerva collected in ground vegetation found in and near orchards, which makes D. minerva a strong candidate for primary spread of X. fastidiosa from alternative hosts to the crop (9,46). Results from surveys reported here, such as high vector abundance and presence of X. fastidiosa in host plants located adjacent to the crop, are in agreement with findings from previous investigations.…”

“…ALSD is found throughout almond production areas of California, with incidence in affected orchards ranging from 0.08 to 17% depending on region and cultivar (5,9). ALSD strains of X. fastidiosa are transmitted by several species of sharpshooters and spittlebugs (35,36), but Draeculacephala minerva Ball (Hemiptera: Cicadellidae) is perhaps the only species that plays a role in pathogen spread to almond in California, as the distribution of D. minerva overlaps with almond production regions.…”

“…In almond orchards, in contrast, the random distribution of symptomatic trees and the absence of distinct disease gradients associated with adjacent vector habitat (20,36) demonstrate that the relationships among proximity to vector habitat, the distribution of vectors in the orchard, and disease incidence are not as clear. Nonetheless, it is known that D. minerva moves between almond orchards and adjacent pastures and alfalfa fields (30), and the pathogen is present in D. minerva and in vegetation both in and around almond orchards (9). Clearly, this species is an important vector of X. fastidiosa strains causing ALSD, and it may be responsible for some level of primary pathogen spread into orchards.…”