Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/7/2010
Publication Date: 7/1/2010
Publication URL: apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-94-7-0827
Citation: Sisterson, M.S., Thimmiraju, S., Daane, K., Lynn-Patterson, K., Groves, R. 2010. Epidemiology of Diseases Caused by Xylella fastidiosa in California: Evaluation of Alfalfa As A Source of Vectors and Inocula. Plant Disease. 94:827-834. Interpretive Summary: Pierce’s disease of grape and almond leaf scorch disease have been chronic problems in California. Both diseases are caused by the xylem-limited pathogen Xylella fastidiosa. The pathogen is vectored by xylem feeding insects. The potential for alfalfa to serve as a source of vectors and inocula was evaluated. Geographic Information Systems maps on the distribution of grape, almond, and alfalfa in California indicated that 94,512 hectares of almond and grape were within one mile of an alfalfa field. Monitoring of needle-inoculated alfalfa plants determined that cool winter temperatures reduced titers of X. fastidiosa to undetectable levels but did not eliminate infections. Incidence of X. fastidiosa in field collected alfalfa during the summer was low with only 6 positive samples out of 1,156 collected. Trapping for vectors in alfalfa fields found that green sharpshooter (Draeculacephala minerva) was the most abundant vector. Analysis of the distribution of green sharpshooters within alfalfa fields determined that green sharpshooters were most abundant in weedy sections of alfalfa fields. Due to the abundance of green sharpshooters in weedy alfalfa fields, the results indicate that alfalfa can serve as a source of vectors. Needle inoculations of alfalfa were successful and infections persisted for up to two years, suggesting that alfalfa has the potential to be a source of inocula. The study provides growers with the information needed to evaluate the risk of planting grapes or almond in proximity to alfalfa fields.
Technical Abstract: Pierce’s disease of grape and almond leaf scorch disease have been chronic problems in California. Both diseases are caused by the xylem-limited, bacterial pathogen, Xylella fastidiosa. The pathogen is vectored by xylem feeding insects and within California key vectors vary by region. The potential for alfalfa to serve as a source of vectors and inocula was evaluated. Analysis of Geographic Information Systems maps on the distribution and abundance of grape, almond, and alfalfa plantings determined that 94,512 hectares of almond and grape were planted within 1.6 km of an alfalfa field. Seasonal trends of X. fastidiosa detection were monitored in five needle-inoculated alfalfa cultivars (CUF101, Moapa69, WL342, WL530, and WL625) over two years. Results indicate that cool winter temperatures reduced X. fastidiosa titers to undetectable levels, but did not eliminate infections. Sampling of alfalfa fields to assess incidence of X. fastidiosa corroborated this result, with positive samples detected in summer only. Incidence of X. fastidiosa in alfalfa during summer was low with only 6 positive samples out of 1,156 samples collected over three years. Trapping in alfalfa fields over three years found that green sharpshooter was the most abundant vector. Within alfalfa fields, green sharpshooter abundance was highest in weedy areas suggesting a preference for weeds over alfalfa. The results confirm that weedy alfalfa fields can serve as an important source of vectors. Incidence of X. fastidiosa in alfalfa was low, most likely due to preference of vectors for weeds over alfalfa. The results suggest that risk of Pierce’s disease and almond leaf scorch disease could be reduced by distancing grape and almond plantings from alfalfa plantings or by managing weeds in alfalfa fields near almond and grape plantings.