ARS | Publication request: Assessment of the importance of alfalfa to the epidemiology of xylellae diseases in the San Joaquin Valley of California

Submitted to: CDFA Pierce's Disease Control Program Research Symposium
Publication Type: Proceedings
Publication Acceptance Date: October 17, 2008
Publication Date: December 15, 2008
Citation: Sisterson, M.S., Groves,, R., Daane,, K., Thimmiraju,, S. 2008. Assessment of the importance of alfalfa to the epidemiology of xylellae diseases in the San Joaquin Valley of California. In: Proceedings of the CDFA Pierce's Disease Control Program Research Symposium, December 15-17, 2008, San Diego, California. p. 239-241.

Interpretive Summary: Xylella fastidiosa is the causal agent of several diseases that are common in the San Joaquin Valley of California, including Pierce’s disease of grape and almond leaf scorch disease. We assessed the role of alfalfa in the epidemiology of these diseases as alfalfa is a known host for X. fastidiosa and alfalfa fields often harbor large numbers of the native vector, Draeculacephala minerva. Laboratory inoculation of fourteen cultivars of alfalfa indicated that all cultivars tested were suitable hosts of X. fastidiosa. In addition, we found that pathogen detection in laboratory inoculated plants held outdoors declined during the winter and increased again during the summer, indicating important seasonal fluctuations in the titer of X. fastidiosa in alfalfa. Seasonal sampling of alfalfa fields found that incidence of X. fastidiosa was low, with only 6 positive samples out of >4,000 collected. All six positive samples were collected during the summer agreeing with the seasonal trends in X. fastidiosa detection observed in controlled studies. Trapping of D. minvera in alfalfa fields indicated that D. minerva abundance in alfalfa fields was often high, but that it preferred weedy field margins. Preference of D. minerva for weeds found in alfalfa fields would limit the spread of X. fastidiosa in alfalfa. The results indicate that alfalfa can indeed serve as a source of vectors, but its role as an inoculum source is unclear. Future work should estimate the incidence of X. fastidiosa in weeds commonly found in alfalfa fields that are preferred feeding hosts of D. minerva. Results of this work will aid in providing practical recommendations for reducing the spread of strains of X. fastidiosa that cause Pierce’s disease and almond leaf scorch disease.

Technical Abstract: The role of alfalfa in the epidemiology of xylellae diseases in the San Joaquin Valley of California was assessed. Alfalfa was investigated as it is a known host of Xylella fastidiosa and often harbors large populations of a native vector, Draeculacephala minerva. Laboratory inoculation of fourteen cultivars of alfalfa indicated that all cultivars tested were suitable hosts. The persistence of infections in alfalfa was followed in four cultivars over one year. For plants held outdoors, detection of X. fastidiosa via PCR declined during the winter and increased again during the summer, suggesting that cool winter temperatures decreased titers of X. fastidiosa. Sampling of alfalfa fields seasonally found that incidence of X. fastidosa in alfalfa was low with only 6 positive samples detected out of >4,000 screened. All positive samples were collected in the summer agreeing with the seasonal trends in X. fastidiosa detection observed in controlled studies. Abundance of D. minvera in alfalfa was high, although the highest numbers were caught on traps located on weedy field margins. Preference of D. minerva for weeds in alfalfa fields would limit the spread of X. fastidiosa in alfalfa. The results indicate that alfalfa can serve as a source of vectors, but its role as an inoculum source is unclear. Future work should determine the incidence of X. fastidiosa in weeds commonly found in alfalfa fields that are preferred feeding hosts of D. minerva.