Submitted to: Plant Disease
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/20/2014
Publication Date: 8/20/2014
Citation: Krugner, R., Sisterson, M.S., Chen, J., Stenger, D.C., Johnson, M.W. 2014. Evaluation of olive as a host of Xylella fastidiosa and associated sharpshooter vectors. Plant Disease. 98:1186-1193. Interpretive Summary: California is the sole producer of olive in the United States with ~17,800 ha planted and a production value estimated at $130 million per year. Increasing occurrences of dieback and leaf scorching symptoms in California olive trees suggested the bacterium Xylella fastidiosa (Xf) as the causal agent of a potential disease threat to olive production in California. Xf is a bacterium that causes numerous diseases of perennial crops, including Pierce’s Disease of grapes and almond leaf scorch. However, how Xf affects olive is poorly understood. A total of 198 samples of olive trees showing branch dieback and leaf scorch symptoms were collected to determine if Xf was constantly associated with the disease. Laboratory tests detected Xf in only 33 samples (~17%), from which six strains of the bacterium were isolated. DNA analysis and laboratory tests using grapevines and almonds indicated that Xf recovered from olive belong to a subgroup known to cause almond leaf scorch but not Pierce’s disease. Bacterial cells from laboratory cultures were inoculated into healthy olive plants and monitored over one year for symptom development and presence of bacteria. Olive test plants remained asymptomatic and Xf infection tended to be self-limiting. Results indicate that Xf is unlikely to be the causal agent of olive leaf scorch/branch dieback. Nonetheless, infected olive may serve as a reservoir for Xf and insect vectors, thereby contributing to the epidemiology of economically important diseases caused by Xf.
Technical Abstract: Olive (Olea europaea L.) trees exhibiting leaf scorch and/or branch dieback symptoms in California were surveyed for the xylem-limited, fastidious bacterium Xylella fastidiosa. Only ~17% of diseased trees tested positive for X. fastidiosa by PCR, and disease symptoms could not be attributed to X. fastidiosa infection of olive in greenhouse pathogenicity assays. Six strains of X. fastidiosa were isolated from olive in Southern California. Molecular assays identified the olive strains as belonging to X. fastidiosa subspecies multiplex. Pathogenicity testing of olive strains on grapevine and almond confirmed that X. fastidiosa strains from olive yield disease phenotypes on almond and grapevine typical of those expected for subspecies multiplex. Mechanical inoculation of X. fastidiosa olive strains to olive resulted in infection at low efficiency, but infections remained asymptomatic and tended to be self-limiting. Vector transmission assays demonstrated that glassy-winged sharpshooter, Homalodisca vitripennis (Germar) (Hemiptera: Cicadellidae), could transmit strains of both subspecies multiplex and fastidiosa to olive at low efficiency. Insect trapping data indicated that two vectors of X. fastidiosa, glassy-winged sharpshooter and green sharpshooter, Draeculacephala minerva Ball, were active in olive orchards. Collectively, the data indicate that X. fastidiosa is unlikely to be the etiological agent of olive leaf scorch/branch dieback, but olive may contribute to the epidemiology of xylellae-caused diseases in California. Olive may serve as an alternative, albeit suboptimal, host of X. fastidiosa. Olive also may be a refuge where sharpshooter vectors might escape intensive area-wide insecticide treatment of citrus, the primary control method used in California to limit glassy-winged sharpshooter populations and, indirectly, epidemics of Pierce’s disease of grapevine.