Studies on olive (Olea europaea) as a host of Xylella fastidiosa in California (US)
From October 2008 to September 2012, olive (Olea europaea) trees showing leaf scorch or branch dieback symptoms in California (US) were tested for the presence of Xylella fastidiosa (EPPO A1 List). Samples were collected from 198 symptomatic olive trees in commercial orchards and urban areas. The objectives of this study were to evaluate the prevalence of X. fastidiosa in olive trees, to characterize olive strains and determine the insect vectors present in Californian olive orchards.
It is recalled that individual strains of X. fastidiosa differ in their host range and may be classified into subspecies according to their molecular characteristics:
- X. fastidiosa subsp. fastidiosa: found on grapevine (Pierce’s disease), and some other hosts including almond.
- X. fastidiosa subsp. multiplex: not found on grapevine but commonly found on almond showing leaf scorch, as well as in peach, plum, and landscape trees.
- X. fastidiosa subsp. pauca: causing citrus variegated chlorosis and coffee leaf scorch in South America.
- X. fastidiosa subsp. sandyi: causing oleander leaf scorch.
In California, the key native vectors of X. fastidiosa were Graphocephala atropunctata and Draeculacephala minerva. In the 1980s, Homalodisca vitripennis was introduced and is now playing a key role in the transmission of grapevine Pierce’s disease.
Results showed that only 17% of symptomatic olive trees tested positive for X. fastidiosa. The prevalence of X. fastidiosa was greater in Southern California than in the San Joaquin Valley and Yolo county. Six bacterial strains were isolated and characterized as belonging to X. fastidiosa subsp. multiplex. Vector transmission assays demonstrated that H. vitripennis could transmit strains of both X. fastidiosa subsp. multiplex and X. fastidiosa subsp. fastidiosa to olive but with a low efficiency. Insect trapping data indicated that both D. minerva and G. atropunctata were present in olive orchards. The authors concluded that these studies showed a poor correlation between symptoms observed on olive trees and bacterial infection. It is felt that in California, X. fastidiosa cannot be considered to be the cause of olive leaf scorch or branch dieback, but that olive trees might contribute to the epidemiology of diseases elicited by X. fastidiosa. Under the Californian conditions, olive is considered to be an alternative and suboptimal host of X. fastidiosa.
Krugner R, Sisteron MS, Chen JC, Stenger DC, Johnson MW (2014) Evaluation of olive as a host of Xylella fastidiosa and associated sharpshooters vectors. Plant Disease 98(9), 1186-1193.