Detection of Xylella fastidiosa in susceptible and resistant field olive trees

Olive quick Decline syndrome (OQDS) is a destructive disease characterized extensive branch and twig dieback, followed by death of the trees. Field surveys have unraveled the coexistence under the same cultivation conditions of olive trees showing severe or conversely very mild and scattered desiccation phenomena. Such evidences prompted investigations on the differential response of the olive cultivars to Xylella-infections. The first outcome of these studies indicated consistent lower bacterial population size (ca. 10^4CFU/ml) associated to the infected trees showing milder and no symptoms. We tested a large number of trees belonging to highly susceptible (Cellina di Nardò and Ogliarola salentina) and resistant cultivars (Leccino), and correlated the susceptibility (presence/absence of symptoms) of the cultivars with the diagnostic results of the ELISA and qPCR tests. All 500 trees belonging to cvs Ogliarola salentina and Cellina di Nardò and showing severe symptoms tested positive for X. fastidiosa in qPCR, with 97.7% testing positive in ELISA. Whereas, when 100 trees of the cv Leccino, mostly symptomless, were tested by both methods, the bacterium was detected in 34,6% of the trees by qPCR and in 14,8% by ELISA. These results while support the finding of lower bacterial population size in the resistant olives, highlight that the implementation of the Xylella-monitoring program for olives should take into account the olive cultivar.