Dissemination of Pseudomonas savastanoi pv. savastanoi populations and subsequent appearance of olive knot disease

Pseudomonas savastanoi pv. savastanoi (Psv) is the causal agent of olive knot disease. The bacterium survives epiphytically and gains ingress through new wounds where infections and colonization result in knot formation. The natural spread of the bacterium and the subsequent appearance of the disease in olive orchards is poorly understood. The aim of this study was to monitor Psv epiphytic populations in inoculated plants with knots versus non-inoculated healthy trees within the same orchard over four years. Additionally, disease severity was measured in both inoculated and non-inoculated control trees. Epiphytic Psv populations moved from inoculated to non-inoculated trees, although average Psv populations were higher in inoculated trees. Olive knot severity increased over the course of the study in all treatments and cultivars, with all plants reaching a high level of disease by the end of the study. However, the delay in the onset of disease was longer in non-inoculated than in inoculated trees. Molecular typing of Psv isolates recovered from non-inoculated control trees confirmed that they were similar to the inoculated strain. These data demonstrate that Psv can move over short distances in olive orchards through dissemination of epiphytic bacteria and suggest a relationship between the presence of epiphytic Psv and the number of knots on trees.