Isolation and pathogenicity of Xylella fastidiosa associated to the olive quick decline syndrome in southern Italy

Abstract

In autumn 2013, the presence of Xylella fastidiosa, a xylem-limited Gram-negative bacterium, was detected in olive stands of an area of the Ionian coast of the Salento peninsula (Apulia, southern Italy), that were severely affected by a disease denoted olive quick decline syndrome (OQDS). Studies were carried out for determining the involvement of this bacterium in the genesis of OQDS and of the leaf scorching shown by a number of naturally infected plants other than olive. Isolation in axenic culture was attempted and assays were carried out for determining its pathogenicity to olive, oleander and myrtle-leaf milkwort. The bacterium was readily detected by quantitative polymerase chain reaction (qPCR) in all diseased olive trees sampled in different and geographically separated infection foci, and culturing of 51 isolates, each from a distinct OQDS focus, was accomplished. Needle-inoculation experiments under different environmental conditions proved that the Salentinian isolate De Donno belonging to the subspecies pauca is able to multiply and systemically invade artificially inoculated hosts, reproducing symptoms observed in the field. Bacterial colonization occurred in prick-inoculated olives of all tested cultivars. However, the severity of and timing of symptoms appearance differed with the cultivar, confirming their differential reaction.

Figure 1

Xylella fastidiosa colonies on BCYE agar medium growing in different stem-prints obtained after imprinting the fresh cut surface of the olive cuttings on the medium. (a) Shows a low number of colonies per spot; (b) and (c) show the high number of colonies growing mostly together.

Figure 2

Inoculated plant of ‘Cellina di Nardò’. (a) Plants 12 months post inoculation, on the left panel a systemically infected plant showing reduced growth, wilting and desiccation starting from the apical portion of the shoot. Mock-inoculated control on the right end side. (b) Severe symptoms of desiccation progressing rapidly in an infected plant 14 months post inoculation. The mock-inoculated plant (right end site) is symptomless.

Figure 3

Stunting and limited desiccation of a X. fastidiosa-inoculated ‘Leccino’ plant. Symptomless mock-inoculated ‘Leccino’ on the right end side.

Figure 4

Olive plants 24 months post inoculation. In all panels the first plant on the left is the mock-inoculated symptomless control. (a) All grafted plants of ‘Cellina di Nardò’ are desiccated except for the suckers pushed by the rootstocks which are symptomless, as it often occurs in the field. Plants of ‘Leccino’ (b) and ‘Frantoio’ (c) much less affected that those of the highly susceptible ‘Cellina di Nardò’.

Figure 5

Inoculated plants of ‘Cellina di Nardò’ 24 months post inoculation (Experiment C). (a) Severely damaged plants on the right of the mock-inoculated control; (b) and (c) infected plants × 7 and × 10 (see Table 3).

Figure 6

Inoculated oleander plants. Symptomless mock-inoculated oleander 14 months post inoculation; (a) leaf scorching in artificially inoculated oleander plants 10 months post inoculation (b) and 14 months post inoculation (c).

Figure 7

Extensive desiccation of artificially inoculated myrtle-leaf milkwort plants 20 months post inoculation. The mock-inoculated control is on the left end side.