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Xanthomonas arboricola pv., a bacterium harmful even for the cherry laurel

Attacks of the bacterium Xanthomonas arboricola pv pruni were detected in Europe since 2006 on plants of Prunus laurocerasus (cherry laurel). The bacterium was found on the most  widely cultivated and sold cultivar as "Otto Luyken", "Rotundifolia", "Novita", "Etna", "Herbergii", "Mischeana" and "Caucasica". It is mainly found in containerized plants, and it can be confused with other diseases and with Pseudomonas syringae, another bacterium that occurs in a similar way. There are reports of its presence in Italy (since 2006), the Netherlands, Germany and Spain.

X. arboricola pv. pruni overwinters on the twigs tips produced during the previous season. Pruned twigs and small branches and the leaves on the ground are considered as possible overwintering sites. In the spring, before budbreak, the bacteria start to multiply in the intercellular spaces of the plants, causing the breakage of the epidermid. Then, a visible lesion begins to develop and this favors the onset of the bacterial cancer. The inoculum is spread by the rain and wind infecting new leaves through the stomatal openings. Pruning is considered to be an important way to transmit the infection because of the use of infected pruners from diseased plants to healthy ones. The lesions developed on the leaf exude bacteria that cause secondary infections. Du Plessis (1983; 1987) suggested that the bacteria can systematically migrate from twigs to leaves. Following leaf infection, summer cancers develop in the green tissue, but usually the plant responds by producing a periderm, thus reducing the viability of the bacteria; Therefore, in plum and peach the summer cancer are of limited importance as wintering sites for the bacterium, or for the start of the infections in the following spring. The late infections that occur during the rainfall season just before and during leaf fall, however, are the primary source of inoculum because of the reduced resistance mechanism of the plants that leads to the production of the periderm.

In summary, the penetration ways of the bacterium are the stomata and the wounds caused by pruning, the leaves on the ground and the wounds caused by hail.

Warm seasons with temperatures between 19-28°C, accompanied by frequent rains with winds and dews are the most favorable for the spread of the infection. In laboratory, bacteria have survived 5 months with temperature from -2 ° C to + 2 ° C. The disease usually does not occur in arid regions.

The infection appears on the lower surface of the leaves, with pale green to yellow circular spots. These spots soon become more apparent by widening, becoming angular and veering to brown. The tissue immediately surrounding the infection can become yellow. The affected areas fall, usually after a color change to brown, but they can fall also before the color change. Often, a dark ring of diseased tissue appears at the edge of the hole. The spots are usually concentrated toward the tip of the leaf.

Quarantine and control. The most serious damages from the ornamental point of view are caused by defoliation that renders the plants not salable. Not only: the trade of diseased plants may be a source of inoculum for fruit-growing areas, given the ability of the bacterium to also attack many Prunus fruit. The bacterium is considered as a quarantine pest, inserted in the EPPO A2 list (http://www.eppo.int/QUARANTINE/bacteria/Xanthomonas_pruni/XANTPR_ds.pdf). There are no products allowed for its control and the only way to eradicate the disease is the destruction of infested plants. To not confuse the symptoms, similar to other diseases, laboratory analysis is required.

Found in some European nurseries, its damage lead to the destruction of affected plants. The diffusion problems related to fruit cultivation

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