Answer: I'm afraid there's no good news. Oleander leaf scorch is a disease found mainly in southern California. It is caused by the bacterium Xylella fastidiosa. As with other diseases caused by X. fastidiosa, the bacterium is vectored by insects, primarily sharpshooters, which feed on the water-conducting tissue (xylem) of the plant.
There is no known cure for oleander leaf scorch. Pruning out the part of the plant showing symptoms may help the appearance of the oleander tree or shrub but will not save the plant. The bacteria by then have already moved throughout the plant via the xylem, and limbs that show symptoms are only the first to become affected. Research indicates that some cultivars of oleander may express symptoms to lesser degrees than others and may live longer than other varieties when infected.
Because of the year-round abundance of the glassy-winged sharpshooter, currently available insecticides are not effective in stopping the spread of the disease. The best management may be early removal of plants infected with the oleander leaf scorch bacteria to reduce the source of inoculum, but there are no experimental data to validate this method. Although only a few plant species have been tested as hosts of the oleander leaf scorch strain of X. fastidiosa, it is possible that other plant species may harbor the bacteria without showing disease symptoms.
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