Oleanders (Nerium oleander) are everywhere in southern California; or at least they were. The showy and fragrant flowers come in several colors, and the low maintenance shrubs are drought and frost resistant, making them ideal as landscaping for parks, freeway median plantings, privacy hedges, and ornamentals for home gardens. Despite the fact that all parts of the plant are poisonous, only three deaths due to exposure to oleander were reported from 1985-2005, and, on the plus side, oleander’s toxicity makes it deer-resistant (source).
Oleanders began dying out in San Diego County in the 1990s, and by 2011, the Parks and Recreation Department of Santa Barbara began removing diseased oleanders throughout the city. The cause of what became known as “Oleander Leaf Scorch” (OLS) turns out to be a plant virus which is spread by a tiny insect known as a leafhopper.
“Leafhoppers, including a subgroup called sharpshooters, are in the family Cicadellidae. All have mouthparts that allow them to pierce the plant tissue and feed on plant juices. Most leafhoppers are about 0.25 inch long and slender. Leafhoppers are pests primarily because some are vectors of plant pathogens. The glassy-winged sharpshooter and the blue-green sharpshooter transmit a bacterial pathogen Xylella fastidiosa that grows in the xylem, or water-conducting tissue, of certain plant species. Xylella fastidiosa can cause a number of plant diseases in a variety of hosts. Thus far, strains of X. fastidiosa that cause oleander leaf scorch, Pierce’s disease of grapevines, almond leaf scorch, and alfalfa dwarf have been identified in California. The strains of the pathogen that infect oleander do not appear to infect grape and are genetically distinct from the other strains” (source).
“Major symptoms of OLS include scalloped chlorosis (yellowing) on the margin of the leaves, followed by marginal or tip leaf scorch. Twigs and branches die back. The bacteria clog the xylem (water-conducting tissue) of the oleanders, cutting off water supply to the leaves. OLS is able to kill oleanders within two years of the first appearance of symptoms. There is no way to cure an oleander of OLS once it has been infected, either with chemicals or with cultural methods. Oleanders cannot outgrow the disease. The disease can be spread through infected cuttings, so never propagate from symptomatic shrubs. The only way to slow the spread of the disease is to destroy infected shrubs. It is unlikely that OLS can be eradicated from California; the disease is already too widespread” (source).