One of my favorite plants here in Encinitas and California for that matter is now under attack and will cost us tax payers millions of dollars in the future because of dead or dying plants.The ubiquitous Oleander used throughout Southern California for it’s color, unusual tolerance to wind and exhaust fumes and a rapid growth rate make it a perfect choice when creating a screen or barrier between oncoming headlights or an unsightly neighboring property.
Growing up with this plant running the length of our property line, (which my dad hated pruning) I can still hear my father’s admonitions ringing in my ears about not eating the leaves since they were deadly poisonous.
And he was right, even bees collecting nectar from the flowers will create a deadly honey that can eventually kill the hive. But that’s another story.
What I am talking about today is the fact that practically all Oleanders are now susceptible to a fatal disease known as leaf scorch.
As a homeowner you might have noticed the Oleanders in your neighborhood or own yard turning yellow and drying at the tips of the branches.
As a tree guy, I originally thought the cause was drought conditions or just old plants reaching the end of their life cycles or both.
Usually, a hard pruning on these types of plants will remove the dead material and with the advent of a good soaking from the hose, fantastic new growth will generate since the newly pruned plants have so much root mass to draw from.
This isn’t the case.
Oleander leaf scorch is a disease found mainly in southern California. It is caused by the bacterium Xylella fastidiosa that is from the same species of bacterium that causes Pierce’s disease on grapevines and almond trees. At this time, there is no cure and it is always fatal.
The strain of this bacterium that kills Oleanders is slightly different from the one that causes Pierce’s disease and removing Oleanders will not reduce the source of x. fastidosa that affects grape and fruit production. Like other systemic plant diseases, the bacterium is vectored by insects, primarily sharpshooters that feed on the water-conducting tissue or xylem of the affected plant.
Most infected Oleanders exhibit signs of yellowing and drooping. Drought stressed leaves will yellow uniformly or along the central leaf vein, whereas in leaf scorch disease, the yellowing of the leaves progresses from the tip or margins of the leaves inward.
Typically, an adult sharpshooter acquires the bacteria from an infected plant and will then carry the bacteria with it for life. These bacteria live within the mouth of the insect and infect each new plant that it lands and feeds upon.
Once inside the tissues of the Oleander, the bacterium is confined to the structures of the water transport system known as the xylem. It will grow rapidly in this nutrient rich environment and will actually plug the xylem tubes transporting the flow of water to the affected branches. Because this is a water related disease, summer heat and drought often compound and hasten the demise of Oleanders infected with this bacterium.
The glassy-winged sharpshooter is approximately 1/2-inch long, not very big and has transparent wings.
It is larger than most other sharpshooters and is dark brown in color. It is interesting that this insect excretes large amounts of liquid when feeding and on heavily infested plants, this will give the leaves or fruit a whitewashed appearance.
These bugs are very prolific and have two generations per year. Most will overwinter as adults and lay eggs in the spring.
A second generation again lays eggs midsummer creating the over-wintering adults that continue the reproductive cycle.
Since there is no known cure for this disease, pruning out the part of the plant showing symptoms may help the appearance of the Oleander tree or shrub but it will not save the plant. Typically, most infected plants will succumb over a three to five year period and be a host for bacteria during this entire time.
Conventional wisdom now suggests the removal of any plants showing signs of infection to help slow down the vector rate of this disease.