Question: My oleander have been dying. The leaves have yellowed and drooped and then turned brown. What could the problem be?
J.D., Santa Ana
Answer: The problem with your oleander sounds like oleander leaf scorch, which was first detected in the early 1990s in the Tustin area.
Typical symptoms are as you described, which include yellowing and drooping leaves on one or more branches and eventual leaf darkening and browning, which is why it is called leaf scorch.
As the disease progresses, more branches are affected and the plant itself eventually dies, typically within three to five years.
Symptoms of this disease can be expressed year-round, but develop more quickly in warm weather and are most noticeable in late spring and summer.
There is no reliable cure known for the disease, which is caused by the same bacteria that causes Pierce's disease in grapevines and almond leaf scorch.
The bacteria are carried by sharpshooters, which are insects that feed on the water-conducting xylem of the oleander. When the bacteria enter the xylem, they may spread to such a degree that the xylem vessels become blocked and water transport is cut off to the affected parts of the plant.
The dominant carrier of oleander leaf scorch in Southern California is the glassy-winged sharpshooter (Homalodisca coagulata), which acquires the bacteria from infected plants while it is feeding on the xylem sap.
When it goes to another plant to feed, it takes the bacteria with it and then deposits the bacteria in that plant, thereby initiating another cycle of disease in the new host.
Other sharpshooters that are likely vectors of oleander leaf scorch in Southern California include the smoke-tree sharpshooter (Homalodisca lacerta) and perhaps the primary source of oleander leaf scorch in the Coachella Valley, the blue-green sharpshooter (Graphocephala atropunctata).
The symptoms of oleander leaf scorch are sometimes confused with those caused either by drought or salt toxicity.
However, the yellowing pattern in the case of drought differs in that all of the leaves yellow and droop at the same time, and the plant will usually recover when watered unless the drought has been protracted and severe.
A plant with oleander leaf scorch will not recover when watered because the bacteria have plugged up the xylem tubes and have restricted water flow to the affected branches.
Drought-stressed leaves also yellow uniformly or along the central leaf vein, whereas with leaf scorch disease yellowing of leaves progresses from the tip or margins of leaves inward.
In the case of salt toxicity, there generally is not any drooping of leaves, even though the leaves characteristically demonstrate marginal browning, and the plant generally will improve if the salts in the soil are leached down beneath the root zone.
Because sharpshooters are abundant throughout the year, and because they are very efficient vectors of the bacteria that causes the disease, insecticides that are currently available have not been effective in stopping the disease from spreading.
The best way to manage the disease may simply be early identification and removal of diseased plants. Pruning out diseased portions of a plant may help its appearance but will not save it.
Written by University of California master gardener Edward A. Shaw of Laguna Beach.
Have a problem in your yard? University of California Cooperative Extension (UCCE) Master Gardeners are here to help. These trained and certified horticultural volunteers are dedicated to extending research-based, scientifically accurate information to the public about home horticulture and pest management. They are involved with a variety of outreach programs, including the UCCE Master Garden hotline, which provides answers to specific questions. You can reach the hotline at (714) 708-1646 or e-mail email@example.com. Calls and e-mail are picked up daily and are generally returned within two to three days. Please include your name and city of residence.