Growing plants is usually an enjoyable and rewarding hobby, but it can be quite discouraging when a tough, generally trouble-free shrub such as an oleander (Nerium oleander) develops a problem that interferes with its growth or flowering. An African native, oleander grows in U.S. Department of Agriculture plant hardiness zones 8 through 10, covering itself in showy, colorful flowers in summer. If you're growing an oleander and it starts doing poorly, an important first step is to identify the cause of the problem, then take appropriate corrective measures to restore its health.
All parts of an oleander are highly poisonous, so it's not a good choice for an area frequented by children or pets. Contact with the plant can also cause skin irritation, so wear gloves when working on the plant.
An oleander bush is susceptible to several different types of bacterial diseases, each causing different symptoms.
Bacterial blight is caused by a bacterium called Pseudomonas syringae. You might see buds or young blossoms die back, along with die-back of green, growing tips. Leaf veins might darken or even turn black, and dark spots could appear on leaves. Dark, soft spots called cankers can also appear on the plant's stems. To solve this problem, use sharp shears to prune off any branches with symptoms, cutting behind the diseased area and into healthy wood. Do this during dry weather, when re-infection is less likely. No effective controls exist for this disease, but if symptoms only appear on leaves and buds and affected parts are removed, the plant should recover; a severe problem that includes main stems might kill the shrub.
Always wipe pruning blades thoroughly with rubbing alcohol between each cut to prevent spreading disease.
Oleander Leaf Scorch
Oleander leaf scorch is caused by growth of a bacterium called Xylella fastidiosa; this organism causes leaves to turn yellow and droop, with leaf margins eventually turning brown. These leaves eventually die and the disease can spread from branch to branch, especially during warm weather. Symptoms resemble those of lack of water, but only affect some branches, helping distinguish it from problems arising during drought that affect the entire plant. Although pruning away affected branches can improve the plant's appearance, this disease has no treatment and will eventually destroy the oleander.
Oleander knot causes knot-like growths with uneven surfaces on stems and branches. Affected stems might die back, but the entire plant usually survives. Remove affected stems by pruning them off as soon as they appear, to help prevent further spread of the problem. Bacteria grow in cuts in the plant's bark and spread when water runs through affected areas. The disease also spreads on tools used to cut flowers or to prune away affected stems. You can help prevent this by cleaning tools as you work, wiping them with alcohol or household bleach, but wear gloves to protect your skin. Also avoid overhead watering; instead, use drip irrigation or a soaker hose to keep foliage dry and prevent spread of bacteria in contaminated water.
When pruning an oleander, always remove cuttings from the area immediately, even if there's no evidence of disease, to help prevent fungal growth or spread of any problem that's not obvious.
An oleander bush might also develop one of several fungal disorders, each with its own set of identifiable symptoms.
Leaf Spot Diseases
Several different types of fungi cause problems collectively called leaf spot diseases. As the name suggests, spots appear on the plant's leaves, sometimes as yellow or brown patches that can be small or cover a large part of the leaf. A severe problem might cause some leaves to fall; if you see this happen, prune off affected branches by cutting into healthy wood behind the problem area, disposing of cuttings as you work. Keeping foliage dry by watering at the plant's base also helps control this problem, as does watering early in the day so foliage dries quickly. Removing plant debris from under the shrub regularly also helps prevent spread of the fungus.
You might also see a blackish layer on the plant's stems and leaves that resembles soot -- called sooty mold. The mold grows where insects that feed on the plant have deposited a sticky substance called honeydew. Mold-coated leaves can slow the plant's growth and some leaves might eventually die. To control sooty mold, destroy the soft-bodied, sucking insects that deposit honeydew by spraying the plant until it's dripping wet with insecticidal soap, diluted at a rate of 5 tablespoons of water per gallon of water, and repeat every two weeks as needed.