By: Mary H. Dyer, Credentialed Garden Writer
As far as oleander diseases go, oleander knot diseases aren’t the worst. In fact, although it can cause plant dieback, oleander knot generally doesn’t result in long-term damage or death of the plant. However, the warty galls cause unsightly, distorted growth. If oleander knot disease has afflicted your oleander plant, read on to learn about treating the disease, also known as nerium canker.
What is Oleander Knot Disease?
Oleander knot is the result of a type of bacteria (Pseudomonas syringae pv. Savastanoi) that enters the oleander plant through wounds and injured or scarred areas. The bacterium is systemic, resulting in the development of knots or bacterial gall on oleander flowers, leaves and stems; and stunted, deformed seed pods. The disease is widespread in Arizona and other areas where oleander plants are popular.
Oleander knot disease is most common after cool, damp springs. The bacteria requires a wound to enter the plant and often finds a convenient route through areas affected by winter damage, or by improper pruning. It also spreads by contact with contaminated water, infected garden tools, or even human hands.
Treating Nerium Canker
Prune infected plant parts, but only when the foliage – and the weather – is dry. Treat the pruned area with a 10 percent bleach solution to prevent entry of the bacteria. Wipe pruning tools with rubbing alcohol or a bleach solution between each cut, and after the job is completed. You can also use a commercial disinfectant, applied according to label recommendations.
Water oleander bushes carefully at the base of the plant to keep the foliage dry. Avoid watering with sprinklers, which can spread the pathogens to uninfected plants. Overhead watering is especially risky after pruning an oleander.
If the infection is severe, apply a copper fungicide or a Bordeaux mixture in autumn. Continue to spray periodically when new growth emerges in spring.
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Grown for its brilliantly hued flowers, oleander (Nerium oleander) is a tough evergreen shrub with narrow, dark green leaves. It is often used for screens, hedges and borders along driveways and paths. Oleander grows in U.S. Department of Agriculture plant hardiness zones 8 through 10. It grows about 20 feet tall and does well in containers on the patio or poolside. The plant may have several main stems or be trained to a central stem for a treelike appearance. All parts of the oleander plant are highly toxic. Don't burn the plant because the smoke is also toxic.
Proper irrigation and fertilization are important to the health of oleander plants. Salt toxicity and excessive water can lead to plant death. Too much water can smother and kill roots. Symptoms of prolonged over-watering include wilted, discolored and dying leaves that drop off prematurely. Salt toxicity is often due to poor drainage or over-feeding. Symptoms of salt toxicity include brittle, brown leaves and stunted plant growth. Roots affected by salt toxicity often die, killing the plant. Proper care of the oleander plant is the best method of control for these potentially fatal issues.
Is Oleander Toxic?
There are multiple poisonous elements to oleander. All of its parts—the flowers, leaves, stems, sap, etc.—are toxic, even when they’re dried or burned. The plant is a potent source of cardiac glycosides, which can cause irregular heart activity. Even eating a single leaf or drinking water from a vase with an oleander flower can be lethal to a small child, though the mortality rate is generally low in humans.
Oleander can affect animals in the same way based on their size and how much they ingest. Poisonings typically occur in farm animals, such as cows and horses, when they are allowed to graze in areas where oleander is present. Plus, dogs and cats can be poisoned when they are allowed to investigate an oleander plant on someone’s property. Even wild birds succumb to the plant’s toxicity.
If you do choose to keep oleander on your property, make sure no children or pets can come in contact with it. Also, wear long sleeves, pants, and gloves when working with your shrub, and wash your hands afterward.
Symptoms of Poisoning
Symptoms generally show up within hours of contacting the plant. And in some cases, especially with small animals and young children ingesting the plant, the first sign that a poisoning took place is, unfortunately, death.
The following are some moderate to severe symptoms that a person or animal might experience from oleander:
- Irregular heartbeat
- Low blood pressure
- Vision disturbances
- Diarrhea, nausea, vomiting, or stomach pain
- Loss of appetite
- Difficulty breathing
- Rash or hives
Moderate cases last one to three days on average severe cases might require hospitalization. If you suspect oleander poisoning has taken place, contact a health care provider or poison control center immediately.
Pests or Problems
Although the toxic properties of oleander protect it against certain enemies like deer, it's not without vulnerabilities. Disease-wise, it can become infected with Sphaeropsis gall, which usually becomes apparent with a proliferation of shoots and branches arising from diseased portions of branches (a "witches broom" effect). If this happens, prune branches at least 6 inches below where symptoms are seen. Prune back further if any discoloration from the fungal growth in the wood is noticed in the cut stem.
Oleander moth. Photo by Chazz Hesselein, Alabama Cooperative Extension System, Bugwood.org
It is also susceptible to false oleander scale, aphids, and the most damaging, oleander caterpillars. The colorful oleander moth (sometimes called the "polka-dot wasp moth" or "Uncle Sam moth") lays its eggs on new leaves at the branch tips, where the larvae will feed. Oleander caterpillars can inflict serious chewing damage if left unchecked they can completely defoliate a plant in as little as a week.
If you find oleander caterpillars, it's not too late. Removing larvae-infested foliage is the most environmentally friendly method of control. Oleander sap can cause skin and eye irritation, so be sure to wash your hands immediately after touching any cuttings, or better yet, wear disposable gloves while pruning. Hand-pick the non-stinging caterpillars or cut off damaged foliage and the larvae feeding on it. Young caterpillars only scrape the leaf tissue, so this initial damage is easy to spot and can help cut short a full-on infestation. Put caterpillars (or the plant matter covered with them) in a plastic bag and freeze for 24 hours to kill the pests.
Mature caterpillars often migrate up walls of nearby buildings and pupate near the eaves. Removing these cocoons can help manage the next generation of this pest.
Oleander caterpillar. Photo by
Anne W. Gideon, Bugwood.org
How to Cure Oleander Leaf Scorch
Cultivated outdoors across U.S. Department of Agriculture plant hardiness zones 8 through 11, oleanders (Nerium oleander) are evergreen shrubs prized for their flowers, available in a range of colors and different forms, as well as an ability to withstand harsh conditions including drought, wind, heat, pollution and salt exposure. Oleander leaf scorch, a disease caused by a strain of the bacterium Xylella fastidiosa, appears first on one or more branches as leaf yellowing and drooping followed by a browning of leaf margins and eventually leaf death. As the disease progresses, more branches exhibit leaf yellowing, browning and death, and the affected oleander eventually dies. No cure exists for oleander leaf scorch, which is spread by sharpshooter insects, but certain care and management practices can at least temporarily improve the appearance of an infected oleander and minimize the potential for further disease spread.
Water the infected oleander deeply during extended dry spells, providing it with 1 to 2 inches of water weekly when weather is warm and dry. Apply the water over the soil around the oleander slowly and evenly, allowing it to penetrate the soil rather than run off the soil surface. Water stress can compound the effects of oleander leaf scorch.
Prune off infected portions of the oleander as soon as you notice the symptoms of the disease, making each cut 2 to 3 feet below visible symptoms. This will not remove the infection, as the bacterium has already spread through the plant's xylem tissue, but it may help to maintain the oleander's attractive appearance and can slow disease progression, giving the shrub a few more years in the landscape.
Wipe pruning tool blades down with a household disinfectant, 10 percent bleach solution or rubbing alcohol, and rinse them with distilled water or let them air dry after each cut and especially between plants and uses. This disinfecting will help to prevent the spread of the disease through pruning cuts.
Remove oleander plants that are completely infected. If you have several healthy oleanders in the landscape at risk for infection, remove and destroy the infected oleander immediately rather than just prune off unsightly portions of the shrub.