Those who care for rhododendrons correctly, discover and combat diseases and pests in a timely manner, will reap a lot of joy with little work. In addition, there is more protection and habitat for insects and birds.
How do I control diseases in rhododendron plants?
To combat rhododendron diseases, pay attention to suitable site conditions, provide the plant with sufficient water and nutrients and remove affected parts of the plant in the event of fungal, bacterial or viral infestation. Protect the plant from frost and sunburn and use targeted pesticides if necessary.
Rhododendrons are considered robust and uncomplicated ornamental trees. However, if the location, water and nutrient supply are not right, deficiency symptoms or pest infestation occur.
The most common causes of rhododendron diseases:
- Location and weather damage
- Mineral or nutrient deficiencies
- Diseases caused by fungi, bacteria or viruses
- Diseases caused by insects
Identify and prevent location and weather damage
Rhododendrons love light, partially shaded places. They thrive on acidic, humus-rich soils with a pH value between 4.5 and 5.5. Location and weather damage such as sunburn or frost can be seen on leaves or buds.
First aid for sunburn
Direct midday sun causes sunburn in sensitive varieties. Recognizable by yellow or brown leaves while leaf surfaces in the shade show no damage.
What to do if there is frost damage?
Extremely cold, dry and windy weather causes frost damage even to winter-hardy rhododendrons. If brown leaves or leaf edges die and flower buds dry up, the plant needs more wind protection. Fir branches or reed mats provide sufficient protection.
Correctly compensate for mineral or nutrient deficiencies
Nitrogen deficiency
At first, older leaves have a uniform light green or yellowish color. Later, younger foliage turns yellow. In summer, premature leaf fall occurs. Soil compaction or insufficient nutrient supply cause nitrogen deficiency with the typical leaf discoloration. Appropriate nitrogen fertilization compensates for the deficiency symptoms and strengthens the plant.
Lack of iron and magnesium
If young rhododendron leaves lose their dark green color and yellow while the leaf veins remain dark, these are signs of too little iron or magnesium.
Rhododendrons react sensitively to calcareous irrigation water or a calcareous soil with too high a pH value. Not only “special rhododendron fertilizers” help here, but above all pH-lowering soil improvement with special rhododendron soil.
Stunting due to nitrogen deficiency
Stunted growth and yellowed leaves indicate a nitrogen deficiency. In particular, “the rhododendron variety Catawbiense” is considered a nitrogen consumer. It needs a lot of humus, nutrients and horn shavings as nitrogen fertilizer.
Nitrogen application through artificial fertilizer should only be done in March and May. If fertilizer is applied later, the shoots will not mature until winter. They do not become woody and freeze to death.
Remedy for diseases caused by fungi, bacteria or viruses
The he althy and generally robust raw dodendron is rarely attacked by fungi, bacteria or viruses. For weakened plants with fungal infestation, it is recommended to immediately remove the affected plant parts, burn them or otherwise dispose of them from the garden! This is the only way to permanently avoid fungal infestation in the garden.
Effectively combat rhododendron wilt, shoot dieback and branch dieback
The fungal genus Phytophthora includes more than 20 species. They cause root, stem, shoot and fruit rot as well as damping-off diseases on rhododendrons and other plants. The fungal spores need water and light to germinate. Waterlogging and warm, humid weather promote infestation.
Dry branches, brown terminal buds and brown leaf spots along the leaf midrib indicate infection with Phytophthora fungi. Gray-brown discolored leaves that curl up without falling off are also included. Among other things, the harmful fungus clogs the ducts and the plant dies entirely or in shoots. The bark in the diseased root area is reddish-brown when cut. He althy tissue, on the other hand, appears bright and juicy.
Powdery mildew on rhododendrons
Little is known about powdery mildew – Microsphaera azaleae – on rhododendrons. The individual varieties also have different susceptibility. Summer green rhododendron leaves are gray-white and hardly grow. Special pesticides act as a countermeasure.
Bud death on rhododendron
Over the course of the winter, the buds of “hardy rhododendrons” brown and die. The shrub does not produce any buds the following spring. The most common cause of this is the harmful fungus Pycnostysanus azaleae. The fungus becomes visible through small black hairs on the fruiting bodies in spring.
It is not clearly clear whether rhododendron leafhoppers are involved in fungal infestation when buds die. Therefore, these animals should not be controlled chemically. Since fungicides do not work against the fungus, the only option is to remove suspicious dry buds by April at the latest. Just in time before the yellowish larvae of the rhododendron leafhoppers hatch in May. As a preventive measure, yellow panels can attract and destroy the insects in summer.
No chance for the voracious insects
Rhododendron leafhopper (Graphocephala coccinea), rhododendron skin or web bugs (Stephanitis rhododendri) or weevils such as the black weevil (Otiorhynchus sulcatus) are among the animal pests. They eat leaves, roots or suck nutrients from the plant and lay their eggs.
Depending on the pest, there are noticeable lesions, deformities and dirt on the leaves. The undersides of the leaves usually appear rusty and you can find droplets of black droppings. In individual cases, the larvae of the weevil cause damage to roots.
From gentle damage limitation to powerful weapons of destruction, everything is available commercially to destroy voracious insects. Strong agents against pests also have strong side effects on beneficial insects. Instead of radical poisons, environmentally friendly measures are recommended to keep the population within a manageable, non-damaging range.
Tips & Tricks
Each rhododendron variety requires specific location conditions. In order to find the right variety for your garden, it is recommended to plant different types of rhododendrons. After a few months you will be able to tell which rhododendrons are growing and blooming profusely without any extensive soil improvement.
