If you take good care of rhododendrons, discover diseases and pests in good time and fight them, you will get a lot of joy with little work. In addition, there is more protection and habitat for insects and birds.
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
Recognize and prevent location and weather damage
Rhododendrons love light, semi-shady places. They thrive on acidic, humus-rich soil with a pH between 4.5 and 5.5. Location and weather damage such as sunburn or frost can be seen on the leaves or buds.
First aid for sunburn
Direct midday sun causes sunburn in sensitive varieties. Recognizable by yellow or brown leaves while leaf surfaces show no damage in the shade.
What to do with frost damage?
Even with hardy rhododendrons, extremely cold, dry and windy weather will cause frost damage. When brown leaves or leaf edges die off and flower buds dry up, the plant needs more wind protection. Fir branches or reed mats offer sufficient protection.
Properly compensate for mineral or nutrient deficiencies
nitrogen deficiency
At first, older leaves show a uniform light green or yellowish tinge. Younger foliage later yellows. Premature leaf fall occurs in summer.
Soil compaction or insufficient nutrient supply cause the lack of nitrogen with the typical discoloration of the leaves. 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 turn yellow while the leaf veins remain dark, these are signs of insufficient iron or magnesium.
Rhododendrons are sensitive to calcareous irrigation water or calcareous soil with too high pH values. Not only "special rhododendron fertilizers" help here, but above all a pH-lowering soil improvement with special rhododendron soil.
Stunted growth due to lack of nitrogen
Stunted growth and yellowed leaves indicate a lack of nitrogen. Especially "the rhododendron variety Catawbiense" is considered a nitrogen eater. It needs a lot of humus, nutrients and horn shavings (32.93€) as nitrogen fertilizer.
Artificial fertilizers should only be used in March and May. If fertilized later, the shoots do not mature until winter. They don't lignify and freeze to death.
Remedy for diseases caused by fungi, bacteria or viruses
The healthy and generally robust raw dodendron are rarely attacked by fungi, bacteria or viruses. In the case of weakened plants with fungal infestation, it is advisable to remove the affected parts of the plant immediately, burn them or dispose of them in some other way 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 fungus genus Phytophthora includes more than 20 species. They cause root, trunk, shoot and fruit rot but also damping-off on rhododendrons and other plants. The fungal spores need water and light to germinate. Waterlogging and warm, humid weather favor an infestation.
Dry branches, brown terminal buds and brown leaf spots along the leaf midrib indicate infection with Phytophthora fungi. This also includes grey-brown discolored leaves that curl up without falling off. Among other things, the harmful fungus clogs the pathways and the plant dies completely or by shoots. The bark in the diseased root area is reddish-brown when cut. Healthy 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 susceptibilities. Deciduous rhododendron leaves have a grey-white coating and hardly grow. Special pesticides act as a countermeasure.
Rhododendron Bud Dying
During the winter, the buds of "hardy rhododendrons" will turn brown and die. The shrub does not sprout 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 body in spring.
Whether rhododendron leafhoppers are involved in fungal attack during bud death has not been clearly clarified. Therefore, chemical control of these animals should not be used. Since fungicides are not effective against the fungus, the only option is to remove suspicious dry buds by April at the latest. In good time before the yellowish larvae of the rhododendron leafhoppers hatch in May.
As a preventive measure, yellow boards can attract and destroy insects in summer.
No chance for the voracious insects
Rhododendron leafhoppers (Graphocephala coccinea), rhododendron skin or mesh bugs (Stephanitis rhododendri) or weevils such as the vine 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 conspicuous signs of damage, crippling and dirt on the leaves. The undersides of the leaves usually appear rusty and black droppings can be found. In isolated cases, the larvae of the weevil cause feeding damage to roots.
From gentle damage limitation to powerful weapons of destruction, everything is available on the market to destroy voracious insects. Strong anti-pest agents also have strong side effects on beneficial insects. Instead of radical toxins, environmentally friendly measures are recommended to keep the population within a manageable, non-damaging range.
tips and tricks
Each rhododendron variety requires special site conditions. In order to find the right variety for your garden, it is advisable to plant different types of rhododendrons. After a few months you will be able to see which rhododendrons grow and bloom luxuriantly without complex soil improvement.