As someone who loves gardening and takes pride in maintaining a healthy garden, I’ve learned that even the most carefully cared-for plants can sometimes fall victim to diseases. Rhododendrons, with their beautiful blooms and evergreen foliage, are no exception. Over the years, I’ve had to deal with various health issues affecting my rhododendrons, and it’s always a bit alarming when I notice something wrong—whether it’s wilting branches, brown leaves, or unexpected spots on the petals. In this guide, I’ll share my personal experiences with recognizing and treating the most common diseases that can affect rhododendrons, along with tips on how to prevent these problems before they take hold. If you’re a fellow gardener, I hope this helps you keep your rhododendrons thriving and looking their best.
Rhododendron: Recognizing and Treating Diseases
If your rhododendrons show wilting shoots, brown leaves, or black buds, a disease could be the cause. We present the most common diseases and offer tips on how to prevent and treat them.
Even with good care, rhododendrons are not always spared from diseases. For example, if a rhododendron shows brown leaves, several fungal diseases might be behind it. Below, we introduce the most common plant diseases and provide tips on how to best combat them.
Contents
Common Rhododendron Diseases
- Shoot Dieback
- Bud Blight
- Chlorosis
- Leaf Spot Diseases
- Rhododendron Rust
- Powdery Mildew
- Lobularia Disease (Ear Lobe Disease)
Shoot Dieback in Rhododendrons
Fungal infections with Phytophthora fungus (left) can be recognized by larger spots with often pale, dry tissue in the center. In the case of a root infection (right), entire branches of the rhododendron begin to wilt.
A serious disease in rhododendrons is shoot dieback, also known as Phytophthora wilt. The symptoms of this fungal disease include brown leaf spots with light, dry tissue in the center and wilting of the branches. The leaves of affected shoots lose their shine, turn gray-brown, and curl. In severe cases, the entire plant may wilt and die. The fungal infection can occur through the shoot tips or the roots.
If the infection occurs through the shoot tips, it’s important to act quickly by cutting back the affected shoots to healthy tissue. Disinfect the pruning tools and treat the rhododendron with a fungicide. A root infection, which is promoted by compacted and waterlogged soil, is more serious. Infected plants must be completely disposed of, and the soil should be better prepared. To prevent wilt disease, it’s important to ensure proper soil preparation and a well-ventilated planting location.
Bud Blight
If the buds of your rhododendron turn gray to brown and die off during the winter, it is likely bud blight, also known as bud dieback. A distinct symptom is the presence of dark hairs on the buds, which are the fungal fruiting bodies. The disease is caused by the fungus Pycnostysanus azaleae, which is transmitted by the rhododendron cicada (Graphocephala fennahi). The insects themselves cause no significant damage. Problems arise from egg-laying under the bud scales in the fall: these small injuries allow the harmful fungus to easily penetrate.
To prevent bud blight, you should take action against the cicadas early. In May, the greenish-yellow larvae hatch from the eggs laid the previous year and primarily live on the undersides of the leaves. Therefore, regularly inspect the undersides of the leaves and, if possible, use biological pest control products such as “Schädlingsfrei Neem” or “Spruzit Schädlingsfrei”. Affected buds should be carefully removed and disposed of in the household waste.
Chlorosis (Iron Deficiency)
Yellowing leaves on rhododendrons indicate chlorosis — a pathological discoloration that is often caused by a nutrient deficiency. In the case of an iron deficiency, the younger rhododendron leaves at the shoot tips lighten and turn yellow, while the veins initially remain green. With more severe deficiency, the entire foliage turns yellow and dries up from the edges. Iron deficiency is often caused by an excess of lime or a high pH in the soil.
For healthy growth, it’s important to ensure acidic, well-draining soil and lime-free watering. In some cases, transplanting the rhododendron may be necessary.
Tip: Choose Inkarho rhododendrons, which are less sensitive to higher pH levels.
If a rhododendron suffers from nitrogen deficiency, older leaves first show signs of pale discoloration and yellowing. Over time, all leaves turn pale green to yellow, including the veins. In the case of severe nitrogen deficiency, older leaves drop, leaving only a yellowish crown of leaves. To combat this deficiency, fertilize the rhododendron with sufficient nitrogen. Organic fertilizers like coffee grounds and horn shavings work well. Use bark compost instead of bark mulch.
Leaf Spot Diseases
Leaf spots on rhododendrons typically occur in warm, humid conditions and can be caused by various fungi. Brown to black spots with reddish borders on the upper and lower sides of the leaves indicate a Colletotrichum infection. Cercospora fungi cause irregular leaf spots in dark brown, which may also be reddish around the edges. A similar pattern of irregular, dark brown spots occurs with Gloeosporium fungi. Initially, the spots are small, but as the infection progresses, they may grow larger and merge.
Leaf spot diseases usually don’t cause significant harm to rhododendrons, but it’s recommended to remove and dispose of the affected leaves. To prevent leaf spots, ensure that watering is directed only at the root area.
Rhododendron Rust
Rust fungi also affect rhododendrons. Like leaf spot diseases, rhododendron rust causes yellow to brown spots on the upper sides of the leaves. A key distinguishing feature is the appearance of yellow to orange, later rust-colored, spore masses on the undersides of the leaves. To prevent further spread of rust, it’s important to promptly remove the affected plant parts and take preventive measures such as ensuring the right location and soil conditions. Water rhododendrons only from below. In severe cases, commercial fungicides can help.
Powdery Mildew
Powdery mildew is characterized by a white, powdery coating on the upper leaf surfaces, which often turns brownish in rhododendrons. This fungal disease primarily affects deciduous azaleas, which have softer leaves than evergreen rhododendrons. The disease is promoted by very dry soils and high temperature fluctuations. To prevent powdery mildew, regular mulching and watering are essential. Affected plant parts should be immediately removed and disposed of in household waste.
Warning: If you want to treat powdery mildew with home remedies, avoid using algae lime on sensitive rhododendrons. Instead, mixtures of milk or baking soda are recommended.
Lobularia Disease (Ear Lobe Disease)
If unusual growths and thickening occur on the leaves of your rhododendrons, it’s likely lobularia disease, also known as ear lobe disease. The fungus Exobasidium japonicum, which primarily affects young leaves of Japanese azaleas, causes the disease. Susceptible varieties include ‘Diamant’, ‘Brilliant’, and ‘Muttertag’. The spread of the fungus is favored by high humidity. Check your azaleas in April/May for potential infections and remove affected leaves. Fungicide treatment is only necessary for severe cases.
By keeping an eye on the health of your rhododendrons and recognizing the signs of common diseases early, you can maintain a vibrant and flourishing garden. Regular inspections and timely treatment can go a long way in preventing and combating these issues, ensuring that your rhododendrons continue to thrive.
Taking care of rhododendrons can sometimes feel like a challenge, especially when diseases strike unexpectedly. However, with the right knowledge and a proactive approach, you can often save your plants and keep them healthy for years to come. From recognizing the first signs of trouble to applying effective treatments, each step you take makes a difference in ensuring your rhododendrons continue to thrive. I’ve found that staying informed and being diligent about care really pays off. I hope this guide helps you feel more confident in caring for your own rhododendrons. Remember, a little attention can go a long way in keeping your garden beautiful and vibrant.
