It’s very worrying when the leaves on a beloved Rhododendron begin to turn black. There are many reasons why this might happen, and it can be challenging to work out what’s causing the issue.
To help you understand what’s gone wrong and fix the problem, I’ll take you through the possible reasons why the leaves on your Rhododendron might be turning black and what you can do to return your plant to full health.
The most common reason for Rhododendron leaves turning black is a fungal disease. Several types of fungal infections can affect rhododendrons, the most common being Phytophthora root rot and Anthracnose leaf spot.
Let’s look at these and the other possible causes of blackening leaves in more detail:
Root Rot and Stem Rot
Rhododendrons are prone to root rot. Their roots sit close to the top of the soil and easily become infected by water and soil-borne fungi such as Phytophthora.
Phytophthora spores become active when the temperature is over 59°F (15°C) and can quickly cause severe damage if the plant’s roots are constantly wet.
When a plant becomes infected with root rot fungus, it may display various symptoms from wilting, yellow leaves to blackened leaves and sudden death. As the fungus attacks the plant’s roots, it can’t absorb the water and nutrients it needs – leading to the symptoms.
Stem rot is caused by fungi in the Rhizoctonia, Fusarium, or Pythium families, which are also soil-bourne. Stem rot causes similar symptoms as root rot – a wilting plant and dying leaves which range from yellow to black.
Once stem rot takes hold, the plant struggles to transport water and nutrients up its stem, resulting in dehydration and nutrient deficiency symptoms.
With both stem and root rot, prevention is better than cure. Once a plant shows symptoms, it may be too late to save it. If your plant has turned completely black, it’s best to dispose of it and try again with a new plant.
If your Rhododendron only has mild symptoms, it might be possible to save it. Immediately re-pot the plant in fresh compost treated with a fungicide developed for the particular problem.
Use clean, sharp scissors to cut away any diseased roots and leaves.
It is vital never to re-use infected compost, as spores of some of these fungi can live for up to five years in soil.
The best way to prevent both root and stem rot is to avoid excess water collecting in your plant’s pot or around the stem.
Never let your plant sit in water for more than an hour at a time, and if you water it from above, make sure that water doesn’t collect around the stem.
A 2-inch layer of pine bark on top of the plant’s compost may also help inhibit the fungi that cause root and stem rot.
Anthracnose is a fungal disease mainly caused by members of the Colletotrichum and Gloeosporium families of fungi. It affects many plant species, as can be a particular problem for Rhododendrons.
It causes dark brown or black spots on the leaves, which eventually spread and cause the leaf to die off. Severe cases can quickly lead to the death of the plant.
Anthracnose spores collect in flower buds, dry organic matter, and plant petioles (the stem connecting a leaf to the main stem). They spread by wind or water splashing onto a plant and activate in warm, humid weather to cause the disease symptoms.
The severity of anthracnose depends on the fungus causing it. Because this can be very difficult to determine, it’s best to get a sample of the infected plant tested in a laboratory to determine the course of action to take.
In the meantime, you can try a fungicide. There are several types of fungicide available to treat anthracnose.
To minimize the threat of anthracnose infection in your plants, remove fallen leaves from the surface of the compost. Avoid allowing water to splash onto your plant by watering from below rather than above.
Sooty mold is a very common problem affecting rhododendrons. Although it does very little harm in itself, it can be a sign of a severe pest infestation.
As the name suggests, sooty mold is a dusty black fungus that grows on the leaves and stem of a plant. It feeds on honeydew, excreted by various types of sap-sucking insects. Sooty mold spores spread through the air before activating when they stick to honeydew on a plant.
If you see sooty mold on your plant, you can wipe it off with a slightly damp tissue or cloth. Make sure you dispose of the tissue or wash the cloth thoroughly afterward to avoid spreading the spores.
The presence of sooty mold indicates a pest infestation, so you should check your plant for sap-sucking pests such as aphids and scale. Most of these pests are easy to treat with horticultural soap or oil sprays.
Bacterial Leaf Spot
Bacterial leaf spot occurs when bacteria of several families, including Erwinia, Xanthomonas, and Pseudomonas, enter an opening in a plant’s leaf and reproduce.
This usually happens when water splashes onto the plant’s leaves, either from rain or watering. Bacteria collect in organic matter on the soil’s surface or on the leaves, stems, and flowers of a plant.
The disease causes yellow, brown, or black spots that start small before becoming larger; often, the spots are ringed with yellow and have a wet or mushy texture. Sometimes the disease manifests as brown or blackened leaf edges that are dry and papery.
The severity of the disease depends on the bacteria in question. Some types are little more than an unsightly nuisance, but others can quickly lead to the death of the plant. All types spread easily and are most active at 77 – 86 °F (25 – 30 °C).
As with fungal diseases, bacterial leaf spot is difficult to treat once it has taken hold, so it’s best to prevent the disease in the first place. Always remove fallen leaves from your plant’s pot and water them from below to avoid splashing water on the leaves.
If you notice symptoms of bacterial leaf spots, immediately remove the infected parts of the plant and dispose of them away from other plants. Don’t put them in the compost bin!
If you catch the disease early enough, a copper fungicide can be an effective treatment. However, once the disease has taken hold, there is no known chemical treatment. It might be best to dispose of the plant to avoid infecting others.
Pests love Rhododendrons! Now, the insect infestation is unlikely to cause your Rhododendron leaves to turn black.
Insects such as aphids, scale, and thrips produce a sticky substance called honeydew, which is a favorite food of the fungi that cause sooty mold.
Treat insect infestation as soon as you notice the signs. Wilting or yellowing leaves could be a sign of pest infestation. Look out for clusters of insects underneath leaves, near the stems. You might also notice sticky honeydew on the plant.
Most sap-sucking insects are soft-bodied and can easily be treated using a horticultural soap solution spray. You can buy these from garden centers or make your own using water and a liquid soap such as Castile.
For more formidable pests like scale, you can use a horticultural oil spray or treat the plant with neem oil which penetrates their waxy armor.
Rhododendrons don’t need a very humid environment to be happy. Still, they need a humidity level of at least 30 – 40% to thrive. If there is not enough moisture in the air, the plant quickly loses water through its leaves.
This causes the leaf cells to collapse and the leaves to dry out and curl before turning brown and eventually black.
Low humidity can kill a plant in a matter of days, so if you notice the signs, take steps immediately.
Keep your Rhododendron in moist, well-draining soil in a position that gets plenty of shade during the hottest parts of the day.
As plants emit water from their leaves, keep your Rhododendron around other plants so that it benefits from the increased humidity.
Humidity trays are a simple way to increase local humidity. Place several large pebbles in a tray of water and stand your plant on top so that the bottom of the pot is not touching the water.
The water will evaporate around your plant and give it the humidity it needs. If the environment is very dry, you could use an electric humidifier.
In extremely dry weather, you could try to increase the humidity level around your plant by misting water nearby.
Make sure you don’t get too much water on the leaves or stem, though, as this can encourage fungal diseases. Because of the risk of spreading disease, you should only mist occasionally.
Heat Stress and Sunburn
Rhododendrons don’t do well in temperatures above 90°F (32°) or full sun. Too much heat or direct sunlight will cause the cells in the leaves of your plant to dry out and collapse. The first sign is wilting, followed by yellow, brown, and eventually dry, black leaves.
While the odd bit of over-exposure to sunlight or heat won’t kill your plant, if it is getting so much sun that it burns the leaves black, it’s essential to remedy the situation immediately.
- Keep your Rhododendron in a situation where it receives minimal direct sunlight
- Don’t keep your plant in a situation that regularly exceeds 90°F (32°C)
- Never keep your plant on a sunny windowsill
Even though Rhododendrons prefer a good amount of shade, they do need some light to thrive. They prefer dappled light and shade, especially when young.
The first indication that your plant might need more light is pale, straggly growth that reaches towards the nearest light source. Eventually, leaves may blacken and die off.
- Make sure that your plant gets plenty of bright, indirect light throughout the day
- A position out of direct sunlight, 2-3 feet away from a window, is ideal
- If you notice pale, straggly growth, move the plant to a brighter position
Over or Underwatering
Although Rhododendrons like moist soil, they don’t like having their roots constantly submerged in water. Not only can wet roots lead to root rot, but they can lead to the plant not being able to absorb nutrients, oxygen, and carbon dioxide through its roots – eventually drowning or dying of lack of nutrition.
If you’ve been watering your plant regularly, but it has dry, yellow, brown, or black leaves, you could be overwatering.
Rhododendrons do like quite a lot of moisture, however. You should aim to keep the compost consistently moist but not waterlogged. If your plant is wilting and has dry, yellow, brown, or black leaves, it might need a drink.
Water your plant only when it needs it – always check first! Never allow your plant to sit in waterlogged soil. If you’ve overwatered once, your plant will probably be OK. Just make sure not to water again until the top inch or two of compost is dry.
To water your Rhododendron, place the pot in a tray of water for an hour or so before removing it. Avoid watering from above as this increases the chances of spreading the bacteria and fungal spores that cause disease.
You can check the moisture content of the soil by using your finger or a moisture meter. You can also get a good idea by picking up the pot – if it feels ‘top heavy’ like the plant is heavier than the pot, your plant needs a drink!
Over or Under Feeding
All plants need several nutrients to survive, but feeding them too much can quickly cause serious problems. Rhododendrons don’t need a lot of feeding, but they do need an acidic soil of pH 4 – 6.5 to absorb nutrients properly, particularly iron.
Most nutrient deficiencies cause yellowing leaves that eventually die off. It takes a long time before the leaves blacken and die off, so they’re pretty easy to treat.
Overfeeding is a bigger problem and can kill a plant quickly. If you’ve recently fed your plant and the leaves are yellowing, turning brown or black, or dropping off the plant, this could be the issue.
- Feed your plant with a fertilizer specially developed for rhododendrons
- Always follow the instructions
- Rhododendron fertilizer acidifies the soil, so if your soil is already acidic enough use a general-purpose fertilizer
Salt Build-up in Soil
Over time, salts from tap water and fertilizer can build up in compost and collect around a plant’s roots. This can prevent the plant from absorbing water, oxygen, and nutrients and, in high concentrations, can even burn the roots.
The plant displays symptoms of dehydration and nutrient deficiency, including blackened leaves, even though you might be giving it plenty of water. You might also see a white crust forming on the top of the compost.
- Water your plant with rainwater, distilled or filtered water if possible
- Re-pot the plant in fresh compost once a year
- Rinse salt build-up from roots when re-potting
How to Prevent Blackening of Rhododendron Leaves
- Keep your plant out of direct sunlight
- Maintain moist but not wet compost with pH 4 – 6.5
- Water your plant from below, never above
- Remove fallen leaves and other debris from the container
- Apply a layer of pine bark to the top of the compost
- Treat pest infestations immediately