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I have found that Rhododendrons are quite hardy plants and except for drooping leaves caused by cold winters, mildew and even vine weevil can be an issue as discussed here. Usually, they are generally problem free.
That being said, after growing rhododendrons for over 20 years, there is a selection of rhododendron pests including scale insects and vine weevils which can become a problem. There are also a number of diseases that can harm them which I’m always on the lookout for, some more serious than others. I cover the main ones to watch out for and discuss what to do about them below starting with pests.
Pests thats attach rhododendrons
The attack of the vine weevil insects is a family affair. The adult weevils eat the leaves of your plant as shown in the picture above while the young larva which looks like grubs head on down to the roots for their feast.
You can spot them not so much by seeing an actual weevil, but by the holes, they leave in the leaves. Their larvae live in the soil so they’re very difficult to see but they are more of an issue with rhododendrons grown in pots. These rhododendron pests are quite common and although the grubs can cause significant damage to the roots with pot grown plants, the adults will make leaves very unsightly and cause possible leaf drop, however, they usually won’t kill the plant.
What to do
Watch out for the adult weevils in the spring and at the end of summertime. When you see them, pick them off the leaves. Use sticky traps around the rhododendrons and the garden in general to trap them. If your rhododendrons are in containers, you can soak the roots in a vine weevil killer solution to kill the larvae before they turn into adults.
Scale insects are a problem on many plants. As rhododendron pests, they suck the sap out of the leaves. but that’s not all. They leave behind a stick, sweet substance called honeydew. Ants and aphids love this stuff and quickly come to your plants, causing additional damage. The honeydew can also lead to powdery mildew which is also a fungus.
What to do
Look on the underside of your leaves and on stems for tiny bumps or scales. Scale insects range from 1mm to 1cm, and they like to live on stems too. Spray the leaves with organic sprays or pesticides. It’s better to catch the scale insects when they’re young as they become more resistant the older they get. They also have a waxy coat which seems to give them some protection against pesticides so it usually takes a couple of applications before they die. You can also try and remove them by hand.
Powdery mildew is another disease that’s easy to live with and usually doesn’t cause any serious problems unlike on most plants, it’s not always easy to notice on rhododendron leaves. The main clue that your rhododendron plant has this disease is a fine white power on the leaves but you can also get brown, orange black spots on the underside of the leaves. If your rhododendron is in a shaded or wet location, you most probably have seen this disease. The issue is usually more the damp environment than the shady position. Don’t forget rhododendrons prefer shade and this is their natural environment.
What to do
Remove any dead or badly infected leaves that have white powder on them. This includes those that have already fallen to the ground. At the first signs of mildew, I usually spray with a fungicide to control the problem but you can also use copper sulphate. If the plant is not badly infected, you can also choose to just live it if it’s not too much of a problem.
As rhododendron like heavy rain, they’re susceptible to root rot. This occurs when the drainage in the soil isn’t adequate and water pools up. Root rot is a soil fungus that can affect plants that are spread out. The roots become waterlogged and start to rot away. The leaves eventually become brown, wilt and then die.
What to do
The main symptoms of root rot appear underground so you might be late to deal with the issue. Improve the drainage of the soil around your rhododendrons. If this doesn’t work, consider lifting them all up and moving them to a better-drained location.
I usually advise that you make sure your soil is free-draining before planting rhododendrons and if the ground is just too wet, consider growing them in large pots. You can even get some stunning dwarf varieties.
Azealia gall is a disease that affects rhododendrons but doesn’t kill the plant. However, it leaves the plant looking an unsightly mess. This is an airborne disease that creates small galls (they’re like boils) on the leaves. These start out a pale green colour and then turn slightly red before ending up white due to fungal spores. The leaf or flower is practically replaced by these ugly galls.
What to do
Remove any infected leaves and flowers. If the disease is widely spread in the plant, you need to remove the plant from your rhododendron bed. Keep an eye on your rhododendrons and do this as soon as you spot the galls. They’re not infectious until they’re white so getting at them early is key to containing the infection. Unfortunately, there are no fungicides to deal with this problem as far as I know.
This fungal disease attacks and kills the roots of plants. It spreads underground through root systems and kills the roots and stems of plants. This disease is one of the most destructive in the UK and isn’t fussy about which plants it infects.
Starting underground means that it becomes well-established before you can even notice the white mushrooms or fungus on the lower-down bark of the rhododendron. Look also for too-small leaves, lack of flowers and the bark cracking. Unfortunately, becoming infected means your plant dies.
What to do
Dig up the damaged plant and burn it with a garden incinerator. Don’t put it on the compost heap as honey fungus disease spreads easily and you don’t want to reintroduce it into your garden.
Bud blast and leafhoppers
If your leaves are turning brown, it could be because of bud blast. This is another fungal disease but this one kills flowers. It’s spread by the Rhododendron leafhopper. The leafhopper is a sap-sucking insect but in itself, it doesn’t do too much damage to your plants. However, it does carry around the bud blast disease. The infected flower buds turn brown and die.
What to do
Nothing can be done about the leafhopper insects. But you can pick off the affected buds and burn them to get rid of the fungal spores. The health of the plant isn’t significantly infected. This is one of the rhododendron pests and a disease that you (and your plants) can live with. Encourage the natural enemies of the rhododendron leafhopper into your garden. These include birds ladybirds, grounds beetle and wasps.
While pruning rhododendrons isn’t necessary, you can keep them in check and healthy by doing a light prune every so often. See my article When and how to prune rhododendrons for more details.