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Rhododendron pests, diseases and other problems
Last Updated on January 21, 2020 by John
Rhododendrons are beautiful evergreen shrubs of which there are hundreds of varieties. They flower mainly in the springtime but can be planted either in spring or autumn and once planted will thrive in well-drained, shaded and wet conditions particularly in places that get ample rainfall. That said there are still some pests, diseases, and other problems you should be aware of when caring for your rhododendron.
Drooping leaves is a common problem which we recently talked about here. Other problems are usually caused by pests such as Vine weevil and fungus such as powdery mildew.
The biggest pest you will likely face is Vine Weevil and its easy to spot simply from the damage they do to the leaves as pictured above. The adult beetles eat the leaves and then it’s baby grubs which live in the soil eat the roots of your rhododendron. In the worst cases, this eventually causes your plant to die, especially if grown in pots.
The adult weevils are most active in the springtime at the end of summer while they’re grubs start to eat the roots between Summer and the subsequent spring. These beetles can attack your plant indoors or outdoors and if you are growing your Rhododendron in containers it will be particularly damaging as we have already mentioned. This is a widespread insect problem across the UK and all you can realistically do is inspect your plant regularly and pick off the weevils you see, trap the adults with sticky barriers around your garden, or introduce natural enemies like birds and frogs which basically means encouraging them into your garden if you don’t already do so.
If you have Rhododendrons grown in pots then you can treat with Provado Vine Weevil killer which is a systemic pesticide that you use as a drench on the roots and it kills the vine weevil larvae.
Scale insects are another problem and they will suck the sap out of your plants whether it’s grown indoors or Outdoors. They leave a sticky substance on your foliage and that eventually causes mould. There are many different species of scale insects that are active all year round. Some are small as 1mm while some are larger than 10mm and if you see scales or tiny bumps on the stems or underside of your leaves, it is indicative of scale insects. You can spray them with organic sprays or pesticides to remove them but they are very persistent and might reappear. There are also pesticides you can use but you need to follow the instructions accordingly and they are best treated when young as pesticides seem to be less effective as that get older.
Powdery mildew is another prevalent disease but its often easier to live with it. As the name suggests you will actually see white powdery growth on the leaves. This can get much more prolific if your Rhododendron is grown in a damp location or somewhere very shaded. And in some situations, this happens just because of the location. It’s recommended that rhododendrons be grown in partial shade with some protection against strong winds but if it’s planted, for example, directly underneath a larger tree it can result in powdery mildew.
It’s very important to remove any dead leaves you see even those that are on the ground around the base of the plant and to improve airflow at all times. If you noticed powdery mildew has already spread on your Rhododendron you can attempt to control it with fungicides.
If you have an evergreen Rhododendron you might get patches of purple, brown, light green, or yellow on the surface of the leaves. Even though the manifestation of the disease is different, it is still a fungal disease and most rhododendrons to still thrive with this disease. If you see any of these symptoms on the leaves from the previous season you can apply a fungicide to prevent them from spreading but you will need to be thorough about the application of the fungicide to effectively prevent the airborne spread. We would not recommend removing the leaves as it would cause severe defoliation. Unless very severe, these types of infections should be tolerated
Rhododendrons need proper rainfall and thrive in areas where there’s a lot of heavy rain naturally but on the opposite end of that, you still have to be careful about root rot. Root rot is a soil fungus that can infect plants in different areas and eventually will cause the leaves to yellow, wilt, and die. If you don’t do anything about the root rot the plant itself will die after a few years. You can rectify this problem by replanting a rhododendron somewhere with better drainage or improving the drainage before planting which is the ideal scenario. With root rot one of the problems is that the preliminary symptoms appear underground and those that appear above ground are very similar to many other diseases and common problems.
Dieback, when you see your leaves wilting and the entire branch dying back typically results of a fungal infection. If you have a very dry season and your Rhododendron gets stressed, you need to be very careful about pruning because any wound, crack in the bark, or scars will allow the infection to enter and spread.
Cut back any affected branches right back to a good healthy section of the stem
Other problems with Rhododendrons
Aside from these diseases and pests rhododendron will experience damage if they are exposed to windy weather or cold weather. Excess frost or winds can damage the leaves and the flowers especially if we get a late frost.
Bud drop or buds not forming
Rhododendrons prefer moist soil, when the soil is not moist at any time of the year this can cause the buds to not form properly or form at all. Often they will simply dry up and just drop off in spring. The only solution to this is to prevent it in the first place. This means watering during times of very little rainfall and adding a good layer of acidic mulch around the base of plants to help retain moisture.
Rhododendron are susceptible to different problems the first of which is called chlorosis. This is effective when the leaves turn yellow and this can happen at any time of the year. In almost all cases it’s completely harmless to the growth cycle of your plant and it’s simply indicative of a nutritional deficiency.
If you look at the plants and you see the veins, in particular, are starting to yellow and down the line, you also notice things like wilting or Leaf Drop, it is often a problem with iron deficiency or magnesium deficiency. It is for this reason that you need to be particularly careful about planting your Rhododendron in the right soil. Rhododendron need acidic soil so you should test of the quality of the soil ahead of time and if you happen to have alkaline soil, it’s better for you to grow them in pots in ericaceous compost.