There are many gardeners who don't understand why their rhododendron has so many black flower buds that have died. The problem was not caused by frost damage but by a fungal disease (Pycnostysanus azaleae). The rhododendron leafhopper, which appears to be completely harmless to humans, is the vector of this disease. Does your rhododendron have black buds? Below we will tell you what you can do against this fungal disease and how you can fight rhododendron leafhoppers!
Rhododendron has black buds – what is it?
If you have an extensive collection of rhododendron plants, you may have experienced the flower buds turning dark brown and eventually black. The problem is the fungus that the rhododendron leafhoppers (Graphocephala fennahi) spread when laying eggs. To do this, the pests cut small gapsinto the flower budsand then lay just one egg in each of these gaps. The virus settles in the resulting wounds, which leads to the so-called “bud tan”. The dying buds are then covered with short, brown-black hairs until the next spring. These fruiting bodies belong to a fungus calledPycnostysanus azaleae, as already mentioned, and can also be found on azaleas.
When will the rhododendron leafhoppers come?
These cicadaswere not always present in Europe. They were introduced to the United Kingdom from North America in the 1930s and from there spread to Central Europe.
The rhododendron leafhopper probably attacked your rhododendrons this time last year and as a direct result, the young buds of your plants were attacked by the fungus, which eventually led to their death. The cicadas arrive in June, so it is important to control the pests at this time if necessary.
Damage to rhododendrons caused by cicadas
In fact, the rhododendron leafhopper does not cause any serious damage to the bushes: the pests, which are only 8 millimeters in size, feed on the sap of the leaves. The larvae of the rhododendron leafhopper lay their eggs on the underside of the leaves. As a result, the leaves of the rhododendrons are not stunted, but instead have a pale green color because some of the leaf cells have been sucked out.
Rhododendron has black buds – fight fungal disease and pests
The affected buds of your rhododendron first turn black, then wither and die, but do not fall off the branches as they are still attached to them. If the disease-causing fungus is not removed from the affected buds in a timely manner, it can spread to other parts of the plant. Therefore, make sure that your rhododendrons are always under observation in the garden: If you notice that brown or black flower buds are developing on the plants, you should remove them as quickly as possible and dispose of them with the remaining waste.
When the buds fall off, it is not entirely clear whether the leafhoppers are responsible for the fungal infection or not. For this reason, the use of chemicals to control these animals is not recommended. Since fungicides are ineffective against the fungus, the only option left is to remove suspicious, dry buds in April at the latest, in good time before May, when the yellowish larvae of the rhododendron leafhoppers begin to hatch. In summer, yellow panels can be used as a preventative strategy that attracts and kills insects.
How should you proceed then?
- Since the rhododendron leafhoppers have no negative effects on the development or vitality of the plants, they can be tolerated.
- It is possible to pick off and dispose of flower buds affected by budding in order to reduce the number of fungal spores released into the environment.
- To control the population of suckers in the garden, it is important to attract natural predators and antagonists such as birds, ladybirds, wasps and ground beetles.
These remedies help against cicadas
The plants can be treated with organic potash soap or diatomaceous earth,about the pest populationsto keep under control. To achieve successful control, both the affected upper and lower leaves must be completely sprayed. You can also use the purely ecological remedy – neem oil.
Pruning rhododendrons in spring
Pruning can be used to reshape an older rhododendron that is losing its lower leaves and becoming bare from the bottom up. However, you should avoid unnecessary pruning of your rhododendron until the flowering shrub is firmly established in the garden. Only then will it be able to cope with pruning. You should also choose the best time to prune your plant. Rhododendrons develop their flower buds the year before, so they should be cut in springto the loss of flowerswould lead. It is therefore advisable to carry out the necessary pruning as soon as possible after flowering. In this way, you can achieve the desired shape of your plant and at the same time stop the spread of the rhododendron leafhopper: In late summer, while laying eggs, the females prefer to transmit the already mentioned fungus to the plants.
Also worth reading:Is your rhododendron not blooming? Find the cause!
