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If the overwhelming flowering of a rhododendron in your own garden suddenly fails to materialize in spring, it is worth taking a closer look. Often an inconspicuous culprit is at work, leaving behind a characteristic damage: the bud tan. At first glance, it is reminiscent of frost damage. In reality, it is a fungus transmitted by rhododendron planthoppers. This article tells you how to fight insects and prevent an infestation.
damage picture
Especially with larger plants, the blooms in spring are often overwhelming. As May progresses and the rhododendron develops only sparse flowers, something is wrong. In many cases, the severe winter is not to blame if the flower buds are not juicy green but dark in color and dead. The real culprit for this clear pattern of damage, the so-called bud tan, is an inconspicuous fungus (Pycnostysanus azaleae). On closer inspection, fine black fungus threads can be seen across the entire bud. In this case, the flower buds have already died completely and can no longer be saved.
- Buds don't open
- brownish-black coloring
- dead
- dark, about 1 to 2 mm long, hairy fungal structures on the surface
- spherical thickening of the threads towards the end
- from April also light spots visible on the upper side of the leaves (by sucking the larvae)
- green-yellow colored larvae on the underside of the leaf
- Moulting residues of the larvae recognizable
Rhododendron planthopper causes bud tan
Rhododendron Leafhoppers
The fungus alone wouldn't do so much damage if it didn't have a helper. The carrier of the rhododendron fungus is Rhododendron Leafhoppers (Graphocephala coccinea or G. fennahi). The approximately one centimeter large, light green insect with the conspicuous red stripes on its back uses the plant to lay its eggs. The green-red cicadas can be found in the garden from the end of July. If you touch the rhododendron, the cicadas jump up and hide on the underside of the leaf. Originally, rhododendron planthoppers are not native pests. They come from North America and were first introduced to Great Britain and from there to Central Europe around 100 years ago. Rhododendron planthoppers have been a problem for us for the first time since the 1970s.
Rhododendron Leafhopper
larvae
While the adult rhododendron leafhoppers are no longer dependent on the rhododendron as a food source, the situation is different for the larvae. After oviposition in late summer and fall, the eggs overwinter cicada in the buds. The yellowish, flightless larvae hatch in April and can then be seen on closer inspection on the underside of the leaves. The larvae are already very agile after hatching and are dependent on rhododendrons as their only source of food at this point. During their development into adult rhododendron leafhoppers, the larvae molt several times until the beginning of July. Although the larvae suck the plant juices on the underside of the rhododendron's leaves, they usually cause hardly any damage.
infection
An infestation with rhododendron leafhoppers does not bother the shrub much. The female rhododendron leafhopper scratches fine slits in the rhododendron buds in order to lay one egg in each. However, the eggs and the larvae themselves do not cause the characteristic damage to the rhododendron buds. The fine wounds, which represent an entry point for the bud blast pathogen, are dangerous for the plants. It's a fungus called Pycnostysanus azaleae. Only the fruiting bodies of the fungus are visible to the eye, which cover the bud as fine black hairs with a spherical thickening at the end and are responsible for the special damage.
The fungus does not necessarily need these wounds to infect the rhododendron. However, if the protective skin of the plant is injured, the fungus can penetrate the rhododendron much more quickly and easily. Unfortunately, the infestation only becomes apparent in the spring, as no visible symptoms appear before then.
black-brown discolored rhododendron bud
combat
In general, the question arises as to which of the two pests involved in infecting the rhododendron is best to combat. Since it is not possible to take direct action against the fungal pathogen, there is only the indirect method of combating the rhododendron planthopper in good time with suitable means.
time
Since the fungus penetrates the plant in the summer when the eggs are laid, it makes sense to start fighting the rhododendron planthoppers in early summer to prevent the plant from becoming infected in the first place. It is helpful when fighting the insect that the cicada only lays eggs once a year. It is therefore important to interrupt this cycle by preventing egg laying.
- Time to fight the larvae: May to mid-July
- Egg laying time: August to September
early detection
In order to discover the rhododendron leafhoppers before they lay their eggs, an occasional look at the rhododendron is necessary from July. The underside of the leaves and the beginnings of the buds in particular should be examined closely. Monitoring is easily possible with glue-covered yellow boards. There are special yellow traps on the market that can be used to catch the adult cicadas that are capable of flying. If the rhododendron is infested with insects, sooner or later cicadas will stick to the board. These should be hung in and over the endangered rhododendrons from June.
crop protection products
In the case of a very severe infestation, you can hardly avoid a pesticide. Action is required before the first cicadas lay their eggs in the buds. There are system toxins on the market that work against rhododendron planthoppers.
- natural fight with pyrethrin
- homemade broths neem oil or neem grist
yellow-brown rhododendron leaves
Squirt right
The crop protection products mentioned above are applied by spraying. It is essential to observe the safety instructions given by the manufacturer in order to keep the risk to humans, animals and the environment as low as possible. In addition, there are a few basic things to keep in mind so that the product can work effectively. In general, adult cicadas are relatively resistant to insecticides. For this reason, you should take action against the cicadas as early as the larvae stage, insofar as this is possible.
- preferably use the early morning hours for control
- the insects are less mobile due to the nightly cold
- also spray the underside of the leaves completely
- Repeat the process every one to two weeks
- a total of at least three times
leaves
Since the leaves of the rhododendron have a water-repellent layer, professional gardeners recommend adding a few drops of washing-up liquid to the spray. The surfactants do not impair the effect of the agent, but only ensure that the surface tension changes and the leaves are wetted more evenly.
Rhododendron leafhopper on rhododendron leaf
Chemicals
Chemical agents should only be used if these pesticides are ineffective in the case of a very severe infestation. In order to minimize the risk of infection, all affected buds should be broken out in spring. Do not dispose of the buds in the garden, on the compost or the bio bin, but put the waste in the household waste so that the fungus cannot spread further.
Prevent
Once a rhododendron has been attacked, the fungus will continue to spread over the years. Doing nothing against the rhododendron planthoppers would therefore be the wrong solution. Ultimately, not a single flower comes to maturity and the whole grove is full of dead buds. It is therefore important that you prevent a new infestation in the period that follows. If you don't want to use poisons, you need a little more patience and the right timing. To prevent this, it is important to break out and dispose of dead buds immediately. This reduces the chance of further or stronger fungal infections.
- regular check of the flower buds from spring
- there are brown or black spots: break out immediately
- Put up yellow boards from mid-June to September
- for smaller shrubs: tie light and air-permeable fleece around the plant
Optimal soil condition
While healthy, vigorous plants are not completely resistant to pests, they are attacked less frequently. If the harmful fungus does occur, the extent of the damage is significantly less than with weakened rhododendrons. It is therefore important to ensure optimal site and soil conditions.
- acidic soil
- ideal pH: around 5
- weathered forest floor
- Soil for ericaceous plants or rhododendron soil
- Check pH regularly
- slightly damp
- avoid dry locations
Rhododendron bush in the garden
Best practice
Getting a rhododendron that has been affected by bud tan completely cicada-free again is usually a tedious affair. However, if you combine all control methods and preventive measures in a sensible way and apply them consistently, you can ideally eradicate the rhododendron planthoppers in just one season. Because if egg laying is prevented, there will be no new generation of cicadas in the following year.
Rhododendron variety
Choose the right rhododendron variety
Fortunately, the occurrence of the fungus with its characteristic damage is limited to the bud. Only in very rare cases, for example in the case of severely weakened plants, does it also spread to the shoot section below. Large-flowered rhododendron hybrids are particularly susceptible to the fungus. However, there are clear differences between the individual varieties. Anyone who buys new plants can therefore consider resorting to more resistant cultivated forms. You may save yourself a lot of hassle and work by planting one of these strains in your garden right away.
Rhododendron with pink flowers
- particularly susceptible: large-flowered Catawbiense hybrids
- less sensitive are: Berliner Liebe, Nova Zembla, Progres and Goldbukett
- Hardly any attack on Japanese azaleas, Williamsianum, Repens and Yakushimanum hybrids
- unfavorable locations promote infestation