Rhododendron – Yellow Leaves and Brown Spots – Causes and Remedies

Rhododendrons are particularly popular in northern Germany, but they can also be found further south in many gardens. A suitable location and a plant substrate that is precisely tailored to them are essential for survival for these plants. If these two criteria are not correct, the woody plants do not feel well and become susceptible to diseases and pests. This must be prevented. In the short term, special pesticides help, but in the long term the conditions must be improved. You can read in our text what you can do with illnesses and how a rhododendron feels.

Problems with rhododendrons

Rhododendrons are actually uncomplicated and less susceptible woody plants, but only if the site conditions and plant substrate are right and the water and nutrient supply are optimally secured. But if this balance is disturbed, either from the beginning or over time, the beautiful flowering trees become vulnerable and their immune system is weakened. This in turn makes the plants susceptible to pathogens and pests, of which there are numerous specimens. It is important to correctly identify harmful pathogens or pests and then to take targeted action!

Tip: In order to remedy illnesses or symptoms of deficiency, there are certainly short-term measures that promise a remedy and often work first. However, if you want to achieve a permanent solution, you have to find out the cause of the weakening and the problem and remedy this. Until this happens, the rhododendron cannot heal.

Yellow leaves on the rhododendron

There are various causes for yellow leaves on the rhododendron. Yellow leaves are not yellow leaves either, there are still differences. Often it is chlorosis or simply a lack of nitrogen that troubles the trees. Short-term aid is not a bad thing, but the rhododendron must be served well in the long term. Only those who change the conditions, i.e. location, plant substrate or irrigation water, will ensure the recovery and good development of this flowering wood.

Chlorosis – yellow leaves with dark veins

At first, the older leaves are evenly light green in color. Over time they get lighter and more yellowish. The younger foliage will gradually turn yellow. There are hardly any flower buds and the foliage falls off prematurely.

  • Lack of magnesium and iron
  • Yellow leaves alone indicate a lack of nitrogen.
  • If you can see a kind of bone pattern on the sheet, iron is also missing.
  • Usually the soil is too rich in lime or the irrigation water contains too much lime
  • This can be checked quickly with a pH test for the garden soil.
  • If the pH value is too high, iron cannot be absorbed from the soil.
  • In the short term, fertilization with aluminum sulfate can help (15 g per m²)
  • Soil improvement is more important.
  • pH 4.2 to 5.5
  • Use special rhododendron soil or peat mixtures (it is better to avoid peat and use substitutes)
  • Bark mulch as a cover

Nitrogen deficiency – yellow leaves, often associated with stunted growth

  • Need plenty of nitrogen, especially rhododendron-Catawbiense hybrids
  • Horn shavings as nitrogen fertilizer
  • Use artificial fertilizer only in March and then again in May
  • Do not fertilize later, otherwise the shoots will not ripen until winter and can freeze to death

Brown spots on the leaves

Brown spots on the leaves can also have several causes. Often it is harmful fungi that like to spread on weakened rhododendrons. There are special sprays called fungicides against fungi. However, these only help in the short term. In the long run, it is important to find out what weakens the rhododendron and makes it susceptible to pathogens. Conditions simply need to be improved. Brown spots can also indicate sunburn. Winter morning sun in particular can cause burns, but so can bright summer sun.

Brown discolorations on the tip of the leaf and the leaf margins also indicate a fungus. The disease is called leaf spot disease.

Damage

  • Cercospora species – smaller, irregular and more angular leaf spots with a dark brown, sometimes reddish border. When the humidity is high, the spots on the upper side of the leaf are covered with a fine down
  • Gloeosporium species – significantly larger, quite irregular dark brown spots on the leaves
  • Colletotrichum species – here brown to black irregular spots appear on the upper side of the leaf as well as on the underside

causes

The causes are usually a wrong location or an unsuitable plant substrate. Too much lime in the irrigation water and a lack of nutrients also play their part in weakening the rhododendrons. This in turn opens the door to harmful organisms such as a wide variety of mushrooms.

Wrong location

  • Too sunny
  • Too close to a shallow-rooted tree or other wood

Inappropriate substrate

  • Too much lime in the soil, too high a pH
  • Too dry substrate
  • Soil must be acidic, a pH value of 4.2 to 5.5 is ideal
  • Too little humus in the soil
  • Too solid ground. The fine roots cannot spread.

What to do in the event of a fungal attack?

Long-term remedies must also be found in the event of fungal attack. The first action should be to remove the infected parts. The cause of the infestation must be found and eliminated. Only that helps in the long run.

  • Cut out infected parts of the plant and dispose of them (not on the compost)
  • Use spray agents, for example, Fungisan Rosen-Mushroom-Free or Saprol New
  • Often the treatment has to be repeated (not in damp weather)
  • Also collect all discarded leaves and dispose of them to break the cycle

Further damage to the rhododendron

In addition to the diseases mentioned above, there are other threats to the rhododendron. Weakened specimens in particular are plagued by all kinds of plagues. It is often the case that you start with an illness and then more and more come because the plant cannot defend itself, has no defenses, simply because the conditions are not right.

Rhododendron bud rot

  • Fungal disease transmitted by the rhododendron cicada
  • Lays eggs in buds
  • Prevention in summer – hang up yellow boards or cherry fruit fly traps

Grauschimmelfäule

  • Often occurs in humid and warm weather
  • Dispose of affected leaves (household waste)
  • Pick up any fallen leaves
  • Spray with a special fungicide

Frost damage

  • Occur after cold, dry, and windy winter weather
  • Usually only on more sensitive varieties
  • Leaf margins or even whole leaves turn brown and die off, flower buds look like they have dried up
  • A windbreak and shading the plants help here
  • Cut out frost damage or cut back the entire plant
  • Think long-term about a new location that is better protected from wind and winter sun

Rolled up leaves

  • Protective measure of the rhododendron against drought
  • Usually occurs in summer during long periods of drought and in winter during long periods of frost (evergreen rhododendrons)
  • Deep watering helps here
  • Do not pour on the leaves – burns
  • In winter only water on frost-free days
  • Don’t forget potted plants
  • A layer of bark mulch protects the soil from drying out too quickly

Spruce needle rust

  • In spring, isolated orange spots, later dusting pustules on the underside of the leaves and shoots with green bark
  • Passes over to spruce trees in summer and forms spore beds there again
  • Infestation in the rhododendron depends on the variety

Frequently asked questions

What can be the reason when whole branches on the rhododendron suddenly dry up?
The cause of this phenomenon is usually a fungus that causes the so-called rhododendron wilt. The fungus (Phytophthora cactorum) blocks the pathways of the plant. There is nothing you can do about it, except to cut the infected shoots back into the healthy wood and remove them. The parts should be incinerated or disposed of with household waste. Under no circumstances do they belong on the compost or in the shredder. The cut does not guarantee that the rhododendron will survive. If the wood is weakened, for example due to an unsuitable location or the wrong substrate, it usually cannot be saved. An old remedy that you could still try is 1% copper lime broth, which should be sprayed on the plant every 10 to 14 days.

What do pale green to yellow and yellowish white speckled leaves indicate?
These noticeable leaf damage suggests various rhododendron skin bugs. These often occur in places that are too warm. The leaf symptoms are a result of their sucking activity. In addition, brown excrement droplets can be found on the underside of the leaf, which dry up to form scab-like coatings. Over time, the leaves wither and fall off prematurely. The whole plant takes care of, sometimes it withers completely. The affected leaves should be picked and discarded. As soon as larvae appear on the underside of the leaves in spring, insecticides are helpful. It is also advisable to transplant the rhododendron to a place that is not so exposed to the sun.

Kira Bellingham

I'm a homes writer and editor with more than 20 years' experience in publishing. I have worked across many titles, including Ideal Home and, of course, Homes & Gardens. My day job is as Chief Group Sub Editor across the homes and interiors titles in the group. This has given me broad experience in interiors advice on just about every subject. I'm obsessed with interiors and delighted to be part of the Homes & Gardens team.

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