The popular and actually easy-care rhododendron can be found in many front gardens, public parks or cemeteries. But as easy to care for as the evergreen shrub that blooms in summer, it can also be susceptible to diseases, fungi and pests. Once the plant has been attacked, this is often reflected in the leaves, flowers and branches. In such a case, action must be taken quickly so that the rhododendron can be saved.
disease patterns
chlorosis
In the case of chlorosis, yellow leaves appear, from which it is not clearly known where they come from. For example, a rhododendron may produce yellow leaves one month, but the next month new, rich, green leaves will grow from the same shoot. Therefore, unless there is a lack of iron or nitrogen in the soil, it is assumed that the soil as a whole is too dry and the plant cannot access the nutrients.
Because the roots of the decorative shrub are weak, the shrub itself usually has a large surface. Some shoots cannot be supplied well enough and the leaves turn yellow here. As a countermeasure, the soil can always be kept moist so that the roots can do their job better.
yellow leaf color on the rhododendron
vine weevil
The vine weevil is a beetle feared by the rhododendron. Neither the leaves nor the roots of the plant are spared from him. The gray to black beetle, about 1.3 cm in size, has a broad, furrowed proboscis and is nocturnal. Therefore, it is very difficult to detect even when infested, because during the day this insect hides on the ground near the plant or even just under the soil surface.
It lays its eggs near the roots, this is also where the larvae hatch in the ground. Therefore, an infestation by the vine weevil is usually only recognized when the first damage to the decorative shrub appears.
These look like this:
- Leaves eaten from the edge
- U-shaped grazing pattern
- Larvae eat the roots
- root bark is also damaged
- the root collar is eaten away
- shows up on the plant
- this one is ailing
- withers and dies
The vine weevil lays several hundred eggs in June, from which the larvae hatch in August. These are curved and creamy white and only visible when the root is dug up. The larvae can overwinter in the soil. If an infestation is suspected, the beetles should be tapped off the plants at night. Roundworms from the garden trade can be used against the larvae, which are put into the soil around the roots with the irrigation water.
iron deficiency
The rhododendron can quickly suffer from iron deficiency, because it needs this substance in order to thrive. Because iron has the important task in the plant of forming the green leaf (chlorophyll), supporting respiration and supporting numerous enzymes as a component. Especially soils that tend to dryness, an excess of lime and phosphoric acid can inhibit the absorption of iron from the soil. Waterlogging can also easily lead to iron deficiency.
This is reflected in the plant as follows:
- Shoot tips on young leaves become light
- Leaf veins remain green
- this is how a vein lattice is formed
- Leaves lemon yellow when severely deficient
- later dry up from the edge inwards
Rhododendron always needs a low pH value in the soil, it usually does not tolerate lime at all. If the pH value of the soil is too high, it does not help to fertilize with iron fertilizer either, since the plant cannot absorb the iron through the roots in such a case. Therefore, an iron deficiency is usually not due to the fact that there is too little iron in the soil, but to a soil with a too high pH value, which in such a case has to be lowered.
yellow leaves on rhododendron due to iron deficiency
bud dieback
Bud death is caused by the fungus Pycnostysanus azaleae, which can invade the plant, especially if it has previously been infested with rhododendron planthoppers. Because they cut small slits in the buds and lay their eggs here. Fungal spores, which actually stick to the insect, get into the buds and thus into the entire rhododendron. The fungus finds the ideal living conditions in the bud scales. However, the full extent of the damage will not become apparent until next spring, because then the flower buds will not sprout.
The damage can be recognized as follows:
- flower buds discolored over winter
- usually brown to grey
- don't fall off
- can remain on shoots for two to three years
- Rods about 2 mm long grow from buds
- these are the fruiting bodies of the fungus
- Infestation on leaves large brown spots
All affected buds on which the visible spores have formed should be removed immediately, otherwise the cicadas will continue to carry the spores in the next year, damaging more buds. The fungus should be treated with fungicides from well-stocked garden shops.
Rhododendron bud death
mildew infestation
Not much is known about a powdery mildew infestation in the rhododendron. Above all, the different varieties also show different types of infestation. The fact is, however, that this fungus likes to spread, especially when the plants have grown very densely or are very close together. Therefore, there should always be enough air circulation between the individual sheets.
The damage caused by powdery mildew is as follows:
- in deciduous rhododendrons
- grey-white coating on the leaves
- growth is impaired
- in evergreen rhododendrons
- yellowish spots on the leaves
If mildew is found to be infested, all affected parts of the plant must be carefully removed. However, like all plant parts that have been damaged by a fungus, these should not be added to the compost. This allows the spores to spread further across the entire garden. It is better to dispose of these parts in the household waste, well sealed. After the shrub has been cleaned, fungicides are ideally applied.
Powdery mildew on the rhododendron
Fungal leaf spots
Various types of fungi can also severely affect the rhododendron. A distinction is made here between three types, each of which makes itself felt differently on the leaves, but which should all be treated equally with fungicides and the affected leaves should be removed. However, fungi usually form on plants when they are weakened by various circumstances or when the weather is damp and cold. It is therefore important to avoid fungal infestation by strengthening the plant from the inside and keeping the humidity low. The fungi show up as described below.
Cercospora fungus
- angular, irregular spots on leaves
- bordered reddish to dark brown
- covered with down on upper leaf surface
Gloesporium fungus
- dark brown leaf spots
- irregular and large
Colletotrichum fungus
- brown/black spots with a reddish-brown border
- on the top and bottom of the leaf
Leaf spots on rhododendron leaf
Rhododendron Leafhopper
The rhododendron leafhopper is a pest that attacks the plant, but usually does not cause any major damage of its own. Nevertheless, this weakens the shrub and there can be consequences from a fungus in the buds, which has already been discussed under the point "bud death". So that these consequences do not occur, the actually harmless cicadas should also be combated so that they cannot lay any more eggs and thus promote the fungus. The rhododendron planthoppers can be identified as follows.
- Larvae hatch in April to May
- live on leaf underside
- Grown into green-orange insects in mid-June
- are very lively and active insects
- fly away when touching the plant
If the cicadas are found on the rhododendron, a suitable insecticide from a well-stocked garden store should be used immediately to prevent damage from a fungus. The sucking activity of the larvae and adult insects, on the other hand, does not leave any significant damage to the leaves, even in the case of a larger infestation.
Cicada on a rhododendron
Rhododendron skin bug
The rhododendron skin bug is a pest that can also damage the graceful shrub. The late-blooming, violet varieties of rhododendron are particularly popular with insects. Varieties with a felty coating on the underside of the leaves, on the other hand, are avoided. Since bugs and their larvae can severely damage the plant's appearance, they should be combated. The eggs of the rhododendron skin bug are laid on the underside of leaves from July until autumn. Here they hibernate under a drop of droppings and hatch from mid-May. The damage to the decorative plant looks as follows.
- Leaf surface mottled dark and light
- dark droppings on the underside of the leaves
- Edges bend
- wither over time
To find out if the rhododendron bug has spread to the shrub, it makes sense to check the leaves for the dropping stains in the fall and winter, before the larvae hatch. Because then the leaves can be freed from the eggs by simply washing them off before they do any damage. If this has already happened in early summer, only the use of insecticides can help.
nitrogen deficiency
The rhododendron reacts very sensitively to a lack of nitrogen in the soil. Because this is required for the leafy green and various compounds in the plant and is therefore one of the basic building blocks of the shrub. But an excess of nitrogen is also not good for the rhododendron and should therefore be avoided. The lack of nitrogen shows up in the plant as described below.
- Yellowing and lightening on older leaves
- later all leaves turn light green to yellow
- also the leaf veins
- in contrast to iron deficiency, in which leaf veins remain green
- if there is a severe shortage, the older leaves will fall off
- only yellowed, younger leaves remain
If a nitrogen deficiency is determined, the soil should be prepared immediately and supplied with new nitrogen. However, all damaged shoots and leaves should be carefully removed so that the shrub can sprout again after the nitrogen application. However, over-fertilization with nitrogen should now be avoided. It is better to spread the gifts over several weeks.
yellowed leaves on the rhododendron bush
Branch dieback (rhododendron wilt)
The so-called branch dieback is also a fungus. Infection occurs via the terminal bud, which is brown in color. The damage is caused by a fungus of the Phytophthora genus, of which there are around 20 different species. Warm, humid weather favors the spread of the fungal spores, which can survive for a long time in the dark soil, but they need light and water to germinate. If the rhododendron has been attacked by this fungus and the following symptoms appear, then all affected shoots must be cut back into healthy wood and the bush and soil must be treated with fungicides.
- Terminal bud turns brown
- Infestation spreads to leaves and branches
- chocolate-colored spots along midrib
- Leaves lose their shine
- turn brown-grey
- curl up
- Twigs turn brown and shrivel
- Withering occurs
The damage caused by this fungal infection initially only affects a few shoots, while the neighboring shoots usually remain healthy. However, depending on which variety of the genus Phytophthora mushroom it is, the entire bush may wither, which can then no longer be saved.
Because, as the Chamber of Agriculture of North Rhine-Westphalia and the plant protection service recently explained, the disease can neither be fought nor cured. However, the infestation can be contained by pruning the affected shoots back to the healthy area. However, if the roots are infested, the shrub must unfortunately be disposed of as a whole.