Well-fed rhododendrons in a reasonably suitable location require little maintenance. Are rather less affected than many other cultivated plants (z.B.B. Roses) attacked by pests and diseases!
Problems that affect the entire plant
Are usually caused by early fall frosts while the plant is still in sap or by late spring frosts when the sap is already rising. For this reason, it is dangerous to fertilize with nitrogen in the summer (after July), because this can stimulate the plants to grow again in the fall. Bark cracks usually heal with difficulty. Can even lead to the death of the plant. When bark cracks are discovered, it is usually too late to take any measures. However, care should be taken to avoid fungal infection or insect infestation.
Curling and drooping of leaves
is normal when rhododendrons are exposed to extreme heat, cold or lack of water. It is a natural reaction that helps to cope with the stressful situation and prevent dehydration. When these extreme conditions are over, plants should regenerate quickly. Since similar symptoms can be caused by root fungi that thrive especially in warm, humid conditions, it is important not to overwater. If the soil is really dry, watering should be done about once a week, in case of sandy soil evtl. frequent. Waterlogging must be avoided at all costs.
Plants wither and die slowly
Root rot caused by the fungus Phytophthora, is the main cause of death of rhododendrons and azaleas. It develops especially in soil that is too wet and not aerated. The roots are blocked by the fungus: the plant withers and dies. When the symptoms appear, it is already too late to take countermeasures. Some rhododendron, z.B. Catawbiense album and Nova Zembla are more susceptible, while others, such as z.B. Roseum Elegans, scintillation And P. J. Mezitt, will hardly be affected by Phtophthora Infest. Peat and bark humus provide good drainage and can prevent root rot, but do not cure it.
Plants wither and die suddenly
The causative agent is usually the larvae of the largemouth weevil Otiorhynchus sulcatus. These nematodes gnaw at the roots and especially at the base of the stem, thus interrupting the flow of sap. Less problematic is the damage caused by the nocturnal adult beetles. The leaves are more or less eaten away in a semi-circle, cove or arch shape from the edge to the midrib. This may not look pretty, but is harmless to the plant. The adult beetles can hardly be controlled effectively. A successful, but not exactly cheap, method of controlling the larvae of the psycophyte weevil for several years. Pupae is the targeted use of insect parasitic nematodes. We do not have our own experience with this, because we have so far been spared from a mass infestation. The nematodes are introduced into the soil by watering at the time of larval and pupal stages, which should be done when the soil moisture is uniform and the minimum temperature of the soil is 10-12 °C. These nematodes are completely harmless to plants and other animals. Good efficacy should be achieved before the beetles hatch (around the beginning of May), so nematode application in the period from mid/late April to late May is appropriate. Another very promising application period is the period from 25. August to 30. September. In the period between these two dates there is no effective application possibility for these beneficial insects.
Bud dieback
is a symptom of a fungal infection that has also been occurring in Europe for several years, caused by the fungus Pycnostysanus azaleae, but can also be caused by late frosts or by too late nitrogen fertilization. In all cases the buds turn brown or black. In case of infection by Pycnostysanus the dead buds are covered with a brown-black, spiny fungal lawn. It is now believed that the fungus enters the bud through the puncture channel of the pretty rhododendron cicada. To limit the damage, all infested and dead buds should be broken out and disposed of in February/March (do not throw them on the compost heap or under the plants)!). The newly hatched cicadas, which start buzzing around the rhododendron area in mid-June, can be destroyed with an insecticide by spraying, which u.U. should be repeated again after 10 days to be on the safe side. Suitable for this measure are z.B. the synthetic pyrethroid "Decis", or preparations like "Roxion" or "Perfekthion". However, success is only guaranteed if the undersides of the leaves are also completely wetted with the respective agent.
Poor flower bud formation
can be a sign that a plant is doing too well (rarely!). Too much shade, a cool, wet summer, inadequate supply of phosphorus or potassium, or unbroken faded inflorescences can also suppress the formation of flower buds. Other reasons for poor flower bud set may be:
Buds are formed in late summer and early fall: If pruning is done at this time or later, then the flower buds are also removed. New leaf buds will still form, but flower buds will not form until the following year.
Some plants almost never flower, or only at an advanced age. This particularly affects rhododendrons grown from seed. Nitrogen promotes leaf in particular-. Shoot growth hardly, on the contrary, the flower buds. Phosphorus, on the other hand, favors the formation of flower buds and resistance. Potassium is for the strength of the shoots. Winter hardiness necessary.
Cold weather can cause flower buds to die off. Normally the buds are then brown in color in the spring. A cold spell in the fall or spring can damage buds that have not matured. If flower buds open prematurely during a prolonged mild period in the fall or winter, they will obviously not be available for normal flowering time.
Most rhododendrons take 2 to 3 years from cuttings to flowering. Some take longer. Others bloom a little earlier. Plants grown from seed need at least 1-2 years more.
Some rhododendrons require full sun to bloom, while others tolerate fairly deep shade. In general, the more sun, the more flower buds. However, there is then also a greater risk of sunburn and desiccation. More shade produces more foliage and fewer flowers.
There are many other parameters that affect the health of the plant and consequently its ability to produce flowers.
Flower buds do not open
The reasons for this can be many.
Bud break: The supposed flower buds could be leaf buds. Leaf buds of some cultivars and especially species are surprisingly large and so can easily be mistaken for flower buds.
Buds open prematurely: Plants that are not mature enough or are exposed to an unusually warm period in the fall or winter can begin flowering prematurely. This bloom is rarely satisfactory and often the flowers freeze before they can fully open. In any case, the following normal flowering is lost. Diseases can also occur before the buds open.
Low temperatures: buds may freeze during winter. For most rhododendrons, there is a low temperature below which flower buds are damaged and therefore do not produce flowers. This temperature is very dependent on the variety resp. Art.
Nutrients: Improper fertilization, can affect cold resistance and flower bud set. Measure pH. Acidify if necessary. Sulfur flower (powdered sulfur) is most suitable for this purpose. Do not use aluminum sulfate or alum, as larger concentrations of aluminum are toxic to plants.
Winter protection: Winter protection (shade cloth or spruce brushwood) allows the plants to tolerate a temperature a few degrees lower.
Small spots on the flowers,
which quickly enlarge and look wet are a symptom of rhododendron blossom rot Ovulinia. The blossoms quickly turn brown and wilt. They remain attached to the plant. Fungicides should be applied preventively just before flower buds open. Infestations can be severe, especially in wet years.
Problems involving individual twigs and branches
Terminal buds and leaves turn brown
The cause is usually fungal diseases, which can be caused by Phytophthora, Botryosphaeria or Phomopsis are caused by. All three diseases spread quickly, but rarely threaten the entire plant. Phytophthora causes the midrib of the leaves to turn brown. Infection spreads outward from there. Leaf wilting Discoloration rapidly spreads to new growth… Some rhododendron z.B. Catawbiense album and Nova Zembla Are more common, while others such as z.B. Roseum Elegans, scintillation and P. J. Mezitt less frequently affected. Disease control is difficult. Precautionary spraying with fungicides can be helpful.
Leaves turn dull green, then brown, curl up and fall off
The cause is a fungal infection caused by Botryosphaeria. Hygienic measures and application of a fungicide after pruning may help.
The symptoms for Phomopsis Can be leaf spots, chlorosis, and browning of the leaves, which eventually wither away. The brown coloration extends down the stem to a wound. Again, you can try to contain the spread with a fungicide.
Foliar problems
If leaf edges become necrotic and brown, then this may be damage from wind and cold weather. Boron deficiency also produces comparable symptoms.
Large-leaved rhododendrons are susceptible to sunburn if the plants have not been sufficiently watered before the ground freezes over. The leaves curl up in the cold. This means that the midribs in particular, but not the leaf margins, are exposed to the sun. These central parts of the leaves, but not the edges, dry out and turn brown. To prevent this, water the plants thoroughly before the first heavy frost and provide wind protection and partial shade.
Leaves with brown areas and white spots are usually an indication of fungal infection with Pestalotia. The infestation often occurs in humid periods. If the infection is discovered, it is usually too late to apply a fungicide.
Irregular spots starting from the leaf tip or even from the leaf margin,
with a diameter of about 30 to 50 mm, reddish brown to ashy gray and dark brown margins, spherical spore deposits in the leaf spots black. These symptoms indicate leaf spot diseases. As the infestation progresses, the leaves die from the tip upwards. Countermeasure: spraying with a broad-spectrum fungicide such as "Dithane Ultra", "Polyram WG" or "Saprol New". If there is no visible success after the first spraying, or if the spray has been washed off during rain, repeat the treatment once or twice at 14-day intervals.
Gray mold rot
In this case, the leaf tip up to about 1/3 of the leaf surface is initially covered with a grayish-colored mold coating, which often gradually spreads over the entire leaf blade to the petiole. This is caused by weak parasites of the genus Botrytis, to which high humidity and warm weather are conducive, and which can appear at the end of June / beginning of July on leaves that are still soft and not fully developed. To remedy the situation, apply a fungicide such as z.B. "Euparene WG" expedient. Repeat spraying after 14 days.
Powdery mildew
is a grayish-white, powdery coating on the leaves. Especially on deciduous azaleas the infestation occurs frequently. Sometimes the underside of the leaf is more affected. Against powdery mildew are a number of effective fungicides on the market.
Uniform light yellow coloration of the leaves
indicates nitrogen deficiency. To remedy the situation, fertilize immediately with the recommended complete fertilizers and horn shavings (s. fertilizer recommendation) is required.
leaf blades are partially or completely thickened,
ear- or spoonlike deformed and later chalky white frosted. This is the earlobe disease (Exobasidium). It occurs only on Japanese azaleas, Rh. ferrugineum and Rh. hirsutum on. Diseased parts should be broken out and disposed of. In addition, treatment with a fungicide such as "Polyram WG" or with "Dithane Ultra", always with the addition of a wetting agent (such as z.B. rinsing agent) to relax the water surface is indicated.
Uneven yellowing
of the leaves (veins remain green). This phenomenon is called chlorosis. It is mostly caused by iron deficiency. Iron deficiency, in turn, occurs especially when the pH is too high, which makes the iron that is sufficiently present in practically any soil unavailable to plants. For young plants, iron chelates such as" can be used as an immediate remedyFetrilon" or"Sequestren" can be applied by spraying or watering. But the success is not permanent. Therefore, think about basic measures to improve the soil. By the way: magnesium deficiency and various other stress situations such as cold, rainy or dry periods can also lead to chlorosis.
Algae coating on the leaves
In deep shade (especially of trees) u.U. in wet years there is often an undesirable but harmless algae coating on the foliage. To remedy this unsightly sight, use the very plant-compatible, non-bee-hazardous sprays "White oil" or"Para-Summer", their oil films smother the algal coating. However, the application is possible only in the months of February to April (before the leaf shoots!) possible.
If the leaves are pale green to yellow on the top side or. yellowish white speckled,
on the underside conspicuously brownish-black soiled, sometimes slightly curled, later withering and falling off, then this is an indication of infestation by the rhododendron bug (stephanitis rhododendri]. Plants in a dry, sun-exposed location are particularly affected. In our experience, the variety 'Flava' particularly susceptible. During the main infestation period, especially in May, June and July, carry out infestation checks, primarily of the undersides of the leaves, so that if necessary, an insecticide can be sprayed in good time during dry weather.