Content
Most rhododendron diseases develop as a result of incorrect, ill-conceived or inept agricultural practices. The plant is vulnerable to infectious, fungal and physiological diseases; it is often inhabited by insect pests. Without timely treatment, the bush dies. That is why the main diseases of rhododendrons and their treatment with photos will be valuable information for connoisseurs of this culture.
Infectious and fungal diseases of rhododendrons
With proper agricultural technology, the bushes are resistant to infectious and fungal infections. Treatment of planting material, the correct choice of location on the site, regular watering and fertilizing allow you to maintain the health of the plant.
Overmoistening, darkening, excess or, on the contrary, lack of nutrients lead to rotting, slower growth, the development of fungus, mold, infection and, ultimately, the death of the rhododendron.
Bacterial root cancer
This is a dangerous infectious disease that continues to develop even after the death of the plant. Its causative agent is the Agrobacterium bacillus, which infects the roots of rhododendron.
If left untreated, the infected plant slows down in growth and drops leaves and buds. The main symptoms of bacterial cancer:
- rotting of the root collar;
- the formation of large round, very dense growths throughout the root system.
As a treatment at the early stage of the disease, the bush is treated with Bordeaux mixture. If the infection is advanced, the rhododendron is uprooted, burned, and the area is disinfected with fungicides.
Tracheomycosis wilt of rhododendron
The causative agent of the disease is the fungus Fusarium oxysporum, which attacks the vascular system of the bush. An infection originates in the roots, which then causes rapid rotting, blocking the movement of nutrients.
If the rhododendron leaves turn brown, this is the first sign of disease. Without treatment, over time the stem becomes thinner, its crown dries out, and a gray coating appears - mycelium. The plant gradually dies.
The bush can be saved if treatment with Bordeaux mixture is started in time. The affected areas are torn off, burned, and the rhododendron is sprayed with Fundazol (0.2%). A little of the drug is poured into the root hole.
Phytophthora root rot
The disease occurs as a result of waterlogging of the plant's root system. There may be several reasons:
- excessive watering of rhododendron;
- insufficient drainage layer;
- heavy, clayey substrate that does not provide proper moisture transmission;
- infection of cuttings in the nursery.
When infected, dark crimson or burgundy spots appear on the leaves of the rhododendron, and the crown of the plant becomes withered and drooping. Stems, cuttings, and shoots acquire a purple tint and become thinner. Without treatment, the growth of the bush slows down and flowering stops completely.
The disease most severely affects the roots of rhododendron. They begin to rot, change color to dark brown, and stop feeding the plant.
Treatment begins by limiting watering and allowing the soil to dry thoroughly. The bush, stems, and root space are sprayed with fungicides (Bordeaux mixture, Fundazol, Quadris). If no improvement is observed within 2–3 weeks of the disease, and the leaves of the rhododendron have drooped, the plant is uprooted and the soil is re-disinfected.
Gray rot of rhododendron
The causative agent of this disease is spores of the fungus Botrytis cinerea. They are volatile and are transmitted by air from an infected bush to a healthy one. Most often they affect dead, dried shoots, buds, leaves, then move on to the rest of the living parts of the plant.
The first sign of infection is brown or brown spots on the rhododendron. Over time, the top layer of leaves dries out and begins to crack.At high humidity, a grayish, fluffy coating is noticeable on the affected areas.
The disease often affects rhododendron after winter. In the photo you can see young shoots with gray rot.
To treat a bush, damaged leaves, buds, and ovaries are removed, and the crown is sprayed with Fundazol once every 2 to 3 weeks until the end of the flowering period.
Rot of shoots, seedlings and buds
Sudden mass wilting of young rhododendron seedlings is observed when seedlings are infected with fungi (Rhyzoctoni Solani Kuhn, Rhyzoctonia, Botrytis or Pythium). If treatment is not started in time, the root collar of the shoot becomes thinner, turns black, and the stem becomes soft. The sprout falls on its side and gradually dies.
Upon careful examination, white or brown fungal spores can be seen on the buds and leaves of the rhododendron, and the surface of the soil is covered with a mesh of thin white threads.
The causative agent of bud rot, Pycnosteanus azaleae, is carried by cicadas. The buds of the plant become brown, dark brown and gradually fall off.
After the buds die off, the disease continues to develop, the mycelium grows into the stems and affects the bush from the inside. Without treatment, the rhododendron withers, stops growing, and ultimately dies.
The causes of the disease most often lie in infected planting material or in non-compliance with the rules of agricultural technology: close proximity of seedlings, impaired air exchange, high humidity in the greenhouse.
For treatment, rhododendron seedlings are covered with fine wood ash or Fundazol. The buds are sprayed with copper-containing preparations 2 times a month until the end of the growing season.
As a preventative measure, seedlings purchased from a nursery are disinfected.In addition, they monitor the frequency of watering, sufficient ventilation and lighting (when planting in a greenhouse).
Dieback of shoots
The disease is typical for rhododendrons growing in the shade. The fungus Phytophtora cactorum attacks young shoots. The buds on them do not bloom, turn brown and fall off.
In the absence of proper treatment, the disease spreads to the stems, and the young leaves of the rhododendron begin to curl. Gradually the bush dies.
To stop the death of shoots, the affected branches are removed, the crown is sprayed with any preparation containing copper every 2 weeks, until the onset of autumn leaf fall.
Root rot
The disease spreads from the root up the stems. Most often it occurs as a result of excessive watering or during prolonged rains with low ambient temperatures.
At an early stage of infection, rhododendron leaves wither for no apparent reason. Then they darken, turn brown, and the young buds gradually die off.
The roots and lower part of the bush begin to rot, darken, and acquire a brown tint.
In most cases, the disease is fatal to rhododendron and cannot be treated. The bush is uprooted and burned.
It is possible to prevent the development of root rot at the stage of planting. To do this, it is worth carefully balancing the acidity of the soil in accordance with the recommendations for a specific rhododendron variety, monitoring humidity and timely removal of weeds.
Swelling of rhododendron leaves
Leaf swelling is also called rhododendron thick leaf disease or wax disease. The causative agents are fungi of the Exobasidium family. When infected, round, fleshy, spherical growths, the size of a pea to a walnut, form on young shoots.
Symptoms of the disease (depending on the type of pathogen):
- white or pinkish-red “pads” grow on young branches;
- The leaf plate of the rhododendron on top becomes yellow-brown, the reverse side becomes covered with a powdery coating;
- dark brown spots appear, white fungal spores are visible;
- rhododendron leaves become pale, abnormally thick and large; Over time, they wrinkle, become moldy, and dry out.
Treatment consists of timely removal of the affected branches and periodic treatment of the bush with fungicides containing copper.
Rhododendron spotting
The disease is common in both domestic and garden varieties. Fungal spores affect adult and young rhododendrons.
You can recognize the pathogen by the shape of the spots:
- Pestalociaceae spotting is localized on the crown and stems. The disease is characterized by the appearance of brown spots of irregular shape with a brown frame. Spore pads are visible over the damaged areas. Treatment: removal of infected shoots, spraying with Bordeaux mixture or Camulus.
- Septoria spotting appears on rhododendron leaves. You can recognize the disease by red round spots with black fungal spores in the center. As the disease develops, the leaf blade dries, turns yellow, and curls. Treatment includes pruning the infected part of the crown and treating the plant with Camulus.
- Anthracnose spotting is determined by brown spots with scattered dark spores on the upper surface of the leaf blade. The reverse side of the sheet turns pale. Gradually, the disease spreads to the stems, weakening the plant. Treatment: pinching damaged leaves, treating branches with Bordeaux mixture.
- Phyllostictous spotting is characterized by the appearance of red affected areas that turn white, dry out and crumble over time. In the advanced stage, black dots are visible on the leaf blade - spores. Treatment comes down to careful pruning with complete removal of infected shoots, spraying with a suspension of Zineb or Captan.
The cause of the development of the disease is most often incorrect agricultural technology: excessive watering, incorrect pruning, untimely application of fertilizers.
Cercospora
The disease develops on the lower tiers of the bush and is caused by spores of the fungus Cercospora. In the early stages of infection, the leaf blades become covered with brown, uneven spots with a pronounced red border. Then a thin gray coating appears on the foliage - this means that the mycelium is growing.
If left untreated, the disease progresses, the entire reverse side of the leaf turns dark brown, the shoots stop growing, and flowering does not occur. Without treatment, the rhododendron dies.
To get rid of the fungus, an integrated approach is used: diseased branches are removed, the bush is sprayed with Ditan and Fundazol.
Rust
The disease affects small-leaved varieties and appears most often in the fall.Rusty, brown, reddish or yellow spots appear on rhododendron leaves. By spring, a cluster of reddish-brown spores will be noticeable in this place.
The infection affects only the crown, without affecting the roots or buds. Rhododendron leaves turn yellow and fall off prematurely. Without treatment, this leads to the death of shoots and death of the entire plant.
At the first sign of rust infection, diseased leaves are torn off and burned. The bush is treated with preparations with a high copper content (for example, Bordeaux mixture).
Mosaic
A viral incurable disease caused by the Rhododendron mosaik virus. Most often it is carried by insects: aphids, bedbugs, and others.
When infected, rhododendron stops blooming and its growth slows down. The leaves of the plant become thinner, turn yellow in places, and brown spots appear on them. The surface becomes rough, rough, and greenish bumps - calluses - form. In the advanced stage, rhododendron leaves darken and become severely deformed. A “mosaic” pattern appears.
It is impossible to cure rhododendron. To save other plants on the site, the damaged bush is uprooted and burned, and the soil is treated with Actellik Confidor.
The disease most often affects alpine varieties.
Non-parasitic diseases of rhododendron and their treatment
In addition to diseases caused by fungal spores, bacterial or viral microorganisms, rhododendron is susceptible to non-parasitic (physiological) lesions. The reason for their development is the incorrect location of the bush, errors in agricultural technology, and unfavorable weather conditions.
To save the bush, you need to know the main non-parasitic diseases of rhododendrons, measures to prevent and control them.
Chlorosis of rhododendron leaves
Chlorosis is detected by pale spots that appear on the leaf blade. At an early stage, the leaf veins remain bright green, then they turn pale. The disease spreads to branches, young shoots, buds, and the bush becomes vulnerable to sunburn.
Chlorosis develops with a lack of nutrients (magnesium and iron), as well as increased soil acidity. If the leaves of a rhododendron turn yellow for no apparent reason, the cause should be sought in depleted soil.
The disease does not require special treatment. To correct acidity, preparations containing magnesium and iron sulfate are added to the soil.
Sunburn
Burns on leaf blades occur in the fall with strong fluctuations in air temperature or in early spring, when the plant emerges from the winter state. If the thermometer drops below 15 degrees below zero, the leaves of the rhododendron curl and freeze. The daytime sun heats the plate on which moisture actively evaporates. As a result, the branches turn yellow, become dry and brittle.
There is no cure for sunburn. To prevent them, the shrub is shaded or moved to a darkened area of the garden.
Winter drying
The disease manifests itself in the spring if the winter was harsh, with prolonged, severe frosts. After the soil has thawed and positive average daily temperatures have established, when the branches should begin to grow, the leaves of the rhododendron remain brown and curled. Gradually they dry out and fall off, the bush dies.
The cause is most often winter loss of moisture, as well as damage to the water supply pathways from the roots to the leaf plates. As a treatment, experienced gardeners recommend abundant watering and frequent irrigation of the crown. Restorative procedures are carried out over 1 – 2 weeks. During this time, the leaves should unwind, restore turgor, and begin to grow. If this does not happen, the rhododendron will die.
The leaves of rhododendrons also turn red in unfavorable weather conditions. If the waterlogged soil does not have time to freeze before heavy snowfall, a greenhouse effect is created and the roots of the rhododendron rot. As the temperature drops, they freeze out, and the plant dies by spring.
Nitrogen starvation
With a lack of nitrogen compounds in the soil, the young leaves of the rhododendron become light, small, grow poorly, and the old ones turn yellow and fall off. By the end of the growing season, only the fresh crown of the current year remains on the shrub, while with sufficient nutrition, evergreen branches are preserved for 4 years.
When the first signs of rhododendron starvation appear, you need to feed it - potassium nitrate or ammonium sulfate. Further treatment consists of timely fertilization of the crop twice a year.
Getting wet
The disease is typical for rhododendrons planted in heavy, clayey substrates, as well as those placed in dark, poorly lit areas of the garden.
Insufficient drainage layer causes waterlogging of the root hole. As a result, the leaf plates first become light green, dull, then begin to turn yellow, fall off, and flowering does not occur.In this case, the root and root collar remain intact. Without timely treatment, the rhododendron will rot and die.
If the soil gets wet, stop watering completely until the soil is completely dry. Sand, straw, and any mixtures that help improve drainage properties are added to the root space.
Insufficient or excessively high humidity
For rhododendron, insufficient or excessive moisture in the soil and surrounding air is dangerous.
Excessive watering leads to rotting of the roots, general weakening of the bush, and infection with spores of pathogenic fungi and microorganisms. With late autumn irrigation, the rhododendron does not have time to slow down its growth, adapt to the drop in temperature and, as a result, freezes out.
Insufficient watering dries out the leaf blades and deprives the bush of nutrition. Such plants do not tolerate winter well, die from drying out, and are often affected by bacterial diseases and pests.
Lack or excess of light
If the rhododendron is placed incorrectly on the site, problems associated with a lack or excess of light may occur. In the first case, the bush stretches out, weakens, and loses its attractiveness. Flowering most often does not occur.
In the second - if the plant is often exposed to direct sunlight - the rhododendron is attacked by diseases and pests. Sunburn appears on it, and the shoots are inhabited by mites and other insects.
Incorrect substrate preparation
Substrate preparation is an important stage in rhododendron planting, on which its growth, development, flowering and viability depend.
High acidity of the soil causes the development of chlorosis, insufficient drainage properties - rot, wilting, and soaking. Sandy soil requires frequent watering, which leads to the leaching of useful substances, especially nitrogen.
Rhododendron pests
The shrub is often attacked by pests. Timely treatment will avoid the death of the plant.
The photo identifier will help you identify the insect that causes rhododendron disease, accurately select the treatment regimen, dosage and the right drug.
The most common pests:
- Furrow weevil – a black beetle, 8–10 mm long, lays white larvae in the ground, which gnaw at the roots. Rhododendron suddenly withers and dies. Adults damage the leaf blades: eaten away areas appear along the edges. Treatment includes spraying with Splender, Iskra, Decis, Actellik.
- Spider mite – actively reproduces in hot, dry weather. It is almost impossible to notice even an adult: the size of the tick does not exceed 0.5 mm. A symptom of its appearance is a thin web covering the underside of the leaf plate, buds, and buds of the rhododendron. Treatment: treatment with Fufanon, colloidal sulfur, Actellik, Fitoverm, Karbofos.
- Acacia false scale – a large (up to 6.5 cm) insect, light brown in color. The pest, attaching its proboscis to young trunks, damages the bark and feeds on plant juices. Rhododendron leaves become sticky. Gradually the bush weakens, loses its decorative appearance, and dies.Treatment: spraying with Fitoverm, Karbofos, Fufan, Aktelik, phosphorus-containing compounds.
- Tobacco thrips - a yellowish-brown winged insect about 1 mm long. An adult female is able to lay up to 100 eggs in the tissue of the leaf blade. On rhododendrons, the pest most often affects the buds. They do not open, turn yellow and fall off. Thrips is a carrier of dangerous viruses. Treatment: disinfection with neonicotinoids, organophosphorus compounds, pyrethroids and other insecticides.
- Rhododendron mite – settles on varieties with a pubescent lower part of the leaf blade. When infected, the plant turns yellow and dark spots appear on it. If the rhododendron leaves turn black and fall off, the disease is already advanced. It is not difficult to see a tick; an adult reaches 3.5 mm, a larva – 2.5 mm. Treatment: manual collection of insects - at an early stage, as well as preventive spraying with nicotine or pyrethrum extract; in case of advanced disease - complete removal of shoots.
- Whitefly – carries viral diseases. The parasite is most often found on large-leaved rhododendrons. Treatment: treatment of shoots with neonicitinoids, nicotine-based pesticides.
- Clams, snails, slugs – appear when there is excessive soil or air moisture. Pests attack buds, young shoots, and buds. Treatment includes: manual collection, treatment with insecticides.
Conclusion
The listed diseases of rhododendrons and their treatment with photos will be useful to gardeners to prevent problems with the development of the crop. If the planting site is incorrectly chosen, unfavorable weather conditions, or non-compliance with the irrigation regime, the bushes are susceptible to severe, incurable viral diseases, and they are often attacked by pests. To save a damaged plant, treat it in a timely manner, select the correct drug and calculate the dosage, it is important to promptly identify the pathogen or insect pest and carry out all the necessary actions, depending on the degree of damage, the age of the bush, and the time of year.
