Contents
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 a photo 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. Processing of planting material, the right choice of location on the site, regular watering and fertilizing allow you to maintain the health of the plant.
Waterlogging, darkening, excess or, on the contrary, lack of nutrients lead to decay, growth retardation, the development of fungus, mold, infection and, ultimately, to the death of 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 untreated, the infected plant slows down in growth, sheds leaves and buds. The main symptoms of bacterial cancer:
- rotting of the root neck;
- the formation of large rounded, very dense growths throughout the root system.
As a treatment at an early stage of the disease, the bush is treated with a Bordeaux mixture. If the infection is running, the rhododendron is uprooted, burned, the site is disinfected with fungicides.
Tracheomycosis wilt of rhododendron
The causative agent of the disease is the fungus Fusarium oxysporum, which affects the vascular system of the shrub. The infection originates in the roots, in which it then causes rapid decay, as a result of which the movement of nutrients is blocked.
If the leaves of the rhododendron turn brown, this is the first sign of the disease. Without treatment, over time, the stem becomes thinner, its crown dries up, a gray coating appears – mycelium. The plant is gradually dying.
The bush can be saved if treatment with Bordeaux liquid is started on time. The affected areas are cut off, burned, 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 root system of the plant. There may be several reasons:
- excessive watering of rhododendron;
- insufficient drainage layer;
- heavy, clay substrate that does not provide proper moisture transmission;
- infection of cuttings in the nursery.
When infected, dark raspberry or burgundy spots appear on the leaves of the rhododendron, the crown of the plant becomes withered, drooping. Stems, cuttings, shoots acquire a purple hue, become thinner. Without treatment, the growth of the bush slows down, flowering stops completely.
The most severe disease affects the roots of rhododendron. They begin to rot, change color to dark brown, stop nourishing the plant.
Treatment begins with the restriction of watering, allow the soil to dry well. The bush, stems, root space are sprayed with fungicides (Bordeaux mixture, Fundazol, Quadris). If no improvement is observed within 2 to 3 weeks of the course 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 agents of this disease are spores of the fungus Botrytis cinerea. They are volatile, 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, 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 the bush, damaged leaves, buds, ovaries are removed, the crown is sprayed with Fundazol once every 2 to 3 weeks until the end of the flowering period.
Rot of seedlings, seedlings and buds
Sudden mass wilting of young shoots of rhododendron is observed when seedlings are affected by fungi (Rhyzoctoni Solani Kuhn, Rhyzoctonia, Botrytis or Pythium). If treatment is not started in time, the root neck of the shoot exudes, turns black, and the stem becomes soft. The sprout collapses on its side and gradually dies off.
On closer examination, white or brown spores of the fungus 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, affects the bush from the inside. Without treatment, the rhododendron withers, stops growing, and eventually dies.
The causes of the disease most often lie in infected planting material or in non-compliance with the rules of agricultural technology: the 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 preventive measure, disinfection of seedlings purchased in the nursery is carried out. In addition, they monitor the frequency of watering, sufficient ventilation and lighting (when planting in a greenhouse).
Withering away of shoots
The disease is typical for rhododendrons growing in the shade. The fungus Phytophtora cactorum infects young shoots. The buds on them do not bloom, turn brown and fall off.
In the absence of competent treatment, the disease passes to the stems, young leaves begin to curl in the rhododendron. Gradually, the bush dies.
To stop the death of the shoots, the affected branches are removed, the crown is sprayed every 2 weeks with any preparation containing copper, 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 a low ambient temperature.
At an early stage of infection, rhododendron leaves wither for no apparent reason. Then they darken, turn brown, young buds gradually die off.
The roots and lower part of the bush begins to rot, darkens, acquires a brown tint.
In most cases, the disease is fatal for rhododendron and cannot be treated. The shrub is uprooted and burned.
It is possible to prevent the development of root rot at the planting stage. To do this, it is worth carefully balancing the acidity of the soil in accordance with the recommendations for a particular variety of rhododendron, monitor the humidity and timely removal of weeds.
Bloated rhododendron leaves
Leaf swelling is also called thick leaf or wax disease of rhododendron. The causative agents are fungi of the Exobasidium family. When infected, young shoots form round, fleshy, spherical growths ranging in size from a pea to a walnut.
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 becomes yellow-brown from above, the reverse side is covered with powdery coating;
- dark brown spots appear, white mushroom spores are visible;
- rhododendron leaves become pale, abnormally thick and large; over time, they wrinkle, mold, dry out.
Treatment consists in the timely removal of the affected branches, the periodic treatment of the bush with fungicides with copper.
Spotted rhododendron
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:
- Pestalocyanic spotting is localized on the crown and stems. The disease is characterized by the appearance of irregularly shaped brown spots with a brown frame. Over the damaged areas, pads with spores are visible. Treatment: removal of infected shoots, spraying with Bordeaux liquid or Camulus.
- Septoria spotting appears on the leaves of rhododendron. You can recognize the disease by red rounded spots with black spores of the fungus in the center. As the disease develops, the leaf plate dries, turns yellow, and curls up. Treatment includes pruning the infected part of the crown, treating the plant with Camulus.
- anthracnose spotting is determined by brown, brown spots with scattered dark spores on the upper surface of the leaf plate. The reverse side of the sheet turns pale. Gradually, the disease passes to the stems, weakening the plant. Treatment: pinching damaged leaves, processing branches with a Bordeaux mixture.
- phyllostictous spotting is characterized by the appearance of red lesions that turn white over time, dry out and crumble. In the advanced stage, black dots are visible on the leaf plate – spores. Treatment is reduced to careful pruning with the complete removal of infected shoots, spraying with a suspension of Cineb or Captan.
The cause of the development of the disease is most often improper agricultural practices: excessive watering, incorrect pruning, untimely fertilization.
Cerkosporoz
The disease develops on the lower tiers of the bush, it is caused by spores of the fungus Cercospora. In the early stages of infection, the leaf blades are 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 becomes dark brown, the shoots stop growing, 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, Fundazol.
Rust
The disease affects small-leaved varieties, it manifests itself most often in autumn. Rusty, brown, reddish or yellow spots appear on the leaves of rhododendron. 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 ahead of time. Without treatment, this leads to the death of the shoots and the death of the entire plant.
At the first sign of rust infection, diseased leaves are cut off and burned. The bush is treated with preparations with a high copper content (for example, Bordeaux liquid).
Mosaic
A viral incurable disease caused by the Rhododendron mosaik virus. Most often it is carried by insects: aphids, bedbugs and others.
When infected, the rhododendron stops blooming, its growth slows down. The leaves of the plant become thinner, turn yellow in places, brown spots appear on them. The surface becomes rough, rough, greenish tubercles are formed – calluses. In the advanced stage, the leaves of rhododendron darken, are 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 wrong location of the bush, errors in agricultural technology, adverse weather conditions.
To save the bush, you need to know the main non-parasitic diseases of rhododendrons, prevention and control measures.
Rhododendron leaf chlorosis
Chlorosis is detected by pale spots that have appeared on the leaf plate. At an early stage, the leaf veins remain bright green, then they turn pale. The disease passes to branches, young shoots, buds, shrubs become vulnerable to sunburn.
Chlorosis develops with a lack of nutrients (magnesium and iron), as well as increased soil acidity. If the leaves of the rhododendron turn yellow for no apparent reason, the cause should be sought in the depletion of the soil.
The disease does not require special treatment. To adjust the acidity, preparations containing magnesium and iron sulfate are introduced into the soil.
Solar burns
Burns on leaf plates occur in autumn with strong fluctuations in air temperature or in early spring, when the plant emerges from its 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 the moisture actively evaporates. As a result, the branches turn yellow, become dry, brittle.
There is no cure for sunburn. To prevent them, the shrub is shaded or transferred to a darkened area of uXNUMXbuXNUMXbthe garden.
Winter drying
The disease manifests itself in the spring, if the winter was severe, with prolonged, severe frosts. After thawing the soil and establishing positive average daily temperatures, when the branches should grow, the leaves of the rhododendron remain brown and twisted. Gradually they dry up and fall off, the shrub dies.
The reason is most often the winter loss of moisture, as well as damage to the water supply paths from the roots to the leaf plates. As a treatment, experienced gardeners recommend abundant watering, frequent irrigation of the crown. Restorative procedures are carried out within 1 – 2 weeks. During this time, the leaves should unwind, restore turgor, and begin to grow. If this did not happen, the rhododendron died.
Leaves turn red in rhododendrons and under adverse weather conditions. If the waterlogged soil does not have time to freeze before heavy snowfall, a greenhouse effect is created, 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 remain for 4 years.
When the first signs of starvation of rhododendron appear, you need to feed – potassium nitrate or ammonium sulphate. Further treatment consists in the timely fertilization of the crop twice a year.
soaking out
The disease is typical for rhododendrons planted in a heavy, clay substrate, as well as placed in dark, poorly lit areas of the garden.
Insufficient drainage layer causes swamping of the root hole. As a result, the leaf plates first become light green, dull, then they begin to turn yellow, fall off, flowering does not occur. In this case, the root and root collar remain undamaged. Without timely treatment, the rhododendron will rot and die.
When wet, watering is stopped completely until the soil is completely dry. Sand, straw, any mixtures that improve drainage properties are introduced into the root space.
Insufficient or excessively high humidity
For rhododendron, insufficient or excessive moisture in the soil and the surrounding air is dangerous.
Excessive watering leads to rotting of the roots, a general weakening of the bush, infection of it with spores of pathogenic fungi and microorganisms. With late autumn irrigation, the rhododendron does not have time to slow down growth, adapt to lower temperatures, and as a result, it freezes.
Insufficient watering dries up the leaf plates, deprives the bush of nutrition. Such plants do not tolerate winter well, die from drying out, they are often affected by bacterial diseases and pests.
Lack or excess of light
With the wrong location of the rhododendron on the site, there may be problems associated with a lack or excess of light. In the first case, the shrub stretches, weakens, loses its attractiveness. Flowering usually does not occur.
In the second – if the plant is often exposed to direct sunlight – the rhododendron is attacked by diseases and pests. Sunburns appear on it, shoots are inhabited by ticks and other insects.
Incorrect preparation of the substrate
Substrate preparation is an important stage in planting a rhododendron, on which its growth, development, flowering and viability depend.
High acidity of the soil causes the development of chlorosis, insufficient drainage properties – rot, wilting, soaking. Sandy soil requires frequent watering, which leads to leaching of nutrients, especially nitrogen.
Pests of rhododendron
The shrub is often attacked by pests. Timely treatment will avoid the death of the plant.
The photodetector will help to recognize the insect that causes rhododendron disease, to accurately select the treatment regimen, dosage and the desired drug.
The most common pests:
- furrowed weevil – a black beetle, 8 – 10 mm long, lays white larvae in the ground, which gnaw the roots. Rhododendron suddenly withers, dies. Adults damage leaf plates: eaten away areas appear along the edges. Treatment includes spraying with Splender, Spark, Decis, Actellik.
- spider mite – actively breeds in hot, dry weather. It is almost impossible to notice even an adult individual: the dimensions of the tick do not exceed 0,5 mm. A symptom of its appearance is a thin web covering the underside of the leaf plate, buds, rhododendron buds. Treatment: treatment with Fufanon, colloidal sulfur, Actellik, Fitoverm, Karbofos.
- Acacia false shield – a large (up to 6,5 cm) insect, light brown in color. The pest, attaching its proboscis to young trunks, damages the bark, feeds on the juices of the plant. Rhododendron leaves become sticky. Gradually, the shrub weakens, loses its decorative appearance, and dies. Treatment: spraying with Fitoverm, Karbofos, Fufan, Aktelik, phosphorus-containing compounds.
- 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 a leaf plate. 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, organophosphates, pyrethroids and other insecticides.
- Rhododendron mite – settles on varieties with a pubescent lower part of the leaf plate. When infected, the plant turns yellow, dark dots appear on it. If the leaves of the rhododendron turn black and fall off, the disease is already running. 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; with 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.
- Shellfish, snails, slugs – appear with excessive soil or air moisture. Pests affect buds, young shoots, buds. Treatment includes: manual collection, insecticide treatment.
Conclusion
The listed diseases of rhododendrons and their treatment with photos will be useful for gardeners to prevent problems with the development of culture. With an incorrectly chosen landing site, adverse weather conditions, non-compliance with the irrigation regime, the bushes are susceptible to severe, incurable viral diseases, they are often attacked by pests. To save the damaged plant, its timely treatment, the correct choice of the drug and the calculation of the dosage, it is important to determine the pathogen or pest in a timely manner and take all the necessary actions, depending on the degree of damage, the age of the shrub, and the time of year.
