Pests and diseases of rhododendrons, their treatment and control
Different types and varieties of evergreen and deciduous rhododendrons are characterized by certain diseases. The pest susceptibility of each generic representative is also special. Evergreens of this genus are most susceptible to damage from insects and diseases when growing in open ground in a sunny place, such perennials are depleted faster and therefore lose their immunity. Compliance with agricultural techniques of cultivation and care of species of the Heather family guarantees their protection. The material in detail describes a variety of symptoms and possible causes due to which they appear.
Rhododendron diseases and their treatment
The main pathogens of culture of the genus Rhododendron are fungi. They arise in connection with poor aeration of waterlogged soil. Plants weaken and die as a result. Regular spraying with Bordeaux fluid can help; infected parts of rhododendrons need to be destroyed. The following are diseases, their manifestations, as well as methods for treating fungal infections.
Spots of different colors on the leaf and stem parts of rhododendron with spotting
The genus is susceptible to several types of pathogens. The symptomatology of spotting depends on a particular type of fungus. The external signs of the four diseases described in the list appear differently and can change over time. Accurate diagnosis will help to recognize the causative agent of the lesion.
Did you know? Joseph Louis Proust mixed lime and copper sulphate, gave a means to the winegrowers for their harvest, so that it looked like mold, and this scared away thieves. So there was a Bordeaux liquid.
The list includes:
- Pestalocium spotting. The fungus infects foliage and stems. Manifestations: small brown curved spots with a border on the leaves, located at their edges. Further formation of gray thickenings (sporulation of the pathogen). Leaf blades turn yellow and dry. On the stems, the marks are large, they are depressed and elongated. Their surface dries and becomes lighter with time. Traces of sporulation appear. The shoots begin to dry out. To overcome the disease It is important to prune all infected parts of the shrub. In spring, you need to irrigate with 1% Bordeaux liquid (blue vitriol in lime milk) prepared according to one of the known recipes (example: dilute 1 kg of vitriol in a small amount of hot water and add cold to 50 liters, then extinguish 1 kg of lime in a different capacity in 50 liters of liquid, mix both solutions). You can use another fungicide and follow its instructions.
- Anthracnose spotting. Brown marks on top of leaves. Stems can also be affected, which after dry up. Peculiar to Pontic rhododendron. Control measures the same as with the previous disease.
- Septoria azalea, or small spotted leaves. The reason is infection with the fungus. Symptoms may be: small red spots that become gray, or yellowish, browning over time. They are on the foliage, and it falls as a result. Fight with septorious infection follows the above method. You can not spray plants in wet cloudy weather.
- Phyllostic spotting. The fungus is determined upon detection of round spots with a red border on the leaves or blurry brown (or ashy), which are located on the edges of the green part of the rhododendron. Control measures such as with other fungal lesions.
Sporangia of different colors with rust
The causative agent is a fungus characteristic of small-leaved species, Adams, Daurian, rusty and myrtle. On the back of the leaves are yellow, red and even brown swellings. The green of the plant falls. They must be collected and burned.
Did you know? The symbol of the US state of Washington is a large-leaved rhododendron species.
At an early stage of the disease, the use of Bordeaux fluid will help (spring spraying with 1% solution). Next, it is worth fighting rust with copper-containing agents (fungicides) twice every week or two.
Yellowness when mosaic
Viral infection, insects carry it. Symptoms of two types:
- the appearance on the leaves of yellow spots in the form of a mosaic, weak flowering, almost complete yellowing of the foliage with the presence of green and brown areas;
- the green part of the plant is mutilated, loses smoothness, a large area of green becomes yellowish, the calloused areas are dense.
Brown leaves and roots with tracheomycosis wilting
A fungal disease destroys the root system, it turns brown and rots. Leaves turn brown, dry, fall off with petioles. A grayish mycelium appears on the cortex. Helps to defeat the infection only complete destruction of plants. Prevention is the irrigation of shrubs and soil in the spring with a 0.2% solution of Fundazole.
Withering of individual leaves with late blight (late blight) rot
The cause of infection with Phytophthora cinnamomi Rands fungus is soil waterlogging or lack of drainage. After a sluggish state of greenery, yellowing of the branches occurs, and the roots gradually turn brown and rot. The root neck is covered with brown spots, and the bark rots. Sporulation looks like dense gray marks.
Important! For late autumn prevention, you can make a 3% solution of Bordeaux mixture.
Brown spots without rims with cracks in gray rot
The gray botrytis mushroom causes this type of rot. The browning is detected on all parts of the plant - even on the petals. Traces dry, crack, necrosis with a large amount of moisture are covered with a coating of sporulation.
All damaged leaves, shoots and buds must be removed. In the industrial cultivation of rhododendrons, they resort to the use of a 0.2% solution of Fundazole for preventive irrigation and irrigation of soil near plants.
Root neck growths with bacterial cancer
The disease is caused by soil bacteria. The root system is overgrown with the resulting seals, which harden and darken over time. Shrubs begin to sluggishly develop, which affects flowering - it weakens.
Next, the rhododendron rots from the root and dies of cancer. If the garden culture is affected to a small extent, it is regularly treated with fungicides until it is completely cured. If it is impossible to save perennials, they must be destroyed.
Swelling with wax disease
From the defeat of the fungus, the leaves are deformed and swell, then covered with red-brown ellipses. On top of the necrotic parts, a waxy layer of sporulation is formed.
Further, all spots crack. Wax disease is characteristic of alpine species of rhododendrons. Sick plants must be removed and burned.
Angular marks on foliage with cercosporosis
Irregular spots have a dark brown hue, have edges of a reddish hue, are noticeable from the bottom of the sheet. Its upper side is covered with sporulation.
The lower half of the bush is most affected by cercosporosis. The Pontic species and its varieties become infected more often. Fungal disease leads to the fact that the plant must be destroyed.
Weevils are furrowed, spider mites, false scutes and bugs often affect rhododendron species. Among the pests that feed on parts of a plant of this genus, there are small whiteflies, various slugs and mollusks, thrips, as well as a mealybug. All these insects are very different and require a detailed explanation of measures to combat them.
Did you know? Residents of Nepal are very fond of rhododendron: its flowers are eaten fresh and pickled, they squeeze juice from them.
The main symptom of the presence of furrowed weevil is eaten leaves. Description of beetle imago:
- black color;
- elytra furrowed;
- body length 8–10 mm;
- do not fly;
- have an elongated head.
The larvae look like this:
- white with a brown head;
- curved shape;
- lack of legs;
- length - 12 mm.
After this stage, a chrysalis appears, and then a bug. Weevil females are able to lay up to 1000 eggs in the ground, from which larvae soon emerge. They instantly begin to eat leaves, eaten areas can be seen at the edges of greenery.
In order to combat this pest, such means are used:
- Decis (0.5–1 g per 5 l of water);
- “Actellik” (dilute 2 ml of the ampoule in 2 l of water and use on 10 m² of plants or from 2 l on the bush);
- "Spark" (1 tablet per 10 liters of water, while 2 liters of solution is enough for 10 m²).
It is difficult to notice this pest, yellowing and drying of the foliage will tell about its presence. Insect sizes are too small. The leaves damaged by them turn yellow, lose their shape, and then turn brown and dry.
To fight with ticks, spraying with drugs helps:
- “Fitoverm” (from 2 to 10 ml of the product per 1 liter of water, which is then diluted in 10 liters of liquid; spraying is two-time, then you need to change the insectoacaricide);
- Karbofos (20 L of solution per 100 m²);
- "Actellik" (see above) and others.
False shield leads to a general weakening of the rhododendron. Insect Description:
- brown female from 3 to 6 mm long without wings;
- the male has a size of about 5 mm; his body is thin red-brown;
- larvae less than a millimeter.
The tick affects the bark, attaches to the shoots. Rhododendron weakens, dries. To control insects, pyrethroids are sprayed. The above insecticides are used. For example, Karbofos, which is dissolved and used in an amount of 90 g per 10 liters of water. In spring, larvae can be collected manually if there are few of them.
Rhododendron mite, or azalea bug (the appearance of black spots)
It affects the rhododendron of Ungern, Smirnov and others with pubescence of the lower part of the leaf. The main symptom of its presence is the appearance of black spots on the leaves. Shiny beetles about 3 mm in size, yellow larvae - up to 2 mm. The yellowing foliage below is covered with black tarry spots, the leaves turn black, curl up and dry.
The bug infects the plant in the summer, carried with peat or pine needles. In the case of a small number of insects, they can be collected independently. At the end of spring, nicotine and soap are sprayed (tobacco is poured with hot water, sedimented for three days, diluted with 2 liters of water, soap shavings are introduced into the infusion, and the affected areas of the bush are plentifully irrigated). If rhododendron is severely affected, its shoots must be cut off.
Otherwise called a white fly, it feeds on large-leaved species and their varietal representatives. The whitefly is small in size, it is a pale white color, covers the foliage from below. If detected, the leaves should be sprayed with nicotine or neonicotinoids in spring and autumn. All affected parts of the plant are burned.
Important! If you systematically use drugs with one main active ingredient, insects will get used to it.
These representatives of gastropods can be collected manually, arrange traps for them (ordinary boards on the ground left overnight). Slugs not only eat plants, but also carry diseases.
There are molluscicides - special chemicals. They include copper sulfate, metaldehyde, and bleach. 15 g of metaldehyde is enough for 5 m² of mollusk-infected area.
Thrips damage the buds. Their size is small, they appear after wintering from the upper layers of the soil and in April begin to feed on weeds and lay eggs. Then the females fly to cultural and ornamental plants. Insects are spreading viral infections.
Pests infect flower buds that cannot open and fall as a result of yellowing. Measures against insects are the same as in the case of combating false shields.
Mealybug (detection of wax plaque on a plant)
Pests settle on shoots, leaves and other parts, slow down the growth and development of the culture, you can identify them by the presence of wax coating on the plant. Shaggy white insects are noticeable, they prefer dry weather. To avoid the appearance of a mealybug, you should inspect the bushes, rinse by hand the foliage and even the stems.
Important! Such drugs as Fitoverm, Confidor, must be used for severe lesions. Processing is carried out at intervals of one to two weeks.
A mild degree of damage allows you to use simple methods:
- with a cotton swab to be moistened in soapy water, clean rhododendron;
- then perform spraying with a solution of green soap (use 10 g per liter of water each of three times, the interval between irrigation is a week);
- You can use the pharmacy tincture of calendula.
Diseases caused by poor conditions and improper care
Fungal diseases appear due to non-compliance with agricultural standards and the rules for choosing the place of landing of rhododendron. Temperature above + 15 ° С with excessive moisture also favors such diseases. A bush thickened with shoots also suffers more than others. Gray mold is characteristic of such conditions. When it leaves turn brown.
Important!Too much moisture, which can be formed due to the lack of a drainage system for shrubs, is attractive to fungi.
If the aphid is not controlled on time, then it can transmit a viral infection to the plant. Unsuitable substrate (improper acidity) exposes the culture to diseases:
- late blight;
- powdery mildew;
- root rot.
After the fight against these fungal diseases (and spotting too), it is necessary to monitor the correct application of fertilizers and the state of the soil. Surviving and strong plants have good immunity if they grow under normal conditions.
Compliance with the agrotechnical requirements of rhododendrons of various species and varieties guarantees excellent cultural development. It is difficult to deal with fungi, bacterial diseases and insect pests, often toxic agents (especially insecticides or improperly prepared Bordeaux mixture) also affect the genus shrubs.
Chemical burns are very harmful to plants. But the list of infections is so great that the gardener, if he wants to grow a beautiful perennial and enjoy it for several years, will somehow learn the rules of planting and care, learn from mistakes, experience and stock up on the secrets of plant growing.