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Issues With Rhododendrons: Dealing With Rhododendron Insect Problems And Diseases

Issues With Rhododendrons: Dealing With Rhododendron Insect Problems And Diseases


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By: Susan Patterson, Master Gardener

Rhododendron bushes are similar to azaleas and members of the genus Rhododendron. Rhododendrons bloom in late spring and provide a burst of color before summer flowers set in. They vary in height and shape, but all produce a plethora of blooms that are perfect for shady, acid-rich areas in the garden.

Issues with rhododendrons are rare as they are relatively low maintenance plants. Rhododendron pests and disease harm plants that are stressed due to environmental conditions or injury. Common problems of rhododendron bushes can be avoided by providing the best growing environment possible and maintaining a consistent pruning, mulching and fertilizing program.

Plant your rhododendron in a shady area that drains well has a pH of 4.5 to 6.0 and provide fertilizer several times during the spring and summer to encourage growth. Mulch to retain moisture and provide protection.

Rhododendron Insect Problems

Of the few rhododendron insect problems that exist, most can be handled first through prevention or subsequent treatment with neem oil. Here are some common pests affecting this shrub:

  • Spider mites – Spider mites feed off of bud and leaf sap, leaving leaves yellow or bronzed.
  • Lace bugs – If the upper sides of leaves are speckled green and yellow, then lace bugs may be at work. The tiny lace bug does most of its damage in the spring and summer and tends to be most problematic on rhododendrons that have been planted in sunny locations. The young insects feed on sap and leave small drops of black excrement in their path.
  • Weevils – The adult black vine weevil is a night-feeding insect that is about 1/5 to 2/5 (5 ml. to 1 cm.) inch in length. It is most prevalent from May through September. The weevil feeds on leaves creating a C-shaped notch around the leaf margin. Although the damage is not attractive, it presents no serious risk to the bush.

Before treating your rhododendron for pests, be sure that you have a professional identify your problem and assist you with a treatment plan. Check with your local Cooperative Extension Office for assistance.

Diseases of Rhododendrons

Few diseases of rhododendrons are also prevalent. These include:

  • Chlorosis – Chlorosis, an iron deficiency, is common in rhododendrons and causes leaves to turn from a rich dark green to a light green or even yellow. New leaves may even emerge completely yellow. Chlorosis becomes a problem when the soil pH is 7.0 or higher. Amending the soil with sulfur and providing an iron fertilizer will help correct the problem.
  • Fungal dieback – Many different fungi cause a disease known as dieback. Leaves and the terminal portion of branches wilt and eventually die back. Soil that is infected, heavy rain and splashing water will spread fungi that enter the bush through weak areas. Cut off all infected areas and destroy them. Spray copper sulfate fungicide after blooming and repeat at least two more times in two-week intervals.
  • Winter burn – Rhododendrons that are exposed to a very dry winter can experience winter burn. Leaves curl up to protect moisture loss and will eventually die. Protect rhododendrons from winter burn by planting in a protected area and mulching heavily. Be sure to water your plants consistently before winter.

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Rhododendron Leaf Problems

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You’ve put in the work and waited all year for the blooms to begin on your rhododendron or azalea. While the bushy evergreen offers a spot of color in the garden all year long, it can be hiding problems that will pop up on the leaves or blooms as the weather warms. The hardy leaves of the rhododendron can lose their lovely appearance due to several diseases attacking the garden favorite.

The main cause of unsightly foliage or a lack of flowers is fungi. Unfortunately, there isn’t a specific time period, as fungi will attack at any time of the year. Winged pests can also create issues that often appear in spring and summer as they migrate from garden to garden.


Rusts

Rusts are caused by a wide range of fungal diseases and tend to create yellow or brown blotches on the upper surfaces of the rhododendron’s leaves and reddish, orange or yellowish-orange spots on the undersides. The leaves will also have orange gelatinous spots and can become distorted or stunted. Treat rhododendrons for rust diseases by raking away infected leaves, pruning off diseased leaves, twigs and branches, and then destroying or discarding the infected plant parts. You may also apply an appropriate fungicide to control the disease in severe infections.

  • Rhododendrons are woody, evergreen shrubs that bloom in large clusters of white, pink or red flowers.
  • Treat rhododendrons for rust diseases by raking away infected leaves, pruning off diseased leaves, twigs and branches, and then destroying or discarding the infected plant parts.

Leaf Gall

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Commonly seen in azaleas, leaf gall is a springtime disease, that may affect other rhododendron species. In this case, the leaves, buds, or stems, develop swollen growths with distorted shapes. Sometimes, these growths may get covered with white spots. This condition is caused by the fungus Exobasidium, and it can be controlled by removal of swollen parts, that have to be burned.


What are the Different Rhododendron Diseases?

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There are a number of different Rhododendron diseases, with the most serious being root rot and dieback of the branches. Other diseases of Rhododendron include petal blight, leaf gall, and leaf spots. Growing these plants under ideal cultural practices can help prevent most Rhododendron diseases and pests.

Root and crown rot is the most serious of the Rhododendron diseases in many parts of the country. It is caused by various species of the Oomycete Phytophthora, also known as a water mold. Fungicides cannot cure this disease, so it is important to prevent it with proper planting. This disease is a problem when the soil has excessive amounts of water.

Rhododendrons should be planted in a soil with a lot of organic matter and not be planted too deeply, so their crown is not covered. It is also important that the soil drains well. The first sign of this disease is often a wilt, with the roots appearing blackened or soggy. It may be possible to save the plant by replanting it with improved drainage and organic mulch.

If the plant dies, it is not a good idea to immediately plant another in its place, since the fungus typically remains in the soil. Even resistant varieties will succumb under these conditions. It may be possible to continue to grow Rhododendrons in this site if the soil is improved.

Another serious Rhododendron disease is dieback of the branches caused by a fungus called Botryosphaeria dothidea. This disease is first visible on seemingly healthy plants when dying branches appear. If the bark is scraped away with a knife, there will be a reddish-brown discoloration underneath on the dying branches.

Dieback is a difficult Rhododendron disease to control. Infected branches should be pruned well below the discolored wood, then disposed of. Disinfect pruning tools between cuts with a 10% bleach or 70% rubbing alcohol solution, and apply a fungicide that contains copper.

Leaf gall is a common springtime Rhododendron disease, but it is not particularly serious. plants affected by it have leaves, buds, or stems that develop swollen, distorted growths in April or May. Sometimes infected leaves become covered with white spots. This is caused by various species of the fungus Exobasidium. Control mainly consists of picking off the galls and disposing of them.

Leaf spot is caused by a group of Rhododendron diseases that consists of various fungi. Species of Septoria, Colletotrichum, Phyllosticta, and Cercospora are responsible for this primarily cosmetic disease. One should remove leaves that have fallen, and take care when watering to keep the leaves dry. Severe cases can cause premature leaf drop and can be treated with fungicides.

Another common Rhododendron disease is petal blight, which is caused by the fungus Ovulinia azaleae. This manifests as spots on the flowers that cause them to rot. It can be easily spread from one flower to another by rain, wind, and insects.

This fungus lives in the soil, so one should replace the ground cover with a mulch that has not been contaminated with petal blight. An additional important treatment and control measure is to destroy any flowers that show signs of infection. Also, avoid watering from above — and only water the plants from below.


Caring for Rhododendrons

Mulch plants in the spring. A couple of inches of pine bark or pine straw will protect rhododendrons' shallow root systems and retain moisture.
Pro Tip: Pine straw and pine bark acidify the soil as they decompose, so pine-based mulch builds soil that's ideal for acid-loving rhododendrons.

Fertilize them sparingly in the early spring, even if they are summer or fall bloomers.

Deadhead after they bloom. This will promote leaf and branch growth. Snip those spent blooms off carefully, though. Next year's flowers are just under those old flowerheads, on the old wood.

In areas with severe winters, put a heavy mulch on evergreen rhododendrons to keep their roots from freezing.

Don't prune them. Rhododendrons are prettier if you let them sprawl into their natural, rambling shape. If you need to trim them back to reduce their size, prune them immediately after they flower or you won't get blooms the following year.

Remove dead or diseased branches. You can do this at any time of the year.


Common Problems of Rhododendron - Learn About Rhododendron Pests And Disease - garden

Insect and Disease Management

When rhododendrons and azaleas are properly planted and maintained, insects and diseases are less of a problem. Usually, when problems do occur, they can be traced back to the plant's environment. Two diseases, branch die-back and Phytophthora root rot in particular, are opportunistic this way.

Die-back of entire branches is usually caused by the fungi Phomopsis or Botryosphaeria. These fungi thrive during dry periods when the plants are stressed. If you notice that the leaves are wilted in the morning and the ground is dry, it is best to water to keep these fungi at bay. Infection takes place through wounds, such as new leaf scars, pruning damage, bark cracks, etc.

Root rot, which can kill entire plants, is usually caused by the fungi Phytopthora. If the leaves are wilted in the morning and the ground is moist, this is a symptom of root rot. It is usually fatal. Hence, it is best to avoid planting rhododendrons and azaleas in areas with poor drainage. In fact, good drainage is one of the most important considerations. Once symptoms are visible it is usually futile to try to stop Phytopthora from progressing in a plant.

Armillaria Root Rot is a common soil fungi on the U.S. west coast and elsewhere. Infected plants grow slower than normal plants, affected leaves may yellow, wilt and drop off. Death may occur after several years. Armillaria infection can be identified by the presence of honey-colored mushrooms at or near the base of the plant, and by a layer of white tissue between the bark and wood on the plant's trunk or on large diameter roots.

Powdery mildew sometimes exhibits the typical white powdery or fuzzy growth, but often takes on a completely different appearance. The white powder form is prevalent on deciduous azaleas. It is more severe on shaded plants and is worse in crowded plantings in damp locations. It is more severe in periods of cool, moist weather. Management includes increasing air flow within and around affected plants, removing dead leaves from the ground and reducing the number of infected leaves. It can be controlled with fungicides which should be applied during periods of new growth on the new leaves. Fungicides won't get rid of existing infections on old leaves. Consider growing mildew-resistant deciduous azalea cultivars. When buying plants look for those specifically mentioning mildew resistance.

On evergreen rhododendrons, light green or yellowish patches on the top of leaves sometimes accompanied by purple-brown areas on the backside of leaves are signs of powdery mildew. A puzzling aspect of this fungal disease is its varying appearance on different cultivars. For instance, the rhododendron cultivar 'Unique' shows almost no upper leaf change other than occasional very faint lighter yellowish areas, while the underside of the leaves is completely covered in brown spots. Another cultivar, 'Virginia Richards,' gets brownish purple spots on both tops and bottoms of leaves. Vulnerable cultivars include 'Elizabeth', 'Lady Chamberlain', 'Unique', 'Virginia Richards', 'Seta,' and 'Mrs. G.W. Leak'. Many rhododendrons, if basically healthy, will coexist with the disease and seem to outgrow or at least survive the symptoms. If you notice the symptoms on last year's leaves, consider protecting the new growth with a fungicide. Apply it to the new growth as it expands, before the symptoms appear. Thorough leaf coverage is necessary for effective prevention. The fungicide, 'Remedy,' which is a potassium bicarbonate (made by Bonide Company), is registered for the problem. Fungicides containing sulfur (such as Safer Garden Fungicide RTU) are also registered, but should not be used when the temperature is over 85°F or within a few weeks of an oil spray application.

Gall is fruit-like growth in a leaf or flower petal caused by spores of the fungus Exobasidium. Fungicide control of the disease generally has not been successful. Removal and disposal of galls before they become white-colored is the most effective means of controlling the disease.

Petal blight causes spots in a flower petal to look like they are wet. Eventually the entire flower becomes slimy and sticks to the leaves. Spores of the fungus Ovulinia cause petal blight. There are sprays for this disease, but the best control method is to remove all diseased material as soon as it is found, and to remove old plant material under the plants to prevent future infections. Also avoid overhead watering and prune plants to keep them open so air can circulate through the plants since prolonged moisture promotes this disease.

Common insect pests on rhododendrons and azaleas include:

Lace bug: If you notice leaves that look diseased, always look at the under side. A common pest is the lace bug. Both adults and nymphs feed on the backside of the leaf resulting in formation of yellow spots. Heavy infestations can cause leaves to turn brown and drop. Lace bugs can be controlled by frequent spraying. There are lace-bug resistant varieties, that are usually labeled as suitable for planting in sunny locations. Lace bugs thrive in sunny locations since their natural enemies avoid these locations. Rhododendron varieties that are susceptible to damage should be grown in more shaded locations.

Weevils: If leaves have notches in the outside edges, these are caused by weevils. Weevils spend the daytime in the ground and come out at night to feed. Since they cannot fly, they can be easily controlled with sticky substances like Tanglefoot that catch the adults when they climb the stem at night. If you use this technique always make sure that no branches touch the ground or the weevils can bypass your trap. Weevils not only eat the edges of leaves, but their larvae feed on the plants roots and stem, often completely girdling (removing a ring of bark) the stem and killing the plant. If a plant is in general decline and weevil feeding is evident, you may need to use a systemic insecticide recommended by your extension service or garden center. Biological control using entomopathogenic nematodes is also effective.

Thrips: Thrip damage is characterized by a bleached, silvery white discoloration on the leaf's upper surface and small black specks of excrement on the leaf's underside. Thrips are a problem in warm and dry climates like California and New Zealand. In cooler climates it's a common pest in greenhouses. Foliar spraying with an insecticides or insecticidal soaps is used to control trips. Insecticide application on the both the top and bottom of leaves is important for best results. Two sprayings one-to two weeks apart may be required for effective control. It is best to remove infected flowers.

Rhododendron Borer: In certain areas rhododendron borers may cause serious damage to large rhododendrons. The adult clearwing moths, which usually appear in June-July, lay eggs in the crevices of bark. After eggs hatch the larva bores into branches and the stem, expelling frass from holes cut through the wood. Weakened stems or branches may break or die. As soon as observed, infected branches should be cut and destroyed. Be sure to cut low enough to eliminate the borer.

Other pests that may cause problems are spider mites that feed on the underside of leaves, scales that feed on stems, aphids that feed on new growth, the red-headed azalea caterpillar that feeds on leaves, and leafminers that tunnel in leaves. If these problems are sufficiently troubling, seek advice from your local extension service or a garden center.

Insecticides and Other Chemicals: Always check with your local extension service or garden center when looking for chemical controls for insects and diseases. These chemicals are controlled substances, and must be labeled for the problem and plant in question. When using chemicals always carefully follow the instructions on the label, and use appropriate personal protection to insure your own safety.


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