Elisabeth C. Miller Library logo Miller Library Home UW Botanic Gardens Home UW Botanic Gardens Home book graphic

3501 NE 41st Street, Seattle, WA 98195 | (206) 543 0415 | Open Monday 9-8; Tuesday - Friday 9-5; Saturday 9-3

Gardening Answers Knowledgebase

Search Results for ' Rhododendrons--Diseases and pests'

PAL Questions: 8 - Garden Tools:

Display all answers | Hide all answers


 

Keywords: Whiteflies, Insect pests--Identification, Insect pests--Control, Root weevils, Rhododendrons--Diseases and pests, Dahlia

PAL Question:

I have a line of Ward's ruby azaleas. The three weakest ones have a lot of tiny notches in the leaves. I seem to remember the notches from the root weevil as being larger than these. Are the tiny notches from something else?

I also noticed that some of my dahlias have splotched leaves and that when I disturb the leaves, white-looking insects fly off the leaves. These flies apparently have spread to tomatoes as well. Are these whitefly? Will they disappear after the winter or is there some control I should use to prevent them from taking over?

View Answer:

First you need to get an accurate diagnosis of your problems. If you are in King County, you can bring samples to a Master Gardener Clinic.

Oregon State University offers this information about root weevils and Rhododendron (which includes Azaleas). It describes using beneficial nematodes as a control.

According to Washington State University Cooperative Extension's publication, How to Identify Rhododendron and Azalea Problems (1984), root weevil damage to foliage is not usually a serious problem. You can check for weevils with a flashlight at night to confirm that they are the source of the notches you are seeing. There are some Neem oil-based products that may be helpful, but they must be used at the correct times of year. See WSU's HortSense page (search under Ornamentals, then scroll down to Rhododendron, and select "weevil").

As for the dahlias and tomatoes, it is important to determine exactly what the insects are before proceeding with treatment. If they are whiteflies, you can put yellow sticky traps around the plants to trap them. University of California, Davis's Integrated Pest Management site has other recommended control methods, including reflective mulch. You may not want to use insecticidal soap:
"Insecticides have only a limited effect on whiteflies. Most kill only those whiteflies that come in direct contact with them. For particularly troublesome situations, try insecticidal soap or an insecticidal oil such as neem oil or narrow-range oil. Because these products only kill whitefly nymphs that are directly sprayed, plants must be thoroughly covered with the spray solution. Be sure to cover undersides of all infested leaves; usually these are the lowest leaves and the most difficult to reach. Use soaps when plants are not drought-stressed and when temperatures are under 80 degrees F to prevent possible damage to plants. Avoid using other pesticides to control whiteflies; not only do most of them kill natural enemies, whiteflies quickly build up resistance to them, and most are not very effective in garden situations."

Season All Season
Date 2008-01-10
Link to this record only (permalink)


Keywords: Root weevils, Rhododendrons--Diseases and pests, Gaultheria shallon

PAL Question:

Last year I had a big problem with weevils in my salal, rhododendrons and a few other shrubs. I am not sure if they returned after putting down beneficial nematodes last fall.

View Answer:

Weevils are tough! You are on the right track with beneficial nematodes. It might take a few seasons to make a difference. Here is a link to information by entomologist Art Antonelli of Washington State University about controlling weevils, especially on Rhododendron. Here is another article from Thurston County Hazardous Waste.

Season All Season
Date 2008-01-17
Link to this record only (permalink)


Keywords: Rhododendrons--Varieties, Root weevils, Rhododendrons--Diseases and pests

PAL Question:

My rhodies are being devastated by root weevils. They have stripped many of the branches clean of their vegetation, and have destroyed ~50% of the remaining leaves. My rhodies look like they will require years to recover, if they ever do.

If I replace them with resistant varieties or plants that are not susceptible to these pests, will this eliminate the weevils?

View Answer:

Root weevils are the most common pest attacking Rhododendrons in the Pacific Northwest so they can only be temporarily eliminated from any garden. If the environment is right and their food source returns, so will the root weevils.

If you want to keep your current Rhododendrons, the weevils can be controlled if you’re diligent (forever!?). An article by Caroline Cox in the summer 2005 issue of the Journal of Pesticide Reform discusses their control. However, it sounds as if you’re willing to remove them and start fresh. Some of the most susceptible (host plants) are Rhododendron and Azalea, Heather, Salal, Manzanita and Kinnikinnick, Pieris, Maples, Viburnum, most Conifers, Astilbe, Cyclamen, Helleborus, Hosta and Primrose.
(Source: Root Weevils in the Nursery and Landscape; Identification and Control, by J. DeAngelis and G. Garth, EC 1485, Oregon State University Extension Service).

The extension bulletin from the Washington State University Extension website has an excellent list of resistant Rhododendron varieties.

Season All Season
Date 2008-01-24
Link to this record only (permalink)


Keywords: Rhododendrons--Diseases and pests, Rhododendron, Master gardeners

PAL Question:

My rhodies have black spot, rust. Is there a plant medicine I can put in the soil so it will get absorbed by the entire plant rather than spraying every other leaf.

View Answer:

I am sorry to hear about your sick Rhododendrons. You should take a take a leaf sample into a Master Gardener clinic for (free) diagnosis. I have linked a list of clinics in Snohomish County below. Their volunteers are trained in identifying plant diseases and suggesting solutions.

If you cannot get into a clinic try the Hortsense webpage from WSU Cooperative Extension. Click ORNAMENTALS, then RHODODENDRONS to see pictures and information on what to do.

The reason why it is vital to get an accurate diagnosis is because some fungal diseases do not have treatments that really work, such as rust, while others "leaf spot problems" are not caused by fungus at all, therefore spraying with fungicides or applying a systemic to the soil would only be a waste of time and money!

Try contacting the Snohomish County Master Gardener Clinics to see if you can bring in samples.

Season Summer
Date 2008-01-31
Link to this record only (permalink)


Keywords: Powdery mildew diseases, Plant diseases--Diagnosis, Rhododendrons--Diseases and pests, Master gardeners

PAL Question:

My rhododendrons have a problem. What appears to be a white powder covers the buds and spreads up the leaves. What is it, and what can I do to stop it?

View Answer:

I cannot be absolutely certain without seeing the plants, but it sounds as if your rhododendrons could have powdery mildew.

Here is an article from the Washington State University Cooperative Extension which describes this disease. One preventive measure you should certainly take is to clean up all the fallen leaves and twigs under your rhododendrons, because the fungus which causes powdery mildew can overwinter there.

You could bring in a sample to a Master Gardener Clinic, and ask if they can diagnose the disease as well (they are at the Center for Urban Horticulture and other locations--see their website.

Season All Season
Date 2006-11-07
Link to this record only (permalink)


Keywords: Rhododendrons--Diseases and pests

PAL Question:

The edges of the leaves on one of my rhododendrons are turning brown and crispy. This condition advances toward the midline until the leaf dies. My older, more established rhododendrons are looking fine. All have flowered this spring.

View Answer:

We can't diagnose the problem via e-mail. However, what you describe could be a couple of things.

It might be marginal leaf necrosis which can be caused by cold damage, and made worse by wind and drought; by drought while the plant is in active growth; by high amounts of salt in the soil, often due to overuse of soluble fertilizers, and seen often if plant is located close to the house, where the eaves prevent rainwater from reaching the soil. Other causes are poor drainage, planting too deep, root damage, root weevils, nutrient deficiency, or disease. (Source: How to Identify Rhododendron and Azalea Problems, Washington State University Cooperative Extension, 1984).

It could also be leaf scorch, which can progress from the edges to the center, or just at the tips of the leaves, as described by Washington State University Extension.

For comparison purposes, see the lists of diseases and problems affecting Rhododendrons from University of California, Davis Integrated Pest Management Online. To get a definitive answer, it may be a good idea to take samples of the affected leaves to a Master Gardener Clinic for diagnosis.

Season All Season
Date 2007-08-01
Link to this record only (permalink)


Keywords: Rhododendrons--Diseases and pests

PAL Question:

I think my Rhododendron is infected with lace bugs. What do you recommend?

View Answer:

According to The Organic Gardener's Handbook of Natural Insect and Disease Control edited by Barbara Ellis (Rodale Press, 1996), this problem occurs more often with plants in full sun. The book recommends using insecticidal soap at the first sign of damage. Spray the undersides of the leaves in particular (and always test the spray on a small area first). Repeat the spraying in mid- to late-summer if the lace bugs reappear. If the problem is severe, use superior oil or pyrethrin as a last resort. There is a recipe for homemade soap spray in the book: 1 teaspoon up to several tablespoons of liquid soap (use unscented, nonchemical soap) per gallon of water. Start with a lower concentration and adjust as needed. You can also add insecticides such as horticultural oil, pyrethrins and BTK to increase the spray's toxicity to pests. Mix 1 tablespoon liquid dish soap and 1 cup of oil (peanut, safflower, corn, soybean, or sunflower), When you are ready to apply the spray, mix 1 to 2 1/2 teaspoons of this soap/oil blend with 1 cup of water. (These are just guidelines for proportions.) You can also use commercial products, examples of which are here: Safer Insecticidal Soap and Stoller Natur'L Oil.

University of California, Davis's Integrated Pest Management website has a page about lace bugs. Below is an excerpt:

MANAGEMENT

Tolerate lace bug damage where possible; in most cases, it does not seriously harm plants. Provide proper cultural care so plants are vigorous. No treatment will restore stippled foliage, which remains until pruned off or replaced by new growth. If damage has previously been intolerable, monitor plants early during subsequent seasons. Take action when populations begin to increase and before damage becomes extensive.

Biological Control

Natural enemies of lace bugs include assassin bugs, lacewing larvae, lady beetles, jumping spiders, pirate bugs, and predaceous mites. These predators may not appear in sufficient numbers until after lace bugs become abundant; their preservation, however, is an essential part of a long-term integrated pest management program. Growing a variety of species, mulching soil with organic material, and shading plants from afternoon sun can reduce lace bug damage to shrubs and increase natural enemy abundance. If applying pesticides, using only short-persistence materials such as oils and insecticidal soaps will minimize the number of beneficial predators and parasites that are killed.

Cultural Control

Grow plants well adapted to conditions at that site. Plants in hot, sunny locations are more likely to be damaged by lace bugs. For example, azaleas grown under partial shade experience less damage by the azalea lace bug in comparison to azaleas that are drought stressed and exposed to bright sun. Provide adequate irrigation and other care to improve plant vigor. Prune out damaged foliage if the discoloring is intolerable and relatively localized. Do not remove more than a small percent of a plant's branches during one season and use good techniques so that pruning does not injure plants, such as by exposing inner branches to sunburn.

Chemical Control

Contact Insecticides. Insecticides will not restore an undamaged appearance, but can reduce or prevent further damage. Almost any insecticide will control lace bugs if it is sprayed directly onto the insects. Azadirachtin (Safer BioNeem), insecticidal soap (Safer), narrow-range oil (Green Light, Volck), neem oil (Green Light Garden Safe), or spinosad (Conserve, Monterey Garden Insect Spray, Spinosad Home and Garden) sprays temporarily control lace bugs if insecticide thoroughly covers the underside of leaves where adults and nymphs occur. These IPM-compatible insecticides have very low toxicity to humans and a less adverse impact on natural enemies than more persistent, broad-spectrum insecticides.

Season All Season
Date 2008-01-25
Link to this record only (permalink)


Keywords: Rhododendrons--Diseases and pests

PAL Question:

The leaves of 3 of my rhododendrons are covered with a fine yellow powder that can be rubbed off, but spraying with a hose does nothing. The plants, which are scattered throughout a large garden, look healthy otherwise. They receive regular water, have good exposure and are spaced well from other plants. There are other rhododendronss in the yard that are not affected. Any ideas?

View Answer:

We can't diagnose the problem remotely, but I can tell you a few common rhododendron problems or conditions which sound a bit like what you describe. I consulted a Washington State University Cooperative Extension booklet entitled How to Identify Rhododendron and Azalea Problems, WSU 1984.

If the yellow coating is more like small speckles on the top leaf surfaces, look for small, tarry black spots on the undersides which could be a sign of lacebugs (this may not be a serious enough problem to require control).

If the leaves have light green to yellow diffuse spots randomly distributed around the leaf, and the underside has yellow-orange powdery spore masses, this might be rust (caused by fungus associated with Sitka spruce).

What seems most likely, based on your description, is algae, which coats the leaves with a yellowy greenish powder, and is easily rubbed or scraped off the leaves. Algae does not harm the plant, but may offend one's sense of aesthetics. The Royal Horticultural Society describes algae on rhododendron leaves.
Excerpt:
Shaded, dense foliage is where algae builds up most on plants, particularly at the base of evergreen hedges, such as holly and yew, or on congested shrubs such as Camellia and laurel. These are areas where water does not quickly dry, creating the damp, shady conditions favoured by algae.
Control:

  • Clipping evergreen hedges so that they taper outwards towards the bottom can help improve light access to the base.
  • Congested evergreen shrubs should be thinned out at the appropriate time of year to increase air circulation around the leaves. This will allow rain to dry more quickly on wetted foliage and reduce the opportunity for algae to build up.
  • Feeding straggly plants in the spring to encourage vigour should also help.

Season All Season
Date 2009-07-23
Link to this record only (permalink)


 

Didn't find an answer to your question? Ask us directly!

Browse keywords or Search Again:

We are continually adding new questions, so be sure to keep coming back.

June 24 2013 12:55:25