Azalea and rhododendron desease #157214 - Ask Extension


Azalea and rhododendron desease #157214

Asked October 15, 2013, 6:06 PM EDT

Hi: Many of my mature azalea and rhododendron bushes acquired a disease: very small black spots form on the back of the leaves after which the leaves start to become yellowish with light brown spots and fall. I could not identify the disease based on descriptions and as a result I do not know how to treat it. A neighbor suggested to try to spray the leaves from underneath with water with dish-washing liquid. Could you please help? Thanks in advance. P.S. I tried to upload 5 pictures (their total is less than 2 MB) but it appears that only one picture can be uploaded... Susan F , Portland, Washington County, OR

Washington County Oregon

Expert Response


Thank you for including an image with your question.  It clearly illustrates damage from the azalea lace bug, a serious pest which arrived in Oregon about 6 years ago.  The small dark spots on the undersides of the leaves are fecal deposits.  These insects are very small, only about 1/8-inch.  And because they are translucent, they’re very difficult to see.

Among the keys to successfully battling lace bugs are these:
- Stressed plants appear to be the most common victims. To relieve stress, thin out individual plants to increase aeration, and ease of applying sprays; irrigate every two weeks through our dry months for good health; and decide if the plants are in excess sun – if so, consider rigging temporary shade or moving the shrub in the fall.
- Start spraying the undersides of the leaves when the first generation hatches, usually mid-May. Monitor to determine the appropriate timing. To do so, check the undersides of the leaves every several days as you look for the small dark colored nymphs (youngsters). The most effective time to spray is right after the hatch, when the nymphs are in a rather tight cluster.
- Insecticide sprays must thoroughly cover the undersides of the leaves. (See the list below.)
- Repeat sprays of these materials are required during the growing season because the lace bugs have multiple generations. You have the best chance to decrease the population – and damage – when new generations hatch in mid-May and again in June, July, August, with perhaps another in September.
- Don’t spray if the temperature is, or will be, above 80F.

The following contact insecticides will temporarily control lace bugs if the product thoroughly covers the underside of leaves where lace bugs live and feed. Repeat the spray according to label directions:
- Azadirachtin
- Insecticidal soap
- Narrow-range horticultural oil. Use in the fall to coat the undersides of the leaves where the eggs are laid alongside the midrib.
- Neem oil
- Spinosad

You also have several options among systemic pesticides: acephate and imidacloprid.

If you decide to replace your azaleas, Encore azaleas have proved themselves to be among those tolerant of lace bugs in other regions. Their performance in the northwest remains to be seen.

You might like to review the recently published “Azalea Lace Bug”

Thank you for using Ask an Expert,

Jean R. Natter Replied October 16, 2013, 1:14 AM EDT
Thanks a lot for the very quick reply. I hope that I can save my mature bushes that have this problem. I will try both suggested approaches. I have to figure out how to effectively spray the underside of the leaves. I hope that the fact that it is already October, is still OK to treat them. Could you suggest which of the suggested sprays is more effective? Thanks again.
The Question Asker Replied October 16, 2013, 1:51 AM EDT
Hello, again.

The publication lists a number of products.  All can decrease the population when used according to directions. Those which aren't systemic must be re-applied according to label directions. 

Perhaps the most important treatment is during May, shortly after you see that the insects hatched and while they are still clustered in the undersides of the leaves.

Jean R. Natter Replied October 16, 2013, 2:17 AM EDT
Thanks again.
The Question Asker Replied October 16, 2013, 10:38 AM EDT
I apologize for bothering you again. I took your replies and the article the link points to to Home Depot and tried to get some products that contain the active ingredients mentioned as most efficient. I was told to come back with some manufacturer/brand names. I looked up the ingredients on the Internet and I get  products that I have no idea how to differentiate and/or use. I am not sure if you can help but at this time I am stuck. I do not know if it would help if I do something now (e.g. spray with an "oily" product on the list for the winter or if I should wait until May. I am not sure if I can reach most leaves from underneath if I buy some product. I do not know if I have to wait for teh rain or avoid the rain for applying these substances. I do not know if I should appeal for help to a company that might deal with these bugs...  I really would want to save the many mature azaleea and rhododenrons that are attacked but I do not know what to do. I would appreciate any follow up suggestion as I didn't have a garden before we moved to Portland.
The Question Asker Replied October 19, 2013, 11:50 PM EDT
Yes, it definitely will be challenge to spray undersides of the leaves.  I suggest you obtain a pump sprayer so that you can direct the wand upward, under the leaves.  Retail garden centers usually carry them in 1- or 2-gallon sizes.  Depending upon how many plants are affected, you may want to hire a pest control company. 

I suggest that you apply the horticultural oil later this fall or early winter to coat the embedded egg-laying sites along the midrib.  Use one of the other products in May, when you see that lace bugs have hatched.  (The first stage is transparent whereas the second stage is dark colored.  Both stages are easy targets because they are clustered as is shown in the publication “Azalea Lace Bug” - the second image on page 4.)  Repeat as needed through the growing season.  

Whatever your choice of insecticide, always read and follow label directions.    

The resource for product suggestions, below, are from the University of California To be effective, the spray must dry on the plant, which usually takes about 12 hours or so.  If rain arrives prior to that, re-apply when the weather is dry.

 Contact insecticides: Must be applied to the undersides of the eaves in order to contact the insects.
a. Azadirachtin (Example: Safer Brand Bioneem Multipurpose Insecticide & Repellent Concentrate)

b. Insecticidal soap (Some examples: E.B. Stone Insect Soap Concentrate Multi-Purpose Insecticide • Earth-Tone Insecticidal Soap • End All Insect Killer • Fruit & Vegetable Insect Killer II • Garden Safe Brand Insecticidal Soap Insect Killer)

c. Narrow-range horticultural oil. Use in the fall to coat the undersides of the leaves where the eggs are laid alongside the midrib. (Some examples: All Seasons Horticultural & Dormant Spray Oil Ready to Spray • Bonide All Seasons Horticultural & Dormant Spray Oil • Bonide All Seasons Horticultural Spray Oil Ready to Use • Lilly Miller Superior Type Spray Oil)

d. Neem oil (Some examples: 70% Neem Oil • Bon-Neem II Fungicide Miticide Insecticide • Bonide Rose Rx 3-in-1 Concentrate • End All Insect Killer • Garden Safe Brand Fungicide 3 • Garden Safe Brand Neem Oil Extract Concentrate • Green Light Neem Concentrate • Green Light Neem II • Neem Oil Fungicide-Miticide-Insecticide Ready to Use • Orchard Rose & Flower 3-in-1 Neem Oil Ready-to-Use)

e. Spinosad (Some examples: Bonide Captain Jack's Deadbug Brew • Green Light Lawn & Garden Spray with Spinosad 2 • Monterey Garden Insect Spray • Orchard Rose & Flower Insect Spray Ready-to-Use)

Systemic insecticides

a. Acephate persists in the plant for about 15 days; repeat the application several times through the season. (e.g.: Lilly Miller Ready-to-Use Systemic Rose, Shrub & Flower Care • Rose Pride Insect, Disease, & Mite Control)

b. Imidacloprid persists within woody plants for years, not necessarily a good thing for pollinator insects.  (Some examples: Bayer Tree and Shrub Insect Control • Bayer Advanced 2 in 1 Rose & Flower Care Ready-to-Use Granules II • Ortho Bug B Gon Year-Long Tree & Shrub Insect Control Concentrate • Ortho Max Tree & Shrub Insect Control Ready-to-Spray)

Please ask if you have additional questions,

Jean R. Natter Replied October 20, 2013, 1:25 AM EDT
Your very detailed answer is much appreciated, particularly on a weekend. Now I am a lot less confused and I know what to look for in terms of products. Thanks.
The Question Asker Replied October 20, 2013, 3:26 PM EDT
Hi again:

Since I am almost 70 I do not think that I can do the job myself so I invited 3 local companies that claimed expertise. One of them came out today and gave me a proposal. I was told that a first treatment (spraying leaves) needs to be done now; winter dormant oil has to be applied in Nov/Dec, followed in 2014 by 4 other treatments (systemic) starting early spring. I was told that systemic means spraying the leaves (not applying the chemical to the root). Somehow this didn't sound correct based on what I read.  He would not tell me what substances they would use and didn't say anything re. observing when the insects hatch and doing the spraying at that time. Does the above description of the proposed approach sound "right" to you? There is no guarantee with the work. Thanks for your help.
The Question Asker Replied October 28, 2013, 8:46 PM EDT
Different companies have varying “packages” they offer for pest management.  So, just as with hiring any business, ask lots of questions of at least three different businesses.  I do wonder about the timing of first treatment they suggest.  When I walked through my neighborhood in 10 days ago (early October), I saw few ALB on damaged plants.   I didn’t see evidence of egg-laying as yet.  So, the population, at least in my neighborhood, is low.

How one applies a systemic pesticide varies with the product at hand.  Some are meant to be sprayed, others are applied to the soil.  The end effect of a systemic pesticide is that the active ingredient is distributed throughout the plant.  Some must be re-applied during the season because they are relatively short-lived in the plant.  Others are longer lived, thus, are applied less often

You’ll hear different opinions about applying a winter dormant oil in Nov/Dec. 

The pesticides I listed for you earlier are for use by homeowners.  A pest control company has a few different options.  Our official resource for insect pest management says this, and emphasizes the importance of spraying when the insects hatch:

- - - - - -
 “Management-chemical : COMMERCIAL USE Use as label directs. Apply in late spring when young nymphs appear. Difficult to control with one application.
  • acephate
  • bifenthrin
  • carbaryl
  • cyfluthrin
  • imidacloprid
  • lambda-cyhalothrin
  • sucrose octanoate
- - - - - -  

I’m not surprised about the lack of a guarantee for treatment of azalea lace bug (ALB).  I doubt any company will offer one.  Treatment suggestions against ALB is based on research completed in other parts of the country whereas research is just beginning here in the northwest. 

In general:
- A pest control company must tell you the active ingredient upon request.
- They must also provide an MSDS (Material Safety Data Sheet) upon request. - The company must be licensed by the Oregon Department of Agriculture to apply a pesticide for hire.  Ask to see their license.
- The company should provide a written estimate detailing the materials they will use, and the timing of treatments.  

“Choosing a Pest Control Company” contains guidelines for structural pest control so isn’t entirely applicable to treating plants.  Even so, the company must reveal the active ingredient to be used upon request.  For landscape applications, a company needn’t be a member of Oregon Pest Control Association.  

The EPA has a very similar document about choosing pest control which is also focused on structural control.

I suggest you consult several Certified Arborists for onsite evaluations to discuss treatments.  Compare their comments, then hire one as such a person will have access to the equipment for any needed treatment.

You can locate Certified Arborists in the Yellow Pages, online or in a phone book.  Tree care companies with such a person will include that information in their ads, either as a brief statement or as a small rectangular logo stating “Certified Arborist.” We suggest Certified Arborists because they are trained in care for trees and shrubs; must pass an exam to acquire certification from the International Society of Arboriculture; and must accumulate mandated continuing education to retain certification.  When you call each company, avoid any unwelcome surprises by asking if they have a fee for the visit and evaluation.

Jean R. Natter Replied October 29, 2013, 1:51 AM EDT
Thanks you very much. I will look for a certified arborist and check the certification of the 3 companies that I invited for an estimate and will request the names of the products they plan to use. I would like to find someone whom I can trust who uses the "right" product that lasts longer as they charge ( a lot) by application. I am wondering for how many days hatching lasts, since they have to do the spraying when that happens, not when they have the time to come out. I am not sure if one can make that a condition for signing up to their program. If I can wait until Nov/Dec (for the oil application) I have a little bit more time to pick a company. Thanks again.
The Question Asker Replied October 29, 2013, 11:33 AM EDT
Hi Jean:

I learned a lot from you and I am continuing the search for a reasonable solution to the infestation. I got the second quote and I am very disappointed. All it states is the following: "A injection to be done next Feb or March; Two foliar sprays to be done in September and October". Based on all the education I received to-date I do not think that this company/technician knows too much about lace bugs... I am still waiting for the 3rd one to come out; he told me that the cost of the average treatment is $100. I sent inquiries to two certified arborists (ISA listed only these two within reasonable distance). In the mean time could you clarify a few more things?

Should the proposed treatments be different for azaleas and rhododendrons?

What happens if the neighbors do not treat their infested bushes? Will I spend a lot of money on my bushes while "their" bugs will fly over to my bushes and infest them again?

What's the usual duration of the hatching period?

Should I reasonably expect from someone who knows what has to be done to time the treatments to when I tell them that I see hatched nymphs?

Under these circumstances (neighbors infested), what's the chance of saving the infested bushes?

Seeing the kind of proposals I got to date I am also considering my gardener to do the spraying with whatever I would buy. He has equipment and the strength that I do not have.

If I'd do that, which of the longer lived systemic substances could be applied to the soil and does this type of application mean an "injection" (deep down) or can the surface just be sprayed around the root?

How would I determine how much insecticide I need?

I tried to find instructions for a few of the ones I could locate on line but apparently the detailed instructions come only with the product.

I apologize for the many questions but this is really over my head. I would love to save the bushes but this really seems to be quite complex from every perspective. Thanks for your time.

The Question Asker Replied November 03, 2013, 12:55 AM EDT
Many people are struggling with the same questions you are.  Treatment is currently based on university-based research completed elsewhere where, I’m told, active management of azalea lace bugs (ALB) limits damage.  Entomologists in Oregon have yet to complete their own research.  Even so, the most effective management strategy combines cultural methods (water during the dry months as well as pruning to allow air circulation within individual shrubs) with chemical treatments.  

Some generalizations:
- Treatment is the same for azaleas and rhododendrons.
- If you want a contact spray applied shortly following the first hatch, I suspect you will need to monitor for that event, then notify the company when it occurs; if so, arrange that sort of scheduling ahead of time.   Systemic materials are applied somewhat ahead of hatching because the active ingredient needs to be distributed within the plant.
- I located the names of a number of nearby Certified Arborists by using an online search with the phrase “certified arborist washington county oregon” – omit the quotes.  Or check the Yellow Pages in a phone book for the Portland Metro area – if you don’t have one, the local library probably does.  Check listings under the heading of “Tree Care” or something similar to find ads stating they have a Certified Arborist; many companies willingly travel to any of our three counties.

- You can request clarification of any estimate given to you.  Ask what product they intend to inject or spray and when.
- Yes, ALB may move from nearby untreated plants but that doesn’t necessarily mean they will decimate your shrubs.  Entomologists say that, while an ALB infestation is seldom fatal, repeated infestations may result in yellowed, sickly plants.
- Entomologists suspect that ALB has two generations a year in our region, with the first hatch the optimal time to decrease the population and, thus, decrease damage latter in the season.  
- Information about dosage and application will be on the label(s) of the product(s).  Repeat treatments are needed because ALB is difficult to control with one application.
- Some systemic insecticides can be injected but this is a technical application which requires special equipment and training.  The injection is into the plant, not the soil. 
- Of the two systemic materials suggested for use by home gardeners, imidacloprid remains within the plant for years, acephate for 15 days.  Acephate is the safer material for pollinator insects as long as it isn’t sprayed while the plant is in bloom. 
- Both Imidacloprid and acephate are available in several formulations: liquids or granules.  The label for each product will state dosage and method of application.  

I hope the above helps resolve your questions.  After you compare estimates, select the one that makes the most sense to you.

Jean R. Natter Replied November 03, 2013, 5:46 PM EST
Thanks for your follow-up. The search approach you suggested produced more results, so I will contact more certified arborists. I am not sure how one can determine ahead of time when hatching will occur, to schedule application of systemic materials  somewhat ahead of hatching. I still do not understand what's the usual duration of the hatching period when one is supposed to apply contact spray.. .Sorry about the many questions but this is uncharted territory for me. I appreciate your patience.
The Question Asker Replied November 04, 2013, 12:35 AM EST
The most important treatment is just after the main hatch sometime between mid-May and early June, at which time the fragile, slow-moving nymphs (youngsters) are easy targets.  Eliminating as many ALB as possible at that time helps to minimize damage through the rest of the season.   Follow-up with two or three more applications, as needed, during the season.

Jean R. Natter Replied November 04, 2013, 2:05 AM EST
Thanks. Probably there is no experience yet to know the duration (not the usual timing) of the hatching period and there is no  info about the method to use to determine ahead of time when hatching will occur (to know when to apply a systemic treatment)...  I will continue my search for the "right" company with  "right" plan or to somehow figure out which of the many available substances is the most effective one, long lasting and easiest to apply and how to combine systemic and contact applications most effectively to have the gardener apply them. If something new is published or will be available to help with the above I'd appreciate the info.
The Question Asker Replied November 04, 2013, 10:25 AM EST
Hi Jean:

Happy New Year!

I wanted to thank you one more time for your guidance and help in dealing with the lace bug. I took my time and today finally I engaged one of the companies that came through as knowing what they are doing (Collier Arbor Care). The person who came out, Lyle Feilmeier appears to be highly qualified. There is one thing that might have changed since I contacted you a few months ago: there is now supposedly a requirement in Oregon to apply Imidacloprid based systemic insecticides only between October and end of January. Is this correct? Best, Susan
The Question Asker Replied January 08, 2014, 4:33 PM EST

Thank you for your concern about the appropriate and safe use of pesticides.

Because I hadn't seen or heard that information, I phoned the Oregon Department of Agriculture, Pesticide Division, to obtain the facts.  As of today (January 8, 2014), no such restriction exists.

Jean R. Natter Replied January 08, 2014, 7:41 PM EST
Thanks, Jean. I asked Lyle if what he told me might be a misunderstanding as I could not find any info about the new requirements... His answer is: "
The Imidacloprid label issue is under review both within the state and nationally. We currently have our lab working on the timing and trying to predict the ending label and rules. We do know that wind pollinated trees, such as the birch will end up having little restriction on timing or delivery of product however the blooming trees and shrubs such as the rhodies will have restrictions. We are working to the safer side because any company licensed to use these products will be heavily scrutinized. That being said, the method of delivery on the birch and the rhodies and azaleas will be a drench. This product will remain in place until the late winter or spring uptake of water as the bubs swell, taking the product with it. In the past we treated with systemic from Jan to July (during the active growth of the plant) with the new predicted label we are just shortening our timing to January so we can cover all systemic. Either the way the product will be doing its job.".

My concern is that with an application in January the rains will wash away parts of the product and most plants being dormant will not absorb enough of the product to be really effective. However, I do not know if that's a good reason for concern. Obviously I would like to make sure that the plants are protected for as long as possible and I would like to get the most value out of the treatment. Would you know if I need to be concerned about this early application? The qualification of the arorist are: "ISA Board Certified Master Arborist MW-0173,
ISA Tree Risk Assessor Qualification". One doesn't know which of these professionals can be trusted...

The Question Asker Replied January 09, 2014, 1:19 AM EST

You received accurate information.  And as stated, 
it's the company’s best guess as to the outcome of the review and potential label changes. 

Imidacloprid is under review as concerns its use and application method. The label will be modified.   As for the timing, a soil-applied drench must be applied a bit ahead of time to allow the roots to absorb it and for the product to be distributed throughout the plant.
Jean R. Natter Replied January 09, 2014, 1:38 AM EST
Thanks, Jean. This makes me feel a little bit better. Although applying the insecticide in January when hatching usually happens in May might be a little bit too early...  I am still concerned about the effect of the rain and the rate of absorption when applied at this time... 
The Question Asker Replied January 09, 2014, 3:02 AM EST
Uptake via the roots to the critical part of the plant -- in this case, leaves -- is much slower than if the product is sprayed directly onto the pests on the leaves.  Once imidacloprid is in the plant, it remains there for an extended time.

Certified Arborists want clients to be comfortable with their service.  They will apply either systemic or contact products according to the situation as well as to the client's preference.  Further, Certified Arborists will explain the pros and cons of systemic and contact materials to the client.  The final decision is up to the client.  

I suggest you review the list of alternative materials mentioned in several of my earlier responses to you. They are contact materials which are applied when the insect pests are present.  

Because they are contact  materials, they must coat the insects to be effective. Thus, they are applied shortly after hatch, then they are repeated several more times through the year, always according to label directions.  

With azalea leaf bug, the time of hatch is determined by repeated observations of the undersurface of the leaves. As said previously, that's sometime between mid-May and early June.  

Jean R. Natter Replied January 09, 2014, 10:37 AM EST
Thanks, Jean. We will go ahead with the treatment as suggested and hope that the application will keep the bugs away. Best, Susan
The Question Asker Replied January 09, 2014, 11:00 AM EST
Hi Jean:

The infested bushes were treated today. Hopefully it will help over time. Sharing with you replies from two vendors of Imidacloprid based products re. recommended timing for application (drenching) in Portland:

"it is best recommended to apply the product when you are not experiencing an abundance of rain. If the product is applied and you have continuous rain as you are having at this time of year, there is a great possibility the product will be washed away. In this case, we would recommend waiting until dryer weather arrives before application."
"Applying in January would put the chemical in place to be taken up as soon as the plant begins moving fluid in the spring, as long as the soil is not deep frozen.  Applying in April would be too late.  Lacebugs feed early in the year, so application timing is critical.  The insecticide takes some time to get into the plant, so even a January application is a little late.  For lacebugs, a fall application is actually best. Will the chemical wash away?  Most likely no.  The chemical bonds to the soil near the plant and stays there.  Water can move past and the insecticide will stay where it was applied.  If the soil itself erodes and moves, then of course the chemical will as well."

Thanks again for all your help.

The Question Asker Replied January 13, 2014, 2:49 PM EST

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