Midwinter Fire Dogwood Shrub - Ask Extension
Hello,
I have 6 "Midwinter Fire" Dogwood Shrubs (cornus sanguinea). They are 10 years old; 5 shrubs are in a grouping and the 1 shrub in qu...
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Midwinter Fire Dogwood Shrub #858813
Asked February 09, 2024, 12:34 PM EST
Hello,
I have 6 "Midwinter Fire" Dogwood Shrubs (cornus sanguinea). They are 10 years old; 5 shrubs are in a grouping and the 1 shrub in question is approximately 45 feet away from the others. I trim the shrubs to half-height every August when they get huge, then I trim to about 2' tall every late March. The shrubs also get new hardwood mulch every Spring.
Sometime in the Summer months of 2023, while the shrubs were still covered in green leaves, this one shrub developed the white substance (in photos) near the base of some twigs. I've seen the white stuff before in small amounts on short-trimmed stems that are dying, and it has never spread and I just remove the dead stems. This was an unusual time of year to see it, plus it appeared on otherwise healthy stems. I thought it might be mold from wet weather, but too many weeks passed. Then I thought the Winter might kill off whatever it is, but no such luck. As far as I can tell, the while substance has neither increased or decreased since it first showed up.
Do you know what this is, and how I should treat it?
Thank you so much for your time!
Baltimore County Maryland
Expert Response
Although we can't magnify the photos enough to be certain, this appears to be the classic sign of a scale insect infestation. Given their appearance, we suspect White Prunicola Scale or its look-alike White Peach Scale. The latter is more commonly seen on shrubby dogwood species than the former, so our best guess is that it's this species.
Scale insects feed on plant sap or cellular juices and can be hard to manage when abundant. Physical removal does help, either by pruning out infestations or gently scrubbing them off the bark, but won't be foolproof. Often, use of at least one insecticide (such as dormant oil before they leaf out again) can kill them or at least suppress populations to a level that they aren't stressing the plants. Beneficial insects, like certain ladybug species, can consume scale, but won't eliminate a population by themselves.
Plants that are stressed may be more vulnerable to scale outbreaks, such as when heavily pruned or experiencing drought without irrigation, but that doesn't necessarily mean your shrubs are being taxed too greatly. It is a routine procedure to cut back redtwig dogwoods to manage size and encourage colorful new regrowth. Still, if this plant can't reach at least half or more of its expected mature size (typically anywhere from 5 to 8 feet depending on variety) each year, then it might be better-off moved to a location in the yard where that full height won't block anything or be in the way. There are dwarf cultivars of redtwig dogwood, though sometimes they have duller red stem color; at least they tend to not need pruning to manage size, though removing old stems every few years can keep their color fresh.
Information about scale management options is given in the web pages linked above and on our Introduction to Scale Insects page. Feel free to send close-up photos of the suspected scale for confirmation if you're not sure that's what the white substance is. We can say that no fungal infection common to dogwoods would create this particular symptom, and any infection that extensive would have killed the wood it is growing on by that point anyway, so you'd be seeing more blatant dieback in that case.
It's possible the scale were present but minimal for years before becoming noticeable, or they happened to arrive more recently on another plant or traveled into the yard on wildlife (squirrel or bird feet can be perpetrators) or the wind. Crawlers, the hatchling stage of scale that are very tiny, can move fair distances in those ways, but older scale life stages stay relatively sedentary and essentially glue themselves to their host plant for the rest of their lives. Since White Peach Scale can have 3 generations per year in Maryland, populations have the potential to build quickly compared to other scale species.
Miri
Scale insects feed on plant sap or cellular juices and can be hard to manage when abundant. Physical removal does help, either by pruning out infestations or gently scrubbing them off the bark, but won't be foolproof. Often, use of at least one insecticide (such as dormant oil before they leaf out again) can kill them or at least suppress populations to a level that they aren't stressing the plants. Beneficial insects, like certain ladybug species, can consume scale, but won't eliminate a population by themselves.
Plants that are stressed may be more vulnerable to scale outbreaks, such as when heavily pruned or experiencing drought without irrigation, but that doesn't necessarily mean your shrubs are being taxed too greatly. It is a routine procedure to cut back redtwig dogwoods to manage size and encourage colorful new regrowth. Still, if this plant can't reach at least half or more of its expected mature size (typically anywhere from 5 to 8 feet depending on variety) each year, then it might be better-off moved to a location in the yard where that full height won't block anything or be in the way. There are dwarf cultivars of redtwig dogwood, though sometimes they have duller red stem color; at least they tend to not need pruning to manage size, though removing old stems every few years can keep their color fresh.
Information about scale management options is given in the web pages linked above and on our Introduction to Scale Insects page. Feel free to send close-up photos of the suspected scale for confirmation if you're not sure that's what the white substance is. We can say that no fungal infection common to dogwoods would create this particular symptom, and any infection that extensive would have killed the wood it is growing on by that point anyway, so you'd be seeing more blatant dieback in that case.
It's possible the scale were present but minimal for years before becoming noticeable, or they happened to arrive more recently on another plant or traveled into the yard on wildlife (squirrel or bird feet can be perpetrators) or the wind. Crawlers, the hatchling stage of scale that are very tiny, can move fair distances in those ways, but older scale life stages stay relatively sedentary and essentially glue themselves to their host plant for the rest of their lives. Since White Peach Scale can have 3 generations per year in Maryland, populations have the potential to build quickly compared to other scale species.
Miri
Hi Miri,
Thank you for your prompt response last week, and your thorough answer. I didn’t think that either link you provided looked like what is on my Dogwood, but I finally took closer pictures today. (And unfortunately, realized I do have it on more than 1 shrub.)
If you wouldn’t mind looking at these two new photos to confirm? I’m guessing White peach scale.
I think I will try scraping as well as dormant oil. Is there a certain brand that you would recommend?
Hello Jessica,
Unfortunately these are definitely scale, and while it would take microscopic examination to separate White Peach Scale from White Prunicola Scale, since the former is more common on redtwig dogwood, that is what we presume they are. (Fortunately, regarding treatment, between these two scale types it won't matter too much which they are.)
Physical removal plus dormant oil is a good first step and might suffice by itself as a treatment regime, though you may need to repeat it next winter if they are still present. (Horticultural oil can be used at the "summer rate," that is not at a dormant oil concentration, during the growing season as well.) We do not collect thorough information about brands on the market and we don't make specific product recommendations, but any product that is labeled for dormant oil use on shrubs in home landscapes should be fine to try. (The label should be read carefully before use, but it's doubtful that redtwig dogwood will be listed specifically among the plants a product can be sprayed on since they don't provide exhaustive lists. If anything, they may just list species known to be hyper-sensitive to oil sprays, but that usually refers to foliage contact, not bark.)
Miri
Unfortunately these are definitely scale, and while it would take microscopic examination to separate White Peach Scale from White Prunicola Scale, since the former is more common on redtwig dogwood, that is what we presume they are. (Fortunately, regarding treatment, between these two scale types it won't matter too much which they are.)
Physical removal plus dormant oil is a good first step and might suffice by itself as a treatment regime, though you may need to repeat it next winter if they are still present. (Horticultural oil can be used at the "summer rate," that is not at a dormant oil concentration, during the growing season as well.) We do not collect thorough information about brands on the market and we don't make specific product recommendations, but any product that is labeled for dormant oil use on shrubs in home landscapes should be fine to try. (The label should be read carefully before use, but it's doubtful that redtwig dogwood will be listed specifically among the plants a product can be sprayed on since they don't provide exhaustive lists. If anything, they may just list species known to be hyper-sensitive to oil sprays, but that usually refers to foliage contact, not bark.)
Miri
