Discula destructiva
I don’t want to talk about dogwood anthracnose aka blight. I’m afraid that even by mentioning its name, it will find my disease-free dogwoods and quietly destroy them.
This disease is constantly lurking around my region, mercilessly destroying our beloved Cornus plants.
So while I’m knocking on wood, rubbing my rabbit’s foot, looking for four leaf clovers, and crossing all my fingers and toes, I am going to talk about this disease.
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I suppose knowing what to expect is half the battle, so we really should prepare ourselves.
Anthracnose leaf blight and canker is a recent arrival in North America, but it has made a big, terrible splash, destroying dogwoods.
If your tree is sickly, you probably want to get right down to identifying the problem and figuring out what to do about it, so let’s jump right in and talk about the following:
What You’ll Learn
What Is Dogwood Anthracnose?
Dogwood anthracnose is caused by a fungus called Discula destructiva, which isn’t indigenous to North America, but that’s where it’s wreaking the most havoc.
It arrived in the US during the 1970s from Asia and was first identified by plant pathologist, botanist, and horticulturalist Pascal Pompey Pirone in 1978.
He first identified and named the disease, which was spreading rapidly in trees in Connecticut and New York.
I have to say, some botanical names are questionable, but “destructiva” is well-chosen, because it certainly is destructive.
Within just a few years, anthracnose had killed a third of the dogwoods in Catoctin Mountain National Park in Maryland. By 1988, 89 percent of them were dead.
In 1983, it was found in the Pacific Northwest, where it spread just as aggressively.
It’s particularly devastating for species that are native to North America, since they don’t have any natural defense against the fungus.
That means flowering (C. florida) and Pacific dogwoods (C. nuttallii) are particularly susceptible.
While other species might become symptomatic, they usually won’t die. But species indigenous to North America can be killed outright.
Thanks to this disease, populations of native dogwoods have been decimated in New England, the South, and the Pacific Northwest.
Other commonly grown species like kousa (C. kousa), tatarian (C. alba), alternate leaf (C. alternifolia), silky (C. amomum), red osier dogwoods (C. sericea), and cornelian cherries (C. mas), are all resistant but not immune.
That means the symptoms, if any, won’t be dramatic. Maybe some spotting on the leaves, but those species usually won’t die.
Identification
When the fungus infects the plant, the first sign will be dark, angular leaf spots that form tan centers and purple margins as they age.
You will also see discolored brown splotches on the foliage and leaf scorch. Eventually dead leaves might drop from the tree.
Dead gray leaves can also hang limply from the tree without falling.
When the flowers develop, the colorful bracts can also develop spots.
As the disease progresses, young shoots can be killed as sunken areas develop that restrict the movement of water and nutrients through the plant.
These sunken areas are known as cankers and can become quite large.
Besides being ugly, the cankers can stop water and nutrients from moving to the plant tissue beyond the canker, causing the tissue to die.
The disease also causes shoots called epicormic sprouts to emerge out of the tree near the cankers.
This results in shading and reduces the air circulation inside the canopy, which creates even better conditions for the fungus to spread.
That covers the visible symptoms, but bad stuff is happening where you can’t see it, as well.
Right underneath the bark is the cambium layer. When this tissue is infected by the fungus it takes on a dark brownish-red color.
Typically, the symptoms start in spring in the lower leaves and move up the canopy from there, so early detection relies on keeping an eye on the foliage.
There are many diseases that can cause similar-looking leaf spots.
Infection from Septoria species may look similar, but these only appear in late summer or fall, not spring, and they usually have a more grayish appearance.
Spot anthracnose, caused by the fungus Elsinoe corni, also causes small spots but only on leaves and fruits.
The spots are usually numerous but don’t grow larger than an eighth of an inch in diameter.
Feeding by dogwood borers can also cause shoots to die off and leaves to drop, but they don’t cause leaf spotting.
If the leaves die as a result of borer feeding, the impacted foliage will be clustered in one portion of the tree and not spread throughout as with anthracnose.
Cercospora fungi causes spots that look similar to anthracnose, with pale brown or tan centers and reddish-purple borders.
Biology and Life Cycle
The pathogen that causes anthracnose needs humidity, shade, and moisture to thrive.
It is most destructive on trees at elevations above 3,000 feet and those that are growing in shady spots, since these areas have cool, moist conditions.
In the spring, the dormant fungus emerges from blister-like structures called conidiomata that are growing in the cankers and from the cambium layer through wounds and other openings. They can also grow on the undersides of leaves.
After they emerge, the spores known as conidia spread on water droplets in the air or splashing water. They can be carried on the wind or by birds, insects, and other animals, including you.
The fungus overwinters on plant material and in cankers. The foliage that falls from the tree in autumn can also harbor the disease.
It’s most deadly on young trees, but it can kill large ones as well.
Organic Control Methods
I wish I could paint a prettier picture for you, but controlling this disease isn’t easy or always successful. Prevention is best and that involves careful plant and site selection.
Cultural Control
In areas ravaged by this disease, it’s generally a good idea to avoid planting susceptible species.
If you have your heart set on a flowering dogwood, Florida hybrids that are crossed with C. kousa have more resistance than the straight species.
Look for those or for resistant cultivars if you know anthracnose is common in your area. Planting a susceptible species is a gamble.
Then, make sure you plant in full sun if the species can handle it. The more sunlight, the less severe the infection since the fungus isn’t able to reproduce and spread as easily.
You also want lots and lots of room around the plants to maximize air circulation.
Also ensure that the soil is appropriately well-draining and that you keep the plant fed and cared for. A healthy tree is more able to withstand an infection.
Add several inches of organic mulch like pine bark or leaf litter and ensure your dogwood is thoroughly watered during hot or dry periods.
Physical Control
If your tree is showing signs of dogwood anthracnose, prune off any symptomatic areas, including shoots, epicormic sprouts, or leaves.
The same goes for any dead areas. Get rid of them immediately and prune down at least a foot beyond the dead area.
You have to make a decision about big cankers, since those growing on the trunk of the tree might not be removable without killing or seriously harming the tree.
Any leaves that fall either during the growing season or in the autumn should be raked up and disposed of as soon as possible.
Don’t put the leaves on your compost pile or use them as mulch anywhere in the garden.
Do your best to prevent damage to the tree. Damage can come in the form of holes from borers or it can be trimmers nicking the trunk.
Biological Control
You can harness the power of beneficial bacteria and fungi to try and slow the progression but you can’t ever actually cure the disease.
Bacillus subtilis is a beneficial bacteria that can help slow the spread of the disease because it pushes out the pathogen and also invades the body of the fungi, preventing growth and reproduction.
You can find products like CEASE that contain Bacillus subtilis available at Arbico Organics.
B. amyloliquefaciens strain D747 is another beneficial bacterium that works similarly to B. subtilis.
Bonide makes a product called Revitalize that uses B. amyloliquefaciens to help slow the progress of diseases like dogwood anthracnose.
Pick up 32-ounce ready-to-use or 16-ounce concentrate at Arbico Organics.
There is also a bacteria known as Streptomyces lydicus WYEC 108.
It occurs naturally in the soil and can be used to fight the fungi that causes dogwood anthracnose.
Actinovate AG is a product that harnesses this beneficial bacteria. You can find it available at Arbico Organics.
None of these will stop the disease, but they can slow it, sometimes dramatically.
If you choose one of these products, combine it with one or two of the following fungicides for the best chance at controlling the disease.
Organic Fungicides
As with biological control, fungicides won’t reverse the course of the disease, but they can slow or stop its progression.
Your usual fungicides like Mancozeb and copper both offer a measure of control. Potassium bicarbonate or hydrogen peroxide can also be useful in the war against anthracnose.
You should try to use multiple different control methods and swap them up every few months. This is to prevent resistance to fungicides.
Thymol is often effective. Guarda contains this ingredient, which is derived from thyme plants.
Purchase two and a half gallons at Arbico Organics.
While I’ve never tried it myself, a neighbor told me that they had great success with a product that contained an extract from the dreaded giant knotweed (Reynoutria sachalinensis).
Giant knotweed is wreaking havoc across North America as an invasive weed that is pushing out native species. Luckily, we can put it to work in controlling dogwood anthracnose.
Regalia biofungicide contains this ingredient, which helps the dogwood develop its own resistance to the pathogen that causes anthracnose.
For that reason, you should use it preventatively or at the very first sign of infection, along with some biological control or other fungicides.
You can find Regalia in a variety of sizes at Arbico Organics.
In the end, you might be able to stop the disease early enough that you can save your tree from certain death.
Or you may just have to eventually give up. With some diseases, it’s important that you remove the plant right away to ensure that the disease doesn’t continue to spread.
But as long as you are treating the tree to suppress the pathogen, you can safely leave it in your garden. After all, the disease is already clearly present in the area.
No Sugarcoating
There’s no way to sugarcoat it, dogwood anthracnose is bad news.
Maybe someday we’ll have a reliable cure, but until then, take as many preventative methods as you can and check your trees regularly for symptoms.
If you do end up losing a tree, be sure to plant something resistant or avoid species in the Cornus genus altogether.
I’m sorry you found yourself here, since it probably means that you at least suspect your tree or shrub is infected.
Or, hopefully, you’re just here to make sure the dogwood you’re going to plant is safe. Either way, let us know what you’re experiencing in the comments section below!
And for more information about growing and caring for dogwoods, add these guides to your reading list next:
