Home & Garden
How to Identify and Treat 9 Common Dahlia Diseases
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When my ‘Labyrinth’ dahlia is in bloom, it’s truly one of my favorite flowers in the garden. How do you top giant, foot-wide flowers in an explosion of pink, salmon, orange, and cream?
I love them so much that I have replaced my plants not once, not twice, but three different times because of disease problems.
I love dahlias, but some diseases that affect them can be a real nuisance.
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Over the years, I’ve learned many techniques to avoid the problems that can plague these plants.
As is the case with many diseases, prevention is always best, and that’s especially true when it comes to dahlias.
Many of the common problems can spell the end of your dahlias. No do-overs, no cures, just digging out and tossing your once wonderful flowers.
A few diseases can be cured or otherwise dealt with, but prevention is the way to go.
In our guide to growing dahlias, we cover how to cultivate these beautiful blooms in your landscape. Healthy specimens are less likely to fall victim to infection from pathogens than those that are stressed or otherwise grown in suboptimal conditions.
In this guide, we’ll discuss nine of the most common diseases you’ll come across.
Here’s the list of the diseases we’re going to talk about:
1. Aster Yellows
Aster yellows is caused by a phytoplasma spread by leafhoppers.
When infected, the plants will have strange growth, such as witches’-brooms, distorted stems, and discoloration of the foliage and flowers.
Unfortunately, once your dahlia is infected, you can’t cure the disease. Since it can spread to other species in your garden, you’ll need to pull the affected dahlias and dispose of them.
Learn more about the symptoms, how it spreads, and how to prevent aster yellows in our guide.
2. Botrytis Blight
Botrytis cinerea causes botrytis blight aka gray mold, and it is a bother on many garden species.
You’ve probably seen gray mold on strawberries that you bring home from the grocery store that are almost instantly covered in a gray fuzz.
During the spring and fall, when the weather is cloudy and wet, the fungus that causes this disease starts to spread.
Once it takes hold, tan spots will appear on the leaves, and gray fungal growth will develop both on underground plant parts and on any flowers.
The damage doesn’t end in the garden. If you dig up the tubers to overwinter your dahlias, the mold can also form on the tubers while they are in storage.
To prevent the disease, you want to deny the pathogen the conditions it prefers. Do this by keeping plans well-spaced for proper airflow and don’t ever water from overhead. Only water at the soil level.
You should also be sure to sterilize your tools as you work in the garden. Never go from one plant to another without cleaning your pruners or other tools first.
You can use a 10 percent bleach solution (one part bleach mixed with nine parts water) which works just fine. I keep a little squirt bottle and some rags in my gardening toolbox so I have no excuse not to clean my tools.
If you know botrytis is a problem in your area, keep a close eye on your dahlias and the second you notice symptoms, start spraying your plants with fungicide.
Copper fungicide works well, but I swear by a product called Mycostop. I use it for all kinds of fungal problems that I encounter in the garden and it has never let me down.
It’s made with a bacterium derived from sphagnum peat moss.
Grab five or 25 grams at Arbico Organics and saturate the dahlias once every two weeks until the weather becomes drier and warmer, and symptoms have resolved.
You should also remove any symptomatic leaves and flowers.
3. Fusarium Wilt
Fusarium wilt is extremely similar to verticillium wilt, which we discuss below. These two diseases cause similar symptoms and they are both incurable.
The first sign, as the name suggests, is wilting in your plant, like you might see during periods of drought or underwatering. The soil might be sufficiently moist and you may be absolutely sure the dahlia has received the right amount of water, but it’s still wilting.
That’s because Fusarium oxysporum fungi have taken up residence in the xylem, or the part of the plant that transports water.
Beyond the aboveground symptoms of wilting, if you dig down, you’ll see a dark stem near the soil level and black roots and tubers. The leaves might turn yellow, as well.
The fungus thrives when it’s hot and temperatures are in the 80s and 90s. It can live in the soil, and on live or dead tissue.
Sadly, once it’s infected, you can’t save the dahlia. You’ll need to dig it up and dispose of all plant debris in the trash. Then, don’t grow anything in that soil that is susceptible to wilt.
Replace your dahlias with asters, carnations, gladioli, sunflowers, or zinnias as these are all immune to this disease.
You can spray your dahlias with Mycostop, which we talk about above, and it will stop the progression. But it won’t cure the disease, so this only works if you catch the problem early on.
4. Impatiens Necrotic Spot Virus
Impatiens necrotic spot (INSV) is caused by a virus that results in necrotic brown or black spots developing on the leaves.
It also causes sunken spots with yellow haloes, stunted growth, shoot and tip dieback, foliage yellowing, lack of flowering, and eventual plant death.
Since fuchsia, impatiens, lettuce, mint, peppers, tomatoes, and a large number of ornamentals can also act as hosts, it’s not just a problem for dahlia growers.
There is no cure for this disease and since it can spread to so many other species, you’ll need to pull and dispose of any symptomatic dahlias.
In the future, the only option is prevention, and that involves cleaning your tools as you work between plants, as we discussed above, and keeping western flower thrips (Frankliniella occidentalis) out of your garden.
Thrips carry and spread the virus, and once they start feeding on your dahlias, it might already be infected.
Our guide to thrips can help set you on the right path, but here’s a brief overview of how to repel them:
Row covers for low-growing species is ideal. Reflective mulch can help, too. Pheromone lures and sticky traps limit their numbers, and trap crops can draw them away from your more valuable garden specimens.
You should also invite or introduce predators like green lacewings, minute pirate bugs, and parasitic wasps into your garden.
You can purchase green lacewing eggs or larvae in quantities from 1,000 to 250,000 at Arbico Organics.
5. Mosaic Virus
Dahlia mosaic virus is a pathogen in the Caulimovirus genus, with the binomial C. dahliae, that has multiple strains, including DMV-D10, DMV-Holland, DMV-Portland, dahlia common mosaic virus (DCMV).
It can be hard to identify the symptoms because they can be vague or even nonexistent. Your dahlias might seem just sort of sickly, with stunted growth or less-than-impressive flowering. Some plants might not bloom at all.
You may also see yellow or cream mosaic patterns on the leaves, or chlorosis. The veins might lack color as well. Flowers might be discolored or exhibit strange patches of discoloration.
Distorted growth can occur on all parts of the plant, or the leaves or stems might have necrotic lesions.
You might need to send a sample of your plant to your local extension or University diagnostic lab to be certain that it’s infected with mosaic virus. If it proves to be, you’ll need to dig up the dahlia and dispose of it, along with any fallen debris.
You can learn more about this disease in detail in our guide to dahlia mosaic virus.
6. Powdery Mildew
Powdery mildew is a common garden disease. It appears on many species, with symptoms that range from unsightly to devastating.
On dahlias, powdery mildew is usually more in the ugly but not deadly category.
Powdery mildew is caused by the fungi Erysiphe cichoracearum and E. polygoni, which are obligate parasites. What does that mean? It means they can’t survive without a host, the pathogens need a live plant to survive and reproduce.
That’s good news for us because it means the pathogens don’t want to kill their host and they can’t hang out in the soil even after your plant is dug up and overwintered out of the garden.
The spores need some humidity, around 60 percent, but not tons of water, and temperatures between 59 to 82°F to germinate. Then, the spores wait for a nice dry day with a breeze or hitch a ride on aphids to spread around the garden to infect all your dahlias.
Once present, you’ll notice a powdery coating on the leaves and maybe some distortion of the foliage if the disease has advanced.
Prune off any symptomatic leaves, unless more than 50 percent of the plant is infected. If it is, go straight to fungicides. A low level infection might be remedied through pruning and treating with milk.
Yes, the stuff in your refrigerator.
A mixture of one part milk and nine parts water will usually be enough to stop or eliminate the disease, though you can increase the mixture until it’s 50:50 for heavier infections.
For heavily infected dahlias that need fungicides, treat them with one or two alternating products to get rid of it.
Sulfur is effective. You can find Bonide Sulfur Fungicide in one-pound bottles or four-pound bags at Arbico Organics.
Spray your plants thoroughly every three weeks.
If you’d like, you can alternate with milk or with copper fungicide.
Arbico Organics also carries copper fungicide in 32-ounce ready-to-use, 16- or 32-ounce hose-end, and 16-ounce concentrate, depending on your needs.
Use this in addition to or instead of sulfur. If you also use sulfur, alternate the product so you’re only using one every three weeks.
You can learn more about how to deal with powdery mildew on dahlias in our guide.
7. Stem Rot
Bacterial stem rot is a gross disease. It’s caused by the bacteria Pectobacterium carotovora subsp. carotovora (previously Erwinia cartovora) and when the pathogen attacks your plants, the lower parts of the stems will turn black and mushy on the interior.
They also give off a distinct, foul odor.
If you walk past your plants and catch a whiff of something extremely unpleasant, crouch down and look at the base of the stem. Give it a gentle squeeze, and if it feels soft, your dahlia is likely suffering from stem rot.
The causal pathogen needs water to survive, and it can’t live long without a host. But it travels easily on tools, shoes, plants, or it can hitch a ride on insects.
There’s no cure, and it will spread to a huge range of other species, from potatoes to celery.
If your dahlias are infected, you’ll need to rip them out and dispose of them. You should also leave the area fallow until the following year to avoid infecting another plant.
In the future, be super careful about garden hygiene and do your best to keep pests out of the garden.
8. Tomato Spotted Wilt Virus
Tomato spotted wilt (TSWV) is extremely similar to impatiens necrotic spot. For a while, they were thought to be the same virus, but genetic research has shown them to be separate species.
In addition to the shoot and tip dieback, lack of flowers, and foliage yellowing, this virus also causes wilting.
It, too, is transported by western flower thrips and, like INSV, there is no cure. The only solution is to pull the plants and try to prevent thrips from infesting your plants in the future.
9. Verticillium Wilt
Verticillium wilt in dahlias is caused by Verticillium dahliae, and while dahlia is in the name, it can infect hundreds of other species, including cabbages, peppers, and melons as well.
The first sign that your dahlia is infected is usually wilting. But it’s not the usual type of wilting that you see when plants are underwatered during dry spells. Typically it will be just one stem or perhaps half of the leaves on a plant.
This wilting happens because the infection prevents the plant from taking up water.
As the disease progresses, you’ll see necrotic spots, yellowing or brown areas, and leaf drop.
Symptoms might also include “rosetting” and/or stunted growth. Rosetting is just a fancy way of saying that the internodes, or the part between the leaves, start to grow shorter than normal. This brings the leaves closer together, creating a “rosette-like” appearance.
Now for the bad news. The pathogen can live in the soil, on hundreds of plants, and on plant debris. Once it’s in your soil, it can stay there for years and years, just waiting for a new host to come along.
Growers have tried to breed resistant cultivars, but even these can succumb to the terrors of “vert” as it is commonly known.
To be sure that you’re dealing with verticillium wilt, you can send in a sample of the plant to your local extension office.
Once infected, the only thing to do is pull the plants and dispose of them. Then, don’t grow anything susceptible to verticillium wilt in that area for at least ten years.
Azaleas, crabapples, ceanothus, dogwoods, ferns, holly, hollyhocks, moss roses, and zinnias are all good alternatives.
Dahlia Diseases Are Dastardly!
Compared to some plants, like roses and hydrangeas, there are relatively few diseases that bother dahlias. But many of them are deadly or incurable.
So when your plants are infected, it’s bad news.
What symptoms are you seeing? Were you able to figure out what’s wrong with your dahlias? If you need additional guidance, please let us know how we can help in the comments section below. Photos help!
Now that you’re better equipped to keep your plants happy and healthy, you’ll need to learn a bit more about your dahlias. We have a bunch of guides that can help you expand your knowledge, including:
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Kristine Lofgren
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