Common Dahlia Pests and Diseases
Dahlias are vigorous plants that are born to bloom. Yet they are not invincible. If you suspect your dahlias are being attacked by a pest or disease, this article can help you identify the problem and figure out how to resolve it. But first, please note:
#1 It’s often difficult to distinguish one plant problem from another — even for an entomologist. If you can offer clarifications or suggestions, please add them to the comments below.
#2 Healthy dahlias are much more resistant to pests and disease. For best results, pamper your dahlias as you would a prized tomato plant. Learn about caring for them here: All About Dahlias, 8 Tips for Growing Better Dahlias, and 10 Questions About Growing Dahlias.
COMMON DAHLIA PESTS
A slug munching on a young dahlia.
Slugs and Snails
These slimy pests are most destructive early in the growing season when dahlia foliage is soft and tender. As the plants grow, their stems and leaves toughen up and become more resistant to damage.
To deter slugs and snails, keep the soil surface as dry as possible. In damp weather, patrol daily (morning and/or evening) to gather and destroy these slimy beasts. You can also protect young plants with an organic slug repellent such as Sluggo.
An earwig feeding on dahlia leaves.
Earwigs
These creepy-crawly insects feed on decaying plant matter. Unfortunately, they also like munching on tender dahlia leaves and flower buds. Earwigs are largely nocturnal and during the day they like to hide among clusters of emerging leaves. You may notice their black frass (shown above) before you see them. Chewed young leaves, buds and flowers are signs of earwig damage.
A dahlia flower that was damaged by earwigs.
To help deter these pests, keep the area around your dahlias tidy. This means removing spent flowers, pruned foliage, weeds and other organic matter. Avoid mulching with shredded leaves or straw (both of which attract them). You can try catching earwigs in an upside-down flowerpot filled with moist, shredded leaves. Or use an earwig-control
Tarnished Plant Bugs
If your dahlia flowers are occasionally distorted in shape or don’t open properly, this may indicate tarnished plant bug damage. These pests (both the adults and the much smaller nymphs) have piercing-sucking mouth parts. As they feed, their saliva leaves behind a toxin that damages plant tissues.
Tarnished plant bugs overwinter in leaf litter, so controlling weeds and mowing nearby areas can help reduce overwintering populations. Insecticidal soaps can help reduce the severity of outbreaks.
Potato Leafhoppers
Leafhoppers usually attack in early summer. They suck plant juices from the undersides of leaves, causing the foliage to become discolored and drop off. “Hopper burn” is a characteristic yellowing and curling of the leaves. Leafhoppers weaken plants, and by moving from one plant to another, they also spread viral diseases.
Spray immature leafhoppers with insecticidal soap, making sure to get the undersides of leaves so the soap comes into contact with these soft-bodied insects.
Dahlias can usually outgrow most pest problems and still put on a beautiful show.
Thrips
These sucking insects cause discoloration and distortion of both leaves and blossoms. Left unchecked, they will weaken the entire plant. Thrips can also transmit bacteria and viruses from one plant to another.
Thrips are tiny and difficult to see. They prefer a dry environment, so keeping your dahlias watered can help discourage them. Insecticidal soaps can temporarily reduce an outbreak. Get more information about this pest and how to control it in this article from the Missouri Botanical Garden: Thrips Outdoors
Spider mite damage showing stippling and webbing.
Spider Mites
These plant pests are true spiders that, like thrips, suck plant juices and cause dahlia foliage to become mottled with yellow. When they are under attack by spider mites, dahlias lose vigor and their leaves begin to wilt and drop off. Spider mites are tiny and difficult to see without a magnifying glass. Look for signs of their webs between the leaves. They thrive in hot, dry weather.
Insecticidal soaps and horticultural oil sprays can be used to control spider mites. For a complete description of this pest and various control options, read: Coping with Spider Mites from the American Dahlia Society.
COMMON DAHLIA DISEASES
Fungal Diseases
The soil is filled with fungi and most of them are good. But some can cause problems for plants, including dahlias. Here are the most common fungal diseases of dahlias. More information is available in this article from the American Dahlia Society.
Powdery mildew on a dahlia.
Powdery Mildew
This disease is common in warm, humid climates. It usually shows up during the second half of the growing season. Though powdery mildew can weaken plants, the damage is largely cosmetic. To help to control it, keep dahlia foliage as dry as possible and encourage good air circulation.
If you live in an area where mildew problems are common, you may have to just live with the damage. Powdery mildew can be controlled with an anti-fungal spray, but the application must begin before the disease appears and continue throughout the growing season.
Botrytis
Also known as grey mold, botrytis is common in peonies and can also affect dahlias. The fungus is encouraged by cloudy, wet weather. Dahlia buds turn dark and may be covered with fuzzy grey mold. The disease eventually infects stems and can kill the plant, so it’s important to remove infected plant parts immediately.
Verticillium Wilt
When this soil-borne fungus infects a plant, it clogs up the water-conducting tissues (xylem) and prevents the circulation of water and nutrients. Leaf margins may turn brown or appear scorched. Stems may turn yellow, display brown or black streaks, or simply wilt. There is no cure for verticillium wilt and the plant will eventually die. This fungal disease typically occurs in weak plants and those that have been stressed by unfavorable weather.
A dahlia succumbing to bacterial stem rot.
Bacterial Diseases
Bacterial diseases are difficult for a home gardener to diagnose. Typically, bacteria enters plants through a wound or natural opening. If a dahlia gets infected during the growing season, overwintered tubers can carry that bacteria on to the next year. Most bacterial diseases are incurable. Affected plants should be removed and destroyed to control spread.
Bacterial Stem Rot
Infection usually starts at the soil level and moves up. Stems become soft and dark, and eventually the entire plant crumples. Unfortunately, there’s no coming back from this one.
A young dahlia with crown gall.
Crown Gall
This peculiar bacterial disease causes clusters of distorted growth (galls) near the base of the plant. This is not curable and plants that show signs of crown gall should be destroyed.
Examples of dahlias with mosaic virus – from Washington State University.
Viral Diseases
There are more than a dozen viruses that can infect dahlias. All have relatively similar symptoms which may include yellow spots or mottling on the leaves, yellowing along the veins, shortened leaf nodes on stems, distorted or twisted foliage, shorter than normal flower stems, poor flowering and overall stunted plants. Unfortunately, these symptoms can resemble other diseases as well as simple weather stress.
The only way to determine if a virus is present is to examine some of the infected tissue in a lab. This isn’t practical for home gardeners and it’s also unnecessary because there is no cure for any of these viral diseases. Infected dahlias should be removed as soon as possible to avoid spreading the disease to other plants. If you suspect your plants have a virus, do not make cuttings and do not overwinter the tubers, as the virus will be present in all parts of the plant.
Dahlia growers in Holland and the US are working to eliminate dahlia viruses, but it’s a big challenge. As with other plant diseases, some dahlia varieties seem to be naturally stronger and better able to resist these viruses. Washington State University has been studying dahlia viruses, and in particular dahlia mosaic virus, for more than 20 years. This website includes lots of relevant information: Virus Diseases of Dahlia.
Hopefully your dahlias will avoid most or even all of these problems. But if something does crop up, being able to identify what’s going on is the the best way to get to a solution.
What about teeny tiny worms? They are very hard to see but I usually can find them when they crawl on the petals.
Hi – If the worms aren’t doing any damage, I wouldn’t worry about them.
I had a cafe au lait suddenly wilt like it was affected by blight but when I dug up the tuber it has multiplied and looks healthy. Is it viable or should I discard it? Are the tubers likely to only grow diseased plants if I store it for next spring?
Hi – I wish I could say for sure, but it takes an expert to accurately diagnose plant diseases. The problem could have been fungal, bacterial or viral. If it were me, I would discard the tubers. Purchasing new tubers will hopefully reduce the chances of having problems again next year. Also consider your planting area. Like all plants (and people!), when dahlias are stressed they are more susceptible to disease.
Have noted the top stem of dahlia is covered with tiny dark / black insects, looks bad and have used water to clear them and it re appears again !
Any advice please
Hi Harry – sounds like it could be aphids. Search for “black bean aphid” and see if it looks like what you have. Aphids feed on tender foliage. They can be dislodged with water, but often come back. Plants that are stressed are more attractive to pests like aphids, so make sure you are providing adequate water and nutrients, and that the plants are getting enough sun. There are some organic insecticides that work well on aphids. You can also try using a mild detergent – mixed with water. Follow instructions for application as tender foliage can be damaged. Good luck!
I have crown gall on my dahlia tubers that I am digging up after the frost. I am discarding these tubers. I wanted to know if I steam the soil that they were planted in can I get rid of this problem in the bed.
Hi Deb – I’m sorry not to know the answer to your question. Looks like you are located in MA, so you should have access to an extension service plant pathologist through UMass. We have a good one here in Vermont (Ann Hazelrigg) and she may be able to either answer your question or direct you to someone else who can. Good luck!
Some of my dahlias are covered with buds which haven’t opened. They feel solid but a bit wet, and look fine except for being
black at the pointed non-stem end. What causes this? How can I control it?
Hi Sylvia – You don’t say where you are located, but regardless, February is very early for dahlias to be forming buds. Probably a good idea to snip off those early buds — sounds like they are damaged. Keep the plants healthy and they should produce many more buds in the future.
Thankyou so much Kate for replying, I live in New Zealand so it is summer here 🙂 I have a long bed full of dahlias, and with the centre dahlias the buds either won’t open, or they have brown spikey flowers instead of the usual gorgeous blooms.
Hi Sylvia – Sorry to hear about this problem. I’m not a dahlia expert, so I can’t say for sure what’s happening. But I have 3 guesses. First thing that comes to mind is thrips, which are very tiny sucking insects that are known to feed on dahlias – especially the buds. They suck plant juices and sometimes their damage isn’t evident until after they are gone. You may see deformed or brown buds and stippling on the leaves. They are hard to control, but can be managed. Second guess is a fungal disease. This can happen in wet weather, high humidity or if you are using overhead irrigation. Cut back the affected areas to increase air circulation and hopefully the plants will have time to rebound. Third possibility is virus. I don’t know if viruses are present in NZ dahlias or not. It’s a slowly growing problem here in the US. The only way to positively diagnose the presence of virus is to have the plant tissue tested in a lab. If your plants fail to thrive, dig them out in the fall and start fresh (don’t save the tubers). I had a miserable dahlia year last year with a whole range of different problems. I’m starting fresh this spring with fingers crossed! Lucky to have been in NZ for two long stays – both islands. Can’t wait until we can travel again so I can return!
Hi Kath, thank you so much for that, you have given me some clear things to go on. I’m going to look for sprays for thrips and fungal disease. As for the viruses, oh no! Nothing is without its challenges even in the garden! So glad you liked our land when you came 🙂 You have a lovely day
I planted tubers in a pot indoors from last year’s dahlias in a fresh bag of good quality potting soil. One pot of plants looks healthy. The plants in the other pot have developed what looks like a huge gall and there are tiny winged insects crawling over the soil and gall. I do not know if I need to discard or quarantine this pot of dahlias so my other plants are not affected. The gall developed after I watered the pot with the galled dahlia tubers about a month ago.
Thanks!
Hi Nan – Leafy gall does seem to be a growing problem with dahlias. I had never seen it until last year. We recommend destroying the affected plant now and also throwing out the soil. Leafy gall is a bacterial disease, so it can spread to other plants.