11 Dahlia Diseases: How to Identify, Prevent, and Treat Them

Dahlias can succum to many different diseases. While there are many different types of disease that try to plague these beautiful flowers, the good news is there are ways you can prevent, and treat them. In this article, certified master gardener Liz Jaros examines the most common dahlia diseases. You'll also learn how to identify, treat, and prevent them.

dahlia diseases

With 42 different species and thousands of colorful cultivars, dahlias have earned a special place in your heart. And most likely your yard. This year, you’ve planned, prepped, and planted your dahlias with care, but something doesn’t look right. And it’s devastating.

Maybe you’ve noticed some stunted growth or twisted stems. Maybe you’re looking at foliage leaves that have started turning yellow, or black. Or, perhaps some of your precious pink petals have turned brown and started to wilt.

Whatever the symptom, you’ll need to get right to the root of your dahlia’s ailment and start triage immediately. Here we look at 11 diseases known to infect dahlias, examine their causes, and discuss the best courses of action.

Bacterial Disease

Since bacteria can be both beneficial and pathogenic, it’s important to remember that not all bacteria are bad. However, in the dahlia world, bacterial disease is almost always deadly and should be dealt with swiftly. Two of the most common bacterial diseases that affect dahlias are bacterial wilt and soft rot.

Bacterial Wilt

Bacterial Wilt
Suitable conditions for the development of the Bacterial Wilt disease are warm weather and high soil moisture.

If your dahlias are suffering from this disease, leaves will appear darker than usual and wilt during the day. You might notice them perking up a bit at night, but eventually, they will turn brown and plants will collapse. Strands of bacterial ooze might be apparent when stems are cut cleanly with a pruning tool. 

Unfortunately, bacteria has clogged up your dahlia’s vascular system and there is nothing you can do but take measures to prevent it from infecting other plants in your garden. Dig it up, including as much dirt from around the infected tubers as possible, and dispose of it in the garbage can.

How to Identify

Dark leaves that wilt during the day are early signs. Brown leaves and soft, collapsing stems are final stage symptoms.

How to Treat

Bacterial wilt is fatal. Excavate dahlias and surrounding dirt, and dispose.

How to Prevent

Remove immediately. Do not plant dahlias near vegetables since cucumber beetles are known carriers. Clean and disinfect tools often.

Soft Rot

Soft Rot
Soft Rot is a very common rotting problem affecting a wide range of flowers and plants, caused by Erwinia carotovora bacterial rot.

Bacterial soft rot causes more crop loss world-wide than any other bacterial disease. Unfortunately, dahlias’ tuberous roots make them particularly vulnerable to this one.

If you’re noticing yellow, water-soaked spots, particularly at your dahlia’s outermost leaves, they might very well be suffering from soft rot. As bacteria continue to break down plant tissue, a foul odor and mushy stems might help confirm this diagnosis. 

Again, with bacterial disease almost always being fatal, the best you can do here is remove the sick dahlias promptly and excavate as much of the soil around them as possible. Water evenly, space properly, and consider planting dahlias (or any other tubers) to a new location next year. You may even want to plant your dahlia tubers in a container, since bacteria can overwinter in most climates.

How to Identify

Yellow, water-soaked spots on leaves that may ooze. Foul odor and mushy stems.

How to Treat

Dispose of the infected dahlia and as much surrounding soil as possible.

How to Prevent

Water evenly, and space properly. Dispose of infected plants. Move dahlias to a new bed next year. Use clean tools.

Aster Yellows

Aster Yellows on Flowers
Aster yellows can cause a flower’s petals to tint green, and can take over an entire plant.

When a dahlia is infected with aster yellows, leaves may appear light green, yellow, or white and be stunted or deformed. Blooms may also be small, oddly shaped and pale in color. This disease is caused by phytoplasma, a small bacteria that can live in the veins of a plant or the body of an insect.

Typically transmitted by leafhoppers, prevention of aster yellows will involve pest control as well as bacterial management. Reflective mulches and light mesh covers can help to exclude leafhoppers from your dahlia beds but can be unsightly. Neem oil coated on leaf surfaces might be a better choice for discouraging leafhoppers without detracting from your dahlias’ natural beauty. 

Image Credit: Estreya via Creative Commons (Image Use Allowed With Attribution)

How to Identify

Deformed light green, yellow, or pale leaves. Small, odd shaped blooms with pale petals.

How to Treat

If aster yellows disease is confirmed, your dahlia will not recover and the entire plant should be dug up and thrown away.

How to Prevent

Discourage leafhopper populations with reflective mulch or horticultural oils. Remove infected plants from the garden.

Fungal Disease

Always present in the soil, fungus is known to wreak havoc in the dahlia garden. Not always apparent until the problem is advanced, fungus comes in many forms and can be difficult to treat. Let’s look at some of the fungal foes that are most likely to infect dahlias, and determine how to handle them.

Powdery Mildew

Powdery Mildew
Powdery Mildew – a whitish, mealy or powdery coating appears on the leaves, quickly covering the entire leaf.

Grayish or white, powder-like spots are the tell-tale signs of powdery mildew. This fungus will be glimpsed first on leaves, but might also spread to stems and blooms. It might also turn black later in the season. Look for it when climate conditions are characterized by high levels of humidity, but moderate day and night time temperatures.

While dahlias are particularly prone to powdery mildew, especially in spring and fall, the disease does not have to be fatal. If leaves, stems, or blooms appear infected, prune out the bad parts and leave the healthy growth alone. If you can live with the way it looks, and fungal growth is not too severe, plants with a little mildew can be left in place until the end of the season. At that point, cut stems to the ground and remove to prevent fungus from overwintering.

How to Identify

Grayish, white spores on distressed leaves.

How to Treat

Prune off infected leaves. Dispose of plants that are completely infected.

How to Prevent

Do not crowd dahlias when planting. Make sure they have good airflow and plenty of sun. Irrigate soil, not leaves. Cut to the ground at the end of the season and do not compost or overwinter.

Botrytis (Gray Mold)

Botrytis (Gray Mold)
Gray Mold disease appears in wet weather. On the leaves, flowers and stems of a diseased plant, gray-brown softened spots can be seen.

Fuzzy gray spores, and petals that are turning brown from the centers outward are indications of botrytis. This fungus is most likely to attack dahlias after an extended rainy period, so keep an eye out for it when the conditions are optimal.

Like powdery mildew, botrytis is a surface disease and not systemic. This means the damage is mostly aesthetic, and infected parts can be pruned off. Cut down and remove all above ground plant parts at the end of the season to prevent multi-season spread.

How to Identify

Look for fuzzy, gray spots on leaves/stems or blooms that fade too quickly from the center out.

How to Treat

Prune off infected parts.

How to Prevent

Plant dahlias in locations with at least 6 hours of sun and good air circulation. Water evenly, avoiding leaves, and delay watering after heavy rains. Disinfect pruning tools regularly.

Verticillium & Fusarium Wilt

Verticillium & Fusarium Wilt
Diseased plants by Verticillium & Fusarium Wilt can be considered dead, they must be immediately removed and burned.

These fungi grow in a dahlia’s xylem and block water transport. Yellow leaves may be an early indication but typical symptoms of this disease include dark stem tissue near the soil level and streaky brown or black roots.

Both fungi thrive when temperatures swing suddenly from cool to warm, so look for signs of wilt when these conditions are met. Since wilt damage will affect the dahlia’s whole feeding system, it’s best to destroy and remove those that are infected.

How to Identify

Yellow leaves with brown/black streaks on stems and roots.

How to Treat

Destroy and remove plants from the garden.

How to Prevent

Do not compost infected tissue. Rotate your dahlias into a new bed next year. Sterilize tools between flowers.

Smut

Smut
Dahila Smut is caused by Entyloma dahliae, which appears as pale spots on the leaves during wet conditions.

Pale leaf spots with dark centers that begin on lower branches and trend upward can be symptoms of dahlia smut disease. Like most fungi, smut presents during wet periods or if irrigation is focused on leaves rather than roots.

If smut is suspected, prune off affected leaves and remove them from the garden. You may also want to prune some of the foliage to create more light and air flow, since crowding is a major factor in smut transmission.

Smut can overwinter in your dahlias’ tuberous roots, so if you’re bringing them in to overwinter, treat them with a fungicide and dry thoroughly before wrapping and storing.

How to Identify

Pale leaf spots with dark centers, seen first on lower branches.

How to Treat

Prune damaged foliage and stems.

How to Prevent

Plant dahlias in full sun and space generously. Water evenly. Watch for fungal disease during wet periods.

Sclerotinia Disease

Stem Rot
Another common dahlia disease is Sclerotinia, also known as “white mold.”

Early symptoms of this fungus include browning and/or wilting stems. If stems are sliced open laterally, a string of tiny black spores can confirm a sclerotina dianosis. The spores spread easily in the air and cling to petals, which then cling to stems, perpetuating the cycle.

Prune off infected stems, taking care to not shake spores onto your soil surface. Remove and destroy them. Deadhead regularly and remove all petals from your garden beds. When tubers are dug up for the season, cut stems down to the roots and examine for sclerotina spores. If some are present, treat with fungicide and thoroughly dry tubers before wrapping them for winter.

How to Identify

Brown, wilting stems with tiny black spores inside or petals with tiny black spores on their surfaces.

How to Treat

Prune off damaged stems. Spray tubers with fungicide before overwintering.

How to Prevent

Monitor during wet periods. Remove spent blooms and petals promptly. Do not shake affected stems or petals over the dirt.

Southern Blight

Dying Flower With Blight
Blight is a disease that feasts on dying dahlia blooms.

In warmer temperatures, this fungus will present with a ring in the soil around your dahlias’ stems. Initially small white spores will turn black and grow together to choke plants out. Dahlias will wilt suddenly and die. Any suspected infection should be dealt with by removing plants completely.

Blight thrives on decaying leaves and blooms, so keep a tidy garden bed to discourage its spread. Monitor dahlias after periods of drought since the first heavy rain after a dry period creates optimal growth conditions. Planting dahlias in a raised bed can help discourage debris piles near their bases.

How to Identify

A white ring or black circle growing in the dirt around stems.

How to Treat

Remove and destroy affected plants.

How to Prevent

Keep plant bases free from leaves, mulch and debris. Monitor when heavy rain follows a heavy drought. Consider planting dahlias in raised beds or berms.

Viral

While some dahlias will show symptoms of viral disease, others will appear just fine and fail slowly over time. Look for stripes and splotchy patterns that vary between light and dark green or leaves that are twisted and yellow. Since viral disease is basically an illness that your other dahlias can catch and transmit, it’s important to know and trust the source of your stock, and to get out in front of any outbreak. Mosaic and ring spot are the viruses you’re most likely to encounter in the dahlia garden.

Dahlia Mosaic

Flower With Mosaic Virus
Look for irregularly colored leaves when identifying this disease.

Look for irregularly colored leaves and pale green bands near your leaves’ veins to indicate the presence of dahlia mosaic virus. In some cultivars, the virus may be almost symptomless but will make itself known through diminished bloom intensity and slow tuber expansion.

While it might take a season or two, mosaic virus will eventually kill your dahlia, so if you suspect this disease is attacking your plant, remove it promptly and throw it away. Disinfect your tools diligently and control aphid populations.

How to Identify

Irregularly colored leaves and pale green bands near leaf veins. Slow growth or diminished flower production.

How to Treat

Destroy and dispose of infected plants. Do not compost.

How to Prevent

Dip pruning tools in bleach solution between dahlias. Discourage aphids with leaf oils or predatory insects. Dispose of suspected carriers promptly.

Ring Spot

Ring Spot
The Ring Spot virus can be found on young foliage as light lemon rings or spots. Sick dahlias are stunted, flowering is absent.

Mottled leaf patterns and wavy or concentric rings are the tell-tale signs of ring spot. The disease is spread by thrips and through cuttings or root divisions of infected plants. To reduce your chances of unknowingly spreading ring spot, always take dahlia cuttings from the tops of fast growing, healthy looking plants. And stay on top of thrips.

If ring spot is confirmed, your dahlia should be destroyed and its bedfellows monitored carefully for early symptoms.

How to Identify

Mottled leaf patterns. Wavy or concentric rings.

How to Treat

Destroy and dispose of infected plants. Do not compost.

How to Prevent

Only take cuttings or root divisions from thriving, fast growing plants. Disinfect tools between plants. Control thrips.

Final Thoughts

It can be difficult to differentiate between the ailments that affect dahlias. Dahlia pest and disease symptoms can be similar and are often interrelated. In most cases, the only reliable way to diagnose a specific disease is by sending a sample out for testing. But most of us are not going to do that every time a yellow leaf shows up in our beds.

It’s most important to know the distinctions between a fungal condition and a bacterial or viral disease since fungal disease can often be treated and is not always fatal. Unfortunately, the hard and fast rule for dahlia disease is always going to be ‘when in doubt, throw it out.’ Keeping transmission in check, and preserving the health of other dahlias in your yard should be your top priority.

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