Powdery Mildew on Dahlias: What It Is, Why It Happens, and How to Keep It Off Your Plants

If your dahlia leaves are looking less “garden goddess” and more “ghost of mildew past,” you’re not alone.

Powdery mildew is one of the most common fungal issues gardeners face with dahlias—especially in warm days/cool nights climates. It’s annoying, it’s clingy, and yes, it can ruin your vibe if left unchecked. But the good news? It’s also totally manageable if you catch it early and play a little defense.

Let’s get into it.

What Is Powdery Mildew?

Powdery mildew is caused by a group of fungi (usually Erysiphe species on dahlias) that produce a soft, white coating... like a dusting of flour on leaves, stems, and even flowers. It thrives when days are warm, nights are cool, and leaves stay a little too moist.

Mild infections mostly look ugly. Severe infections can stunt growth, reduce blooms, and defoliate your plants.

👉 See it up close? UMass has photos of it on dahlias here.

How to Spot It

• White or gray powdery spots on leaves—usually starting with older growth

• Yellowing, curling, or distorted leaves

• Reduced flowering or plant vigor

• Leaves dropping prematurely if infection spreads

It doesn’t usually start with a bang. It creeps in. Catch it early, and you’re golden.

Why Dahlias Get Powdery Mildew

This fungus is basically the plant version of a fair-weather friend: it only shows up when conditions are just right.

Main Causes:

• Warm days + cool nights

• High humidity + poor air circulation (hello, southern United States)

• Shady spots or overcrowded plants (whoops)

• Consistently wet leaves or overhead watering

• Lack of airflow from tight spacing (looking at you, overenthusiastic planters)

Pro tip: Spacing dahlias too close, like under 18" but definitely under 12", is a recipe for mildew drama. We plant close but continuously strip the lower leaves for good airflow.

How to Prevent Powdery Mildew

Most mildew prevention is just good garden hygiene. Like brushing your teeth, but for plants.

 1. Improve Air Circulation

Space your dahlias generously and avoid planting them in tight rows or against fences without airflow. If you're planting Fuirk Home tubers, we suggest spacing them 18–24" apart. But... let's be realistic, 12" happens all the time. Just keep the lower leaves stripped and stay on top of plant management. 

2. Give Them Full Sun

Mildew hates full sun. Aim for at least 6–8 hours a day. I grow in a mix of part shade to part sun and am easily able to manage powdery mildew. Shade is not a death sentence. By the way - visit this page next if you want to know more about how to grow dahlias in the shade.

3. Water Smart

Water at the base of the plant in the morning, not on the leaves. Wet foliage at night is essentially a magnet for fungus. With that said, I overhead water my plants to keep them cool. I water multiple times each day, and my last watering session is at 3pm to give the plants time to dry. On days under 90 degrees, I often end a bit earlier. Just know that if you decide to overhead water, you have to be extra diligent when it comes to plant management, especially the next step.

4. Clean Up Leaves

Remove infected leaves promptly. When you strip infected leaves, you need to bag them and haul them out of your garden. Stripping the leaves and leaving them on the ground doesn't remove the problem. Bag them and trash them. Also, don’t compost infected material—unless you’re really into breeding spores.

Treatment Options (When Prevention Fails)

Begin treatment before symptoms appear especially if you’ve had mildew in past years. My suggestion is to start with BioWorks Millstop or a copper-based fungicide before you see signs of powdery mildew and treat weekly until the end of the season.

• Neem oil – works as both prevention and treatment (note: Neem oil has been ruled toxic in some European countries, so use with care).

• Milk spray (1 part milk to 2–3 parts water) – surprisingly effective in sunny conditions

• Sulfur or copper-based fungicides are effective, especially when used preventively. Captain Jack's Liquid Copper Fungicide is a good choice. 

• Dr Earth Final Stop uses rosemary oil, clove oil, and peppermint oil, and it is OMRI approved. 

• BioWorks Millstop - this is my favorite treatment! It uses potassium bicarbonate to disrupt fungal growth and is OMRI listed. 

• Rotate products
  

For more advanced details, the Pacific Northwest Plant Disease Handbook has all the sciencey bits.

What About Biologicals?

Biological fungicides like Actinovate SP or Serenade can help suppress mildew naturally and are friendly to bees and other beneficials.

Quick Recap

Need Supplies?

If you’re prepping for a mildew-prone season, check your stash:

• Copper fungicide? ✅

• Sharp snips for cutting back infected growth? ✅

• Mulch to prevent splash-up? ✅

• Healthy dahlia tubers with good genetics? Obviously.

Make sure you're subscribed to our newsletter at the bottom of our home page so you can shop  field-tested dahlia tubers and rooted cuttings. Your garden should start with fantastic dahlias!

Still Seeing Spots?

Shoot us a message or tag us on Instagram @fuirkhome. We’re happy to help you troubleshoot with photos. No judgment here—powdery mildew’s like glitter: it shows up where you least expect it and never totally disappears unless you get serious.

Sources:

• American Dahlia Society: Fungus Control

• Gardening Know How: Dahlias with Powdery Mildew

• PNW Plant Disease Handbook

• UMass Agriculture Floriculture