What is Dahlia Mildew, What Causes It, And How To Control It?

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The dahlia (DAL-ee-a) has seen unheard-of success worldwide, with its 42 species boasting over 57,000 registered cultivars and counting.

You can find one of these plants with just about any shape and size of flower, from tiny buds to ones that rival a sunflower, round pop-pons, flat single flowers, and more!

Vibrantly Purple DahliaPin

The plants are also quite easy to grow and relatively free of pests and diseases.

However, you might have noticed a strange white powder forming on some of your dahlia’s leaves.

This is powdery mildew, a condition that may only be a symptom of a larger problem.

What Is Dahlia Powdery Mildew?

Powdery mildew may resemble talc, but it’s a type of fungal infection caused by a wide range of fungi.

The species of fungus which affect dahlias most are:

  • Golovinomyces cichoracearum
  • Golovinomyces orontii
  • Golovinomyces spaciceus

However, it also commonly attacks other plants, including azalea, crabapple, delphinium, dogwood, euonymus, lilac, phlox, rhododendrons, sunflowers, and vegetables like beans, cucumbers, lettuce, melons, peas, pumpkins, peppers, tomatoes, and others.

It most often appears due to infestation or when there’s an especially high humidity of at least 95% percent in conjunction with temperature fluctuating between 68° to 86° degrees Fahrenheit and low light.

This combination of factors means you’ll most often run into powdery mildew in the spring and autumn when weather conditions are most unstable.

What Damage Does Powdery Mildew Cause?

Powdery mildew doesn’t cause any major harm to dahlias, but it can still be a big problem.

Infected leaves will appear to have white mold covering them or on their undersides as if someone sprinkled the leaf with talc or confectioner’s sugar.

Infected leaves will lose their ability to properly photosynthesize if left untreated, resulting in distorted or discolored leaves that will eventually wilt and fall off.

The mildew can easily spread to nearby leaves, allowing it to infect much of the plant in time.

However, the species which affect dahlias are unlikely to spread to other genera of plants nearby.

More importantly, powdery mildew may be a symptom of a pest infestation.

A wide range of sap-drinking bugs such as aphids, mealybugs, and scale left behind partially digested sap is called honeydew.

This honeydew is a perfect breeding ground for powdery mildew.

How To Control Dahlia Powdery Mildew?

Treating powdery mildew is a three-step process:

  • Stop the spread
  • Treat the plant
  • Prevention

Stopping The Spread

Immediately remove any leaves showing signs of infection, and be careful not to bring them into contact with healthy leaves. 

Don’t forget to remove diseased or damaged foliage, flower buds, stems, and fruits through pruning.

Make sure your shears or pruners are sterilized between each cut by dipping them in some rubbing alcohol.

You will also want to locate and eliminate any pest infestations at this time.

Treating Your Dahlia

Once you’ve removed the visible infection, it’s time to treat the invisible spores of powdery mildew fungi.

Chemical fungicides work well for this step, as they’re usually quick and effective.

The biggest downside to using fungicides is that you must change it frequently, so fungi are less likely to develop resistance.

Thus, if you use fungicides with a specific active chemical once or twice, switch to different active chemicals for the next 1 to 2 treatments before using the first chemical again.

However, fans of natural remedies can turn to every gardener’s ally: neem oil.

Powdery mildew anchors itself to the Dahlia leaves, piercing the surface.

As a result, it’s both an internal and external infection.

Neem foliar sprays applied at dusk or dawn can not only help fight off the mildew on a leaf’s surface but will suffocate piercing insects on contact.

Apply it every other day for 14 days or until all signs of infection are gone.

Meanwhile, a neem soil soak will attack the internal portions of the fungus, and the poison will pierce the insects, so they can no longer reproduce and may even starve themselves to death.

The soak should be applied every 14 days until the Downy Mildew or any accompanying infestation is gone.

Finally, you may make a homemade sulfur spray solution to treat your dahlias.

The sulfur makes it impossible for the fungus to survive on treated leaves.

However, treating a dahlia with sulfur in temperatures above 85° degrees Fahrenheit can actually harm the plant, so reserve this treatment plan for cooler days.

To control this, you can also spray the plant using insecticidal soap, ultra-fine horticultural oil, or malathion. Baking soda solution will also prevent powdery mildew.

Ensure to provide good air circulation and enough sunlight to reduce infection.

Preventing Future Outbreaks

You can incorporate several preventative measures into your daily care routines that will help prevent future outbreaks of powdery mildew and help prevent other problems as well.

Neem foliar sprays may be applied safely every 14 days to help protect plants from infestations.

Likewise, neem soil soaks may be applied every 2 to 3 weeks. It will remain an effective systemic pesticide for up to 22 days and can also attack any fungal or bacterial infections that break through the plant’s surface.

Water is another major vector for powdery mildew on dahlias, so try to avoid getting the leaves wet when watering. 

Related: Details on watering Dahlia flowers.

Try to prune around the base of the plant to get more air circulation, and try not to use sprinklers or misters around the plant.

You can also introduce Bacillus subtilis, a bacterium with an active ingredient called Serenade that helps prevent the powdery mildew from infecting the plant.

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