Powdery mildew is a fungal disease affecting a wide array of plants and rose varieties.
While most types of fungus plant diseases thrive in cool, damp settings, all species of powdery mildew thrive in dry, warm climates.
The fungus lies dormant when the humidity is low, but when the relative humidity rises, mildew spreads quickly, producing mildew spores and spreads rapidly from plant to plant.
Powdery mildew infestation can stunt the plants’ growth or even kill it as it blocks sun contact with the leaves and puts and into photosynthesis.
This disease was formerly known as Sphaerotheca pannosa, a fungus which overwinters as mycelium in infected buds, rose leaves, twigs, and branches.
In this article, we discuss powdery mildew on roses and share advice to help you prevent and deal with this problem.
Read on to learn more.
What Does Powdery Mildew Look like?
Powdery mildew infestation usually begins late in the growing season.
You’ll begin to notice small, powdery round black spots on the leaves and stems.
If left untreated, the mildew you will spread and cover the entire plant. That is what causes white spots on rose leaves.
If your rosebush is badly infected with powdery mildew, it may look as if someone has come along and dusted flower over it.
It’s important to inspect your rose bushes frequently for signs so it never reaches this point.
You’ll notice initial mildew growth most on young leaves.
Usually, the mildew appears on the topside of the leaves; however, it grows on the undersides too.
If the fungus is left to thrive, the rose leaves will begin to dry out and turn yellow.
They may even begin to grow in a disfigured manner, to twist and to break.
This effect may spread to flower buds, as well as the growing tips of stems.
How Do You Treat Powdery Mildew?
If caught early, begin by ruthlessly pruning back all affected plant parts of your rose bushes.
Dispose of these pruned off parts by placing them in a sealed plastic bag and putting them out with the trash.
Don’t put foliage affected by powdery mildew into your compost heap because it will simply stay there, grow and reinfest your garden when you use the compost.
Very badly infested plants should be removed entirely.
The soil surrounding the plant should also be dug up and disposed of in sealed plastic bags.
The remaining soil should be treated with an antifungal solution, and all new soil should be introduced.
Place a layer of mulch over the soil in the spring to cover any fungus spores on the surface of the soil.
After removing affected plant tissue, spray liberally with a disease control fungicide such as:
- Potassium Bicarbonate
- Lime Sulfur
- Neem Oil pesticide mixture
- Sulfur
- Horticultural Oil spray
- Myclobutanil
Homemade Powdery Mildew Solutions
Make an all-natural antifungal solution using water and baking soda in the garden.
Mix a quart of water with a teaspoon of baking soda and spray the solution liberally over your plants.
For larger gardens add 4 parts water with 4 teaspoons of baking soda and create a gallon of water mix.
This sort of natural solution is more effective as a preventative than as a treatment.
Natural treatments will kill the fungus present but will not prevent it from returning.
Chemical treatments can both kill existing fungus on contact and prevent it from returning and spreading to other plants.
Set a schedule to reapply mildew treatments every couple of weeks to kill off existing fungus and prevent reinfestation.
When using fungicides, be sure to follow packaging instructions carefully.
Apply the products on a dry full sun day when rain is not expected.
Be sure to protect yourself against chemical fungicides by wearing gloves, long sleeves, a painter’s mask, and eye protection.
How Do You Prevent and Control Rose Powdery Mildew?
Maintaining good air circulation is one of the main keys to preventing powdery mildew infestation.
Be sure to keep your rose bushes well-pruned so air can circulate freely through them.
Avoid touching unaffected limbs with pruners used to prune away infected limbs.
Keep your pruning implements clean by wiping them frequently during use with a clean cloth soaked in rubbing alcohol.
When you are finished pruning, clean your pruning shears thoroughly with a bleach solution and wipe them dry.
Inspect your rose bushes regularly for any signs of powdery mildew and deal with signs of infestation promptly by removing affected foliage and treating your plants with an antifungal solution.
Use ground watering only.
Never water your roses from overhead as this will raise the relative humidity around the plants,
High humidity encourages new growth of powdery mildew above leaf surfaces and on lower leaf surfaces.
Choose varieties of roses resistant to fungus. Some resistant varieties examples include:
- Knockout Roses
- Rosa Radrazz
- Rosa rugosa
Remember roses having waxy leaves or shiny leaves will be less likely to contract powdery mildew.
If you know a plant is affected by this fungal infection year after year, as is the case with many:
- Monarda
- Zinnias
- Phlox
- Lilacs
… then spraying early in the season may prevent fungal growth for the year.
No matter what type of rose bush you choose, be sure you thoroughly understand how to provide it with the best care.
Roses appreciate a deep weekly watering; bright, full sunlight, and excellent soil drainage.