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.
If you see just a light case of powdery mildew, like white powder on rose leaves, spray it off with water on a warm, dry day.
The fungus lies dormant when the humidity is low. But when the relative humidity rises, mildew spreads quickly, producing mildew spores and spreading rapidly from plant to plant.
Before your roses fail to bloom and the leaves fall off, you’ll see some early symptoms like white powder on roses.
Make sure all dead leaves, stems, and decaying matter are removed from the bush and around its base.
As well as plant growth being inhibited, stunted growth of the shoots and the formation of deformed leaves can also occur.
Powdery mildew infestation can stunt the plants’ growth or even kill them as it blocks sun contact with the leaves and puts them into photosynthesis.
The white, powdery fungal growth can be very disfiguring, with repeated heavy infection reducing plant vigour.
This disease was formerly known as Sphaerotheca pannosa, a fungus that overwinters as mycelium in infected buds, rose leaves, twigs, and branches.
Rose powdery mildew spreads during the growing season by means of microscopic, air-borne spores produced on the powdery growth.
In this article, we’ll discuss powdery mildew on roses and share advice to helping you prevent and treating powdery mildew on roses.
What Does Powdery Mildew Look Like?
Powdery mildew infestation usually begins late in the growing season as environmental conditions change.
You’ll begin to notice small, powdery, round black spots on the leaves and stems.
If left untreated, the mildew 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 flowers over it. It can cover flower stalks, calyces, stems, and petals.
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 top side of the leaves; however, it grows on the undersides of the leaves, too.
If the fungus is left to thrive, the rose leaves will begin to dry out and turn yellow.
Both leaf surfaces can be affected. There may be discoloration (yellow, reddish, or purple) of the affected parts of the leaf, and heavily infected young leaves can be curled and distorted.
They may even begin to grow in a disfigured manner, to twist and to break.
This effect may spread – sporulation [fungi produce masses of spores (conidia), which become airborne and spread to other plants. source] to flower buds, as well as the growing tips of stems.
Now you might be asking, what causes powdery mildew on roses?
Rose powdery mildew is a disease caused by the fungus Podosphaera pannosa that produces airborne spores from infected stems or buds on roses.
How To Treat Powdery Mildew on Roses?
Understanding the disease’s life cycle is vital to maintaining control of your crops.
If caught early, begin by 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. It will simply stay there on stems or dormant buds, grow and reinfest your garden when you use the compost.
- Remove badly-infested plants 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
Before using commercial chemical controls, try making an all-natural antifungal solution using water and baking soda in the garden is an effective powdery mildew on roses home remedy.
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 mat of 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 rose mildew treatment every couple of weeks to kill off existing fungus and prevent re-infestation.
When using fungicides, be sure to follow the packaging instructions carefully.
Apply the products on a dry full sun day when rain is not expected.
Always read the label before applying sprays. Be sure to protect yourself against chemical fungicides by wearing gloves, long sleeves, a mask, and eye protection.
Related: How To Control Downy Mildew
How Do You Prevent and Control Rose Powdery Mildew?
Maintaining good air circulation is one of the main keys to preventing powdery mildew infestation.
Due to the advent of new, disease-tolerant roses and efficient pesticides that should be used for serious infestations, the technique of pruning infected leaves to stop spreading has become outdated.
Be sure to keep your rose bushes well-pruned so air movement circulates 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 the new growth of powdery mildew above leaf surfaces and on lower leaf surfaces.
Choose varieties of roses that are more resistant to fungus. Some examples of 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 deep weekly watering, bright, full sunlight, and excellent soil drainage.