What Causes White Spots On Rose Leaves?

Take a close look at your rose bushes, and you may notice what looks like flour dusted over the leaves. Looking closer, you may find pests like aphids, scale insects, leafhoppers, or other bugs.

Still, it’s more likely that the white spots on rose leaves are a result of the fungal disease powdery mildew.

white spots on rose leaves (powdery mildew)Pin
White spots on leaves of rose plants | aRTI01-Adobe

Powdery mildew is a common problem in gardens. It can infect most plants. But vegetables, roses, lilacs, and legumes are more susceptible.

If left uncontrolled, the fungi can weaken rose bushes. It can cause disfigurement, leading them to wither and die in severe cases.

Powdery mildew can spread to other leaves and plants, too, so it’s important to nip the problem in the rosebud.

Here are some things to know about those pesky white spots, what causes them, and how to prevent and control them.

What Is Powdery Mildew?

There are different species of powdery mildew, and each species can infect a range of plants. The plants with the least resistance are:

  • Cucurbits: Cucumber, Squash, Pumpkins, Melons
  • Nightshades: Tomatoes, Eggplants, Peppers
  • Flowers: Roses (even Double Knockout Roses), Lilacs, Phlox
  • Legumes: Beans, Peas

Once the fungus infects the plant, the white mildew made up of spores creates a layer across the tops of the leaves. The wind carries these spores from plant to plant and slows down growth.

In severe cases, the mildew can change the flavor of the fruit or vegetable and decrease yield. Infected rose bushes grow slower or stop blooming.

Related: Learn more about growing Double Knockout Roses here.

What Causes Powdery Mildew on Roses?

The fungus known as sphaerotheca pannosa var. rosae causes the white spots seen on rose leaves and buds. This fungus loves warm, humid climates and fresh foliage.

When the conditions are right, the mildew latches onto and feeds off the plant. This causes disfigurement and slow if any, growth.

How To Identify Powdery Mildew?

The mildew starts as chalky, white spots on the upper part of the rose leaves. It may also spread to the undersides or other parts of the plant. Marks can also show up on the stems and flower buds.

The rose leaves dry out and turn yellow, and some may twist or break. The individual white spots will cover the leaf surfaces and look more like a blanket of flour.

If the fungal infection goes untreated, the rosebuds and plant-growing tips will twist, break, and become disfigured as the growing season continues.

How To Prevent and Control White Spots on Roses?

Active prevention is the best way to control an outbreak of powdery mildew on roses. Some of the most common prevention options include:

  • Choose disease-resistant rose bush varieties
  • Plant rose bushes in sunny spots where they can receive full sun. The mildew flourishes in shady, wet environments.
  • Prune back rose plants to increase air circulation and decrease humidity.
  • Remove leaves with signs of mildew. 
  • Spray the rest of the plant, as well as any nearby plants. For treatment, use a mixture of one teaspoon of baking soda in one quart of water. Evenly cover the top and underside of each leaf and stem.
  • Prevent mildew or treat infected plants with organic fungicides like sulfur, neem oil, lime sulfur, and potassium bicarbonate.

Keep in mind that baking soda can sunburn rose leaves. It’s important to thoroughly water your plants before applying the baking soda solution. Do not apply the mix during daylight hours.

You can also use dish soap, rubbing alcohol, horticultural oil, spinosad, insecticidal soap, or a combination of fungicide and insecticide to remedy this.

When using neem oil, mix 1 gallon of water and two tablespoons of neem oil. 

Spraying this solution will help control this disease but won’t harm natural predators or beneficial insects like birds, bees, and butterflies. 

If the disease has already taken hold, remove the infected rose bushes by burning or throwing them in the trash.

Do not keep infected plants around or compost them. Mildew spores can thrive inside compost and spread to other plants through the air.

It’s natural to get discouraged when you see white spots on rose leaves or appear on any of your plants. Gardening takes time and effort. To have it all go to waste because of some fungus isn’t fun.

The best way to prevent powdery mildew is to take preventive measures.

  • Inspect your rose bushes daily to look for signs and mildew symptoms
  • If you spot anything unusual, take control by using the ideas listed above.

You may lose a few plants, but you’ll have a beautiful, healthy rose garden as a result.

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