How to Identify and Control Powdery Mildew on Roses

I spent many years admiring other gardener’s roses before I was brave enough to start my own. I was always told that there are many things that go wrong, but my fears didn’t match reality. The biggest issue I face each year in my rose garden is powdery mildew.

It can be minor at times or cause serious problems depending on the conditions and time of the year.

If you want your garden to be pristine (think English rose garden) make sure that powdery mildew doesn’t take up residence. But it’s not the end of the world if it does. Here’s how to deal.

What is Powdery Mildew on Roses?

Powdery mildew on roses is caused by a fungus called Podosphaera pannosa. It’s commonly called rose powdery mildew. The disease normally strikes roses in summer and it loves humid conditions when the weather is dry.

Powdery mildew on roses is spread by way of spores from infected plants. The fungus will overwinter on the roses and appear in summer when the conditions are right and will infect all aerial parts of the rose plant.

Symptoms of Powdery Mildew on Roses

The first thing you normally see is a white powdery covering on the leaves. Powdery mildew may be on both the top and bottom of the foliage. It’s easy to wipe off with your finger.

You may see the same growth on the stems, petals, flower stalks, or calyces. The calyces are the individual green sepals that encase the developing flower and end up as collars under the formed flower.

Flower buds will likely fail to bloom or open if the plant is heavily infected. If the infection is severe, the leaves can become curled and disfigured.

Some leaves will be covered in white powder, but may also be discolored and look purple, red, or yellow instead.

As the powdery mildew ages, it will likely turn dull white and then brown. Severe infections cause the foliage to drop.

Difference Between Powdery Mildew and Downy Mildew

It’s important to know that there is a similarly named, but different disease that a lot of people confuse with powdery mildew, and it’s called downy mildew. Before we go on, we should look at the difference.

Downy mildew (Scattered downy mildew) is a water mold (or oomycete), while powdery mildew is what is known as sac fungi.

Rather than the distinctive white powder look, you will see spots of red, brown, yellow, or purple on the tops of the leaves. Leaves may also drop off, even if they don’t show any symptoms. Buds may also dry out.

The disease usually starts at the top of the plant rather than the bottom, as black spot does, though downy mildew can be confused with black spot because they look similar.

Downy mildew is more likely to cause plant death and is a serious and destructive disease on roses.

Lifecycle of Rose Powdery Mildew

Unfortunately, rose powdery mildew isn’t something that appears one season and disappears the next. It survives on the plant in various forms depending on the time of the year.

The first thing to know is powdery mildew needs living tissue to grow on. It won’t live on dead tissue or in the soil.

On roses, powdery mildew lives season to season as what are called vegetative strands of tiny buds or spheres. The disease survives on the stems and leaves.

Spores are within the powdery film you see, which is known as mycelium. If you have a strong magnifying glass, take a look at the powder and you may see the individual or groups of spores. They form chains on the upper or lower parts of leaves.

Wind carries the spores from plant to plant. Water splash can throw spores around an area as well.

Unlike many other diseases, rose powdery mildew can germinate, travel to, and infect plants in the absence of water. It loves dry and humid conditions.

If water sits on the foliage for extended periods, it stops the powdery mildew disease cycle. This is why in rainy seasons, you often see other water-loving diseases, but not powdery mildew.

Direct sunlight and temperatures over 95ºF will likely kill the spores.

Conditions For Powdery Mildew on Roses

Sometimes it doesn’t matter what you do to prevent diseases like powdery mildew on your roses, it still happens. These are the conditions that fungi like, so do your best to disappoint them.

• Shady and low light conditions. Like many fungi, powdery mildew spores don’t like lots of sunlight. The good news is your roses do. Provide at least six hours of sunlight a day and powdery mildew is already on the back foot.
• Lots of weeds surrounding the roses. This can prevent airflow and provide overwintering opportunities.
• Water splash. Water the base of your plants as much as possible.
• Long dry summers. Unlike many fungi spores, powdery mildew doesn’t require a set amount of time of wetness on the foliage. This is why you see powdery mildew in dry summers, especially if the rose is dehydrated or struggling in the heat for any reason.
• Combined with the conditions above, powdery mildew prefers temperatures around 60-80ºF during the day, and cool nights.

How to Prevent Powdery Mildew on Roses

Prevention is better than the cure for powdery mildew (and most diseases), so you want to do as much as you can to avoid your roses getting it. After all, we grow roses for their beauty and anything that ruins that is bad news.

• Don’t water from above, soaking the foliage. Water the base of the plant.
• Make sure there is good airflow
• Good drainage for your roses is important. During hot, dry times, make sure you water frequently so that the rose doesn’t become stressed and more susceptible to powdery mildew.
• Use a rose-specific fertilizer and don’t overfeed. You want strong growth, but if you give a rose too much nitrogen, the growth will be soft. Soft growth is more likely to succumb to rose powdery mildew.
• Prune roses in the dormant season for shape. Pruning during the growing season opens up cuts and wounds to infection.
• Roses planted in the open with good airflow resist powdery mildew better than those planted against walls or corners.
• Apply a good layer of mulch in dry periods. Retaining moisture is a great way to help the rose to resist powdery mildew.
• Use neem oil as a preventative during times when the conditions are just right for the disease to develop.

There are disease-resistant cultivars too numerous to list, so the best thing to do is to ask your local rose plant seller. Many heirloom varieties are resistant to powdery mildew and a lot of other pests and diseases.

Knock Out and Drift brands, as well as those bred by Griffith Buck, are all known to be powdery mildew resistant.

Management of Powdery Mildew

So, what do we do when we end up with rose powdery mildew? Don’t panic.

Non-Chemical Control

Although I’ve said not to water the foliage from overhead, there is a time and place. If you have a mild case of rose powdery mildew, you can overhead water in dry weather in mid-morning.

The spores don’t like direct contact with water, and doing this mid-morning gives plenty of time for the foliage to dry out. Don’t use this method if the roses are in low light or a shaded area and if the plant will remain wet for too long.

Remove infected leaves and dispose of the garbage. Be careful not to infect other rose bushes as you work, so always use clean clippers and clean them when you’re done using them.

Homemade Remedies

Luckily there are a few homemade remedies you can choose from. Experiment and test first to make sure you don’t damage the roses.

• Milk: Mix milk with water to about a 50/50 ratio. Spray over the infected areas. If the infection is severe, use straight milk. You can use a 1:5 mix of milk and water as a preventative, as well.
• Baking Soda: This has been considered a fungicide (amongst many other things) since the 1930s. In a gallon of water add one tablespoon of baking soda and a teaspoon of liquid dish soap. Mix it well and spray liberally.
• Vinegar: This is one method you must test first. Add three tablespoons of vinegar to a gallon of water and spray a tiny part of your plant to see how it reacts after a day. All good? Go to town.
• Mouthwash: This is one some people swear by, though who knows what the science says since studies haven’t been done. Mix one cup of mouthwash with three cups of water and spray away.

With any application, you have to spray the top and bottom of the foliage, as well as any other parts affected by the powdery mildew. Always spot test to make sure the ingredients you’ve chosen to use aren’t too harsh for the rose plant.

Biofungicides

There are many different microbes utilized by companies to deal with rose powdery mildew. Check with your local supplier if they have any. The beauty of using biofungicides is they don’t leave toxic residue and are generally considered organic.

Bacillus subtilisa beneficial bacteria, is the most common and most effective. Products like Cease contain this useful ingredient and work well against this disease.

Chemical Control of Rose Powdery Mildew

A benefit of the nature of rose powdery mildew is it is a surface disease, so it’s hit hard by chemical control.

There are many types of fungicides available. Just make sure you are comfortable using the chemicals they contain. Naturally-derived chemicals like Geraniol work just as well as those synthetics out there.

Products that contain copper fungicide, neem oil, and sulfur also work well.