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I spent many years admiring the roses of other gardeners before I had the courage to create my own. I was always told that there were a lot of things going wrong, but my fears did not match reality. The biggest problem I face every year in my rose garden is powdery mildew.
This can sometimes be minor or cause serious problems depending on conditions and time of year.
If you want your garden to be pristine (think English rose garden), make sure powdery mildew doesn’t take hold. But it’s not the end of the world if that’s the case. Here’s how to treat.
Contents
What is powdery mildew on roses?
Powdery mildew in roses is caused by a fungus called pannosa podosphere. This is commonly called rose powdery mildew. The disease normally strikes roses in the summer and likes moist conditions when the weather is dry.
Powdery mildew on roses is spread by spores from infected plants. The fungus will overwinter on roses and will appear in summer when conditions are favorable and will infect all aerial parts of the rose bush.
Powdery mildew symptoms on roses
The first thing you normally see is a white powdery coating on the leaves. Powdery mildew can be both on the top and bottom of the foliage. It is easy to wipe off with your finger.
You can see the same growth on stems, petals, flower stalks, or calyxes. The calyxes are the individual green sepals that envelop the developing flower and eventually form necklaces under the formed flower.
Flower buds will probably not bloom or open if the plant is heavily infected. If the infection is severe, the leaves may curl and become deformed.
Some leaves will be covered in white powder, but may also be discolored and look purple, red, or yellow instead.
As powdery mildew ages, it will likely turn dull white and then brown. Severe infections cause foliage to drop.
Difference Between Powdery Mildew and Downy Mildew
It is important to know that there is a disease of the same name, but different, which many people confuse with powdery mildew, and which is called downy mildew. Before proceeding, we should examine the difference.
Mildew (Sparse Peronospora) is a water mold (or oomycete), while powdery mildew is what are known as bag fungi.
Rather than the distinctive white powder look, you’ll see red, brown, yellow, or purple spots on the tops of the leaves. Leaves may also fall off, even if they show no symptoms. The buds can also dry out.
The disease usually starts at the top of the plant rather than the bottom, as black spot does, although late blight can be confused with black spot because they look alike.
Downy mildew is more likely to cause plant death and is a serious and destructive disease of roses.
Life cycle of pink powdery mildew
Unfortunately, rose powdery mildew is not something that appears one season and disappears the next. It survives on the plant in various forms depending on the time of year.
The first thing to know is that powdery mildew needs living tissue to grow. It will not live on dead tissue or in the soil.
On roses, powdery mildew lives from season to season in the form of so-called vegetative strands of tiny buds or spheres. The disease survives on stems and leaves.
The spores are found in the powdery film that you see, called mycelium. If you have a powerful magnifying glass, take a look at the powder and you will be able to see the individual or groups of spores. They form chains on the upper or lower parts of the leaves.
The wind carries the spores from one plant to another. Splashing water can also throw spores into an area.
Unlike many other diseases, rose powdery mildew can germinate, travel and infect plants in the absence of water. It likes both dry and wet conditions.
If the water remains on the foliage for long periods of time, it stops the disease cycle of powdery mildew. This is why during rainy seasons you often see other water-loving diseases, but not powdery mildew.
Direct sunlight and temperatures above 95°F will likely kill the spores.
Conditions for powdery mildew on roses
Sometimes no matter what you do to prevent diseases like powdery mildew on your roses, it still happens. These are the conditions the mushrooms like, so do your best to disappoint them.
- Shade and low light conditions. Like many fungi, powdery mildew spores don’t like a lot of sunlight. The good news is that 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 air circulation and provide overwintering opportunities.
- water splash. Water the base of your plants as much as possible.
- Long, dry summers. Unlike many fungal spores, powdery mildew does not require a set amount of moisture on the foliage. This is why you see powdery mildew during dry summers, especially if the rose is dehydrated or struggling with heat for some reason.
- Combined with the above conditions, powdery mildew prefers temperatures around 60-80ºF during the day and cool nights.
How to Prevent Powdery Mildew on Roses
Prevention is better than cure for powdery mildew (and most diseases), so you should do everything you can to keep your roses from catching it. After all, we grow roses for their beauty and anything that ruins is bad news.
- Do not water from above, drenching the foliage. Water the base of the plant.
- Make sure there is good air circulation
- Good drainage for your roses is important. During hot, dry periods, be sure to water frequently so the rose does not 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 sluggish. Soft growth is more likely to succumb to pink powdery mildew.
- Prune the roses during the dormant season to give them shape. Pruning during the growing season opens cuts and wounds to infection.
- Roses planted outdoors with good air circulation resist powdery mildew better than those planted against walls or corners.
- Apply a good layer of mulch during dry periods. Retaining moisture is a great way to help the rose resist powdery mildew.
- Use neem oil preventively during times when conditions are conducive to disease development.
There are too many disease-resistant cultivars to list, so the best thing to do is ask your local rose dealer. Many heirloom varieties are resistant to powdery mildew and many other pests and diseases.
The Knock Out and Drift brands, as well as those bred by Griffith Buck, are all known to be resistant to powdery mildew.
Powdery Mildew Management
So what do you do when you end up with rose powdery mildew? Don’t panic.
Non-chemical control
Although I said not to water the foliage from above, there is a time and a place. If you have a mild case of pink powdery mildew, you can water overhead in dry weather mid-morning.
The spores don’t like direct contact with water, and that mid-morning gives the foliage enough time to dry out. Do not use this method if the roses are in low light or in a shady area and the plant will stay wet for too long.
Remove infected leaves and dispose of trash. Be careful not to infect other roses while you are working, so always use clean clippers and clean them when you are finished using them.
home remedies
Fortunately, there are a few home remedies you can choose from. Experiment and test first to make sure you don’t damage the roses.
- Milk: Mix milk with water in a ratio of about 50/50. Spray on infected areas. If the infection is severe, use plain milk. You can also use a 1:5 mixture of milk and water as a preventive measure.
- Baking soda: It’s been considered a fungicide (among many other things) since the 1930s. In a gallon of water, add a tablespoon of baking soda and a teaspoon of liquid dish soap. Mix well and spray generously.
- The vinegar: This is a 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: It’s the one that some people swear by, but who knows what the science says since the studies haven’t been done. Mix one cup of mouthwash with three cups of water and spray.
Whatever the application, you must spray the top and bottom of the foliage, as well as all other parts affected by powdery mildew. Always do a spot test to make sure the ingredients you’ve chosen to use aren’t too harsh on the rose bush.
Biofungicides
There are many different microbes used by companies to control rose powdery mildew. Check with your local supplier if they have any. The beauty of using biofungicides is that they leave no toxic residues and are generally considered organic.
Bacillus subtilis, a beneficial bacteria, is the most common and effective. Products like Cease contain this helpful ingredient and work well against this condition.
Chemical control of pink powdery mildew
An advantage of the nature of rose powdery mildew is that it is a surface disease, so it is hard hit by chemical control.
There are many types of fungicides available. Just make sure you’re comfortable with the chemicals they contain. Naturally occurring chemicals like geraniol work just as well as synthetic ones.
Products containing copper fungicide, neem oil and sulfur also work well.
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