Downy mildew can be a relentless disease, wreaking havoc and causing unappealing blemishes and damage to your beloved roses.
Finding downy mildew on your roses can be disheartening, but worry not- it can happen to even the most seasoned of rose gardeners.
In this article, we’ll learn how to identify downy mildew and give you the tips and tricks you need to eradicate downy mildew and prevent it from happening again.
The Symptoms Of Downy Mildew On Roses
- Purple/Reddish spots on the leaves and branches. These spots are irregularly shaped and sometimes will have a yellow border around the edges. Additionally, you may also notice fuzzy gray/white mildew growing on the underside of the leaves.
- Discolored and deformed leaves. Downy mildew can cause leaves to become misshapen and lose their color. Affected leaves may turn yellow, eventually causing the leaves to drop.
- As the disease progresses, infected rose plants will display reduced growth and an overall decline in health.
What Causes Downy Mildew?
Downy mildew is caused by a fungus-like organism called the Pseudoperonospora cubensis.
It spreads through moisture and thrives in cold, wet conditions.
The Difference Between Downy Mildew And Black Spot On Roses
Both downy mildew and black spot are two common (yet dreadful) rose diseases.
Look out for these critical distinctions when determining if you have downy mildew or not.
| Downy Mildew | Black Spot | |
| They Look Different | Downy mildew can be identifiable by its spots. They are reddish-purple in color with a yellow perimeter. These spots commonly appear near the veins of the roses plant’s leaves, but can also be seen on the branches. In some cases, you may find a gray fuzz developing on the underside of the leaves. | True to its name, black spot manifests itself as black splotches that primarily affect the leaves and branches of the plant. As time progresses, these black spots will increase in size, draining the leaves of their color and consequentially causing them to drop. |
| They Thrive In Different Conditions | Downy mildew thrives in cold, wet conditions; especially during the night when temperatures are at their lowest. | In contrast, black spots on rose leaves thrive in warm, wet conditions; especially in climates with high rainfall. |
Downy Mildew On Roses Vs Powdery Mildew On Roses
Downy mildew and powdery mildew are both types of rose diseases caused by different groups of fungi.
While they share some similarities, they are distinct in terms of their appearance, symptoms, and the fungi responsible for causing the diseases.
| Downy Mildew | Powdery Mildew | |
| Causative agent | Downy mildew on roses is caused by the fungus-like organism Peronospora sparsa. | Powdery mildew on roses is caused by various species of the Erysiphales group of fungi, including Sphaerotheca pannosa and Podosphaera pannosa. |
| Appearance | Downy mildew typically appears as angular, purplish-red or purple spots on the upper surface of rose leaves. On the lower leaf surface, you will find a characteristic downy, fuzzy growth, which consists of spores and sporangia of the pathogen. | Powdery mildew forms a powdery, white to grayish coating on the upper surface of rose leaves, stems, and flower buds. Unlike downy mildew, this growth is typically more superficial and does not involve the lower leaf surface. |
| Symptoms | Infected leaves often show yellowing and wilting around the affected areas, and in severe cases, defoliation may occur. The disease is more prevalent in cool, humid conditions. | Infected leaves may become distorted, curl, and show signs of yellowing or browning. The disease can occur in both humid and dry conditions. |
| Management | Cultural practices such as providing good air circulation around the plants, avoiding overhead watering, and removing infected plant material can help control the spread. Fungicides may be used preventively to manage downy mildew. | Reducing humidity around the plants and providing adequate spacing to allow air circulation can help prevent powdery mildew. Fungicides may also be applied preventively or curatively to manage the disease. |
Both downy mildew and powdery mildew can weaken the roses and reduce their overall health and vigor.
Early detection and proper management are essential to prevent severe damage.
The Best Treatments For Downy Mildew On Roses
Just because you’ve found downy mildew on your roses, does not mean your plants are doomed.
There are multiple steps you can take to eradicate the disease for good.
Downy Mildew Cultural Care
Keep in mind that downy mildew spreads via water, so it is imperative you avoid any unnecessary moisture by following some simple cultural care practices.
Water your roses as close to the base as possible, and do not water your plants overhead.
Additionally, please direct any automatic sprinklers away from your rose plants, as this will just haphazardly splash water all over your plants.
Whenever you can avoid getting the leaves wet, please do so.
The time of day you water your plants also has a huge impact on the likelihood of contracting downy mildew.
We highly recommend watering your roses in the morning.
This will give any excess moisture the chance to evaporate in the afternoon sun, reducing surface moisture.
You can also increase the air circulation in your plants by maintaining a regular pruning schedule and cutting down excess branches that cross over.
This allows air to pass through the roses, keeping downy mildew at bay.
Chemical Treatment For Downy Mildew
1. Fungicide spray containing copper is a potent substance used to treat cases of downy mildew. However, it should be worth noting that not every spray containing copper is certified as organic.
2. Neem oil is a powerful and natural fungicide that can be used to eradicate downy mildew.
To prepare the neem oil spray, mix 1-2 tablespoons of neem oil with a gallon of water and a few drops of liquid soap (to help the oil disperse in the water).
Spray this mixture onto the affected plants, making sure to cover both sides of the leaves. Apply the spray early in the morning or late in the evening to avoid burning the leaves in direct sunlight.
3. Bacillus subtilis is a fungicide that is safe for organic use. It is derived from chemicals originating from microorganisms.
4. Baking soda is another effective natural remedy for downy mildew. It creates an alkaline environment that inhibits the growth of the fungus.
Mix 1 tablespoon of baking soda, 1 teaspoon of vegetable oil, and a few drops of liquid soap with a gallon of water.
Shake the mixture well and spray it onto the affected plants. Repeat the treatment every 1-2 weeks, or after rainfall.
However, it must be mentioned that the use of chemical treatment or fungicide for downy mildew can affect the local (and oftentimes beneficial) insects that frequent your garden.
We strongly urge gardeners to weigh the pros and cons of every product and to follow the individual manufacturers’ instructions carefully.
How To Prevent Downy Mildew
Natural Prevention
Downy mildew can be prevented naturally by following two rules:
1. Increase Air Circulation:
- When you plant your roses, ensure that you are giving each individual plant ample room in order to grow without becoming overcrowded by the other plants.
- Once a year, perform a heavy pruning of your rose plants. Any branches that are dead or diseased should be removed in order to prevent the chance of infection. However, you should also cut away any branches that have grown clustered or are crossing over one another. This thick growth stifles evaporation and reduces airflow to the branches, increasing the risk of mildew or other disease.
Related: How To Prune Roses (+Techniques By Rose Type)
2. Reduce Excess Moisture:
- Water your rose plants as close to the base of the plant as possible. Please refrain from splashing water onto the foliage, as this only creates more unnecessary moisture and can pose a risk to your plant (downy mildew spreads via water).
Chemical Prevention
Many of the fungicide and chemical sprays that are used for treating an existing case of downy mildew can also double as preventative.
We recommend reading the instruction label on each spray and following directions carefully.
Improper use of chemicals/fungicides whether it is organic or not can have counterproductive effects on your rose plants.
Conclusion
By practicing the tips discussed in this article, you can strengthen your rose plants against downy mildew.
With a proactive approach, a thriving rose garden that remains resistant to mildew is not far away!
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