Article by contributing editor Jonathan Leger
The first thing to learn about rose plant diseases is that the faster they get diagnosed and treated, the better the outcome is likely to be. In order to keep your roses blooming and healthy for many years in the future, it is worth it to take the time to learn about the signs, symptoms and diseases of these plants and of course, how to treat them.
Black Fungus - There are a few different names for this disease. Sometimes it is called black spot, leaf blotch, or sooty mold. It first appears on the top of the surface of the leaves and it is fairly easy to identify because of the black splotches that it causes.
As the disease progresses, the spots will get larger. You will begin to see a yellow halo around the outside of the dark spots, too. In time, the entire leaf will turn yellow and drop off of the plant. In time, it can weaken, defoliate and eventually kill a rose bush.
Sadly, this disease is a global problem. Even after the plant has been treated, the impacted leaves won't lose their spots, but new foliage should grow in green and healthy once again.
To treat this disease, you should keep the area below your roses clean by removing any foliage or debris that has fallen. Be sure to mulch often. You can apply an organic fungicide such as sulfer on a weekly basis.
Powdery mildew - Some gardeners abbreviate the name to PM. This is another very widespread and serious rose plant disease. Like black spot, it is also caused by a fungus. In the case of powdery mildew, the first symptom is a white and powdery substance on both the top and bottom of the plant's leaves.
If this fungus is not treated, the bush will never do well and the leaves will become wrinkled. Eventually, the impacted leaves will die and drop off of the plant. Even after treatment, the affected leaves will not improve, however, the new leaves should grow in with a healthy appearance.
To treat powdery mildew, use one teaspoon of baking soda, one teaspoon of cooking oil into a spray bottle with one quart of water. Sulfur dusts will help to control this disease as well.
Downy mildew - This is one of the fastest and most destructive of all of the fungal diseases that roses are prone to. It first shows up on stems, blooms and petals. It looks dark purple, red-purple or brown.
This disease can kill a rose bush rather quickly if it isn't treated soon enough. In addition, more than one treatment might be needed to get control back. With downy mildew, the faster you can act, the better chance you have to save your rosebush.
Rose canker - This disease is sometimes called cankers. It starts out as a black, gray or brown patch on the stem of the bush. In some cases, it might be caused by severe cold or some other kind of damage that the bush has suffered.
One thing to be very careful of is that this disease can spread from one plant to another if tools aren't cleaned properly after they are used to prune a diseased bush. Gardeners suggest using a disinfectant wipe or having a bucket of water and bleach to use as a rinse. Let the tools dry before using them on another bush.
To treat canker disease, clean the area around and under the plant raked up and clean. Be sure to get rid of any infected parts of the plant right away. You should not prune or fertilize this plant until you have established control once again because the new growth will be susceptible to this disease. A copper-based fungicide will help to treat this disease.
Rust - As the name suggests, this disease actually looks like little spots of rust on the bottoms of leaves. As time passes, the rust will move to the tops of the leaves. This is another rose fungus.
To treat rose rust, be sure that you dispose of the infected leaves and spray regularly with a copper sulfate fungicide. Maintaining good airflow around the rose bush can prevent this disease.
Mosaic virus - This is not a fungal disease. It is a virus. Affected plants will not flower or produce healthy leaves. The leaves may become wrinkled or curled and could contain yellow spots or stripes. The only way to know for sure is to get the plant tested.
Once a plant is infected, there really is no cure for this disease. The best cure is prevention. You will need to discard the plant to keep the virus from spreading.
Some gardeners purchase rose plants that are considered disease resistant. These plants have been bred to be hardier. However, that does not mean that they are immune to all diseases. Any successful rose plant gardener will always keep an eye on these lovely plants to make sure they remain healthy.
Jonathan Leger is a member of the Garden Writer's Association and a gardening enthusiast. He runs a site dedicated to the history, education and care of Knockout roses. Visit his website here.