The Diplocarpon rosae fungus causes black spot, one of the diseases that most commonly affects roses [Source: Watt]. Black spot begins as small brown or black pinhead-sized spots on leaves. The spots grow and the leaves turn yellow and fall off. The disease weakens the plant, making it produce fewer blooms and more susceptible to winter kill [source: Campbell].
Black spot spores germinate when it's around 65 degrees Fahrenheit (18.3 degrees Celsius). The disease spreads rapidly when the temperature reaches around 75 degrees Fahrenheit (23.9 degrees Celsius). Temperatures above 85 degrees Fahrenheit (29.4 degrees Celsius) will keep the disease from spreading [Source: Watt].
The best way to control black spot is to prevent it altogether. Here's how to prevent black spots on your roses.
- Choose resistant varieties of roses. Some varieties are more resistant to black spots than others.
- Grow your roses in a sunny spot, so the foliage will dry rapidly. Spores only grow if they've been wet for at least seven hours [Source: Watt].
- Leave room between your rose plants so the air circulates freely [source: Campbell].
Here's how to control black spots on your roses:
- Remove infected leaves as soon as possible, so the disease won't spread.
- Prune and discard any canes that are obviously infected.
- Avoid wetting the foliage.
- Rake and discard all fallen leaves. This is essential because the fungus can survive the winter on fallen leaves. It cannot survive in the soil.
- Spray your roses with fungicide regularly throughout the growing season. (Liquid sprays work better than dust formulations.) Don't wait until you see black spots on your roses. Preempt them by using fungicide before the problem begins [Source: MSUcares].
The following fungicides will help keep black spots at bay.
If you don't want to use chemical fungicides in your garden, here are some alternatives:
Because rose leaves are waxy in nature, adding a spreader to the spray will help give better coverage.