Downy Mildew is a common rose fungus that will cause significant defoliation of your rose plants. This rose fungus is a rose bush disease, that affects roses throughout the U.S.
This disease is most prevalent in the Spring, in moist coastal regions of California, and the Pacific Northwest.
Once the plant is affected with Downy Mildew, young leaves will develop dark purplish to dark brown spots on them. The spots/blotches are defined by leaf veins on the upper surface of the leaf. The symptoms of Downy mildew sometimes look like burns.
This disease is often confused with the disease (Black spot).
Infected leaves may turn yellow, and will often drop off quickly once the infection appears.Young shoots may wilt and die.
You should collect and discard all of the infected leaves but (Do not compost them)
Downy mildew can also affect the stems of the roses.
This rose fungus requires high humidity (over 85%) and cool temperatures to get started. Spring time is the perfect weather for this rose bush disease to take hold of the rose.
The best mildew control, involves trying to avoid the problem before it starts! You can do this by checking your plants regularly to identify the problem early on.
If your climate is moist with high humidity, you might begin a spraying as a preventative in early spring. The organic spray is a mixture of Baking soda, water and horticultural oil (or dish soap). Be sure to spray the undersides of the leaves. Early in the day is the best time to spray.
(As a rule, I never spray in the heat of the day or late afternoon).
This is one of the more common diseases of rose plants for those rose growers that grow cutting roses under glass, but not so much for garden roses.
Here are some tips for good garden hygiene that could help you avoid problems.
Drip irrigation is the best way to water your roses. It puts the water right at the roots of the plants without the splashing that causes many problems for the rose bushes.