Rose Bush Diseases & Black Spots

Black Spot

• Black spot is a fungal disease caused by Diplocarpon rosae spores. This disease causes the entire rose bush to defoliate by the fall. Defoliated rose bushes are at a greater risk for winter injury. In addition, gardeners may notice reddish-purple spots develop on stems. To control black spot, prune off all diseased canes. Rake up debris to prevent the fungal spores from continually infecting your rose bush. Apply a fungicide in the spring to prevent black spot from damaging your rose bush, advises University of Minnesota Extension.

Powdery Mildew

• Powdery mildew is one of the most widespread fungal disease, infecting numerous plant species, according to Colorado State University. Unlike other fungal diseases, powdery mildew's symptoms are worse during dry warm temperatures. However, high humidity is needed for this disease to germinate and spread. Gardeners may see infected rose bush leaves exhibit white powdery fungal spores on leaves. Prune off infected leaves and canes. Apply a fungicide to reduce powdery mildew damage on rose bushes.

Brown Canker

• Brown canker is caused by Cryptosporella umbrina fungal spores. This fungal disease attacks any part of the rose plant above the ground. Symptoms of brown canker include red-to-purplish spots appearing on stems, which later develop into white lesions. Most of the damage occurs on current year's stems. Use sterilized bypass shears when removing damaged portions of the rose bush. To sterilize bypass shears, spray them with a mixture of 70 percent denatured alcohol and 30 percent water.

Botrytis Blight

• Botrytis blight cause flower buds to droop, turn brown and decay. You may see gray-black lesions appear right under flower buds. Gardeners reduce the risk of botrytis blight infecting their plants by watering at the base of the rose bushes. Overhead watering provides enough moisture for fungal spores to germinate and spread. Cut off infected flower buds. Apply a fungicide if the disease has severely damaged your rose bush.