Black Spot is a fungal disease that has characteristic black blotches surrounded by yellow patches. This is a disease mostly found in the spring and summer, but if you have been fighting black spot this year, now would be a good time to consider buying and replanting with resistant cultivars.
The fungus produces infectious spores throughout the year, causing repeated infections in warm, wet weather. Spores are readily spread by water.
PLANTS MOST AFFECTED
Small to large circular black spots on leaves. The spots have fringed margins and may run together to form large irregular spots. Severely affected leaves may turn yellow and drop. Under humid conditions, particularly susceptible roses can be almost entirely defoliated.
MEANS OF CONTROL
Plant resistant cultivars. Plant where shrubs can get good air circulation. Prune to maintain open foliage and avoid wetting leaves.
Remove and destroy infected leaves; spray with fungicial soap, sulfur or Neem Oil; repeat at 7-10 day intervals, or after every rain.
You can also try a home made solution of baking soda and water (1 teaspoon baking soda to 1 quart of water.)