Roses like full sun, good air circulation, and moist soil that drains well. Deviate from these and there will be trouble in the form of disease. Unfavorable weather can also create problems.
Black Spot is one of the most serious diseases on roses, and our continued humid weather with frequent thundershowers helps it to develop and spread.
Symptoms start as small black spots on the upper leaf surfaces which enlarge. The spots have fringed margins and can join together to form larger blotches. Areas around the spots turn yellow or the whole leaf can turn yellow. Leaves will start dropping prematurely and the whole plant can defoliate quickly. Eventually twigs will die back. Flower production is usually poor and there is a loss of vigor to the plant.
Try to intervene as soon as possible. Stop any overhead watering. When practical, pick and collect any spotted or dead leaves. Clean up any fallen leaves at the base of the plant. Begin spraying every 7 to 10 days with a fungicide to protect new growth. A good organic spray to use would be copper, such as Bonides' Liquid Copper Fungicide. Chemical choices would be Bonides' Fung-onil Multi-Purpose Fungicide (Daconil) or Bonides' Infuse Systemic Disease Control.
Like all problems with roses, it is better to prevent this disease than to try and control it. If your roses are prone to Black Spot, a season long preventative fungicidal spraying program would be beneficial. Starting from when the plant first leafs out in early spring, periodic spraying of a fungicide should be carried out. Call or come visit Doctor see to have any of your gardening questions answered!