Flower petals may be distorted and red flecks may occur. Expanding leaves between 6 and 14 days of age are most susceptible to blackspot infection. Conidia, the infecting spores, must be continuously wet for at least seven hours for infection to occur. The fungus overwinters in infected canes and in fallen leaves. Protective fungicidal sprays and planting of resistant cultivars are the best means of blackspot control.
Powdery mildew is another widely distributed and serious disease of roses.
Young tissues are the most susceptible and the disease is typically diagnosed when white, powdery patches of fungal growth appear on young leaves. These leaves will often fold inward or become twisted and distorted. New stem growth and flowers can also be attacked. Although high humidity encourages disease development, powdery mildew will not develop well when leaves and other plant parts are wet. Conidia, the infecting spores, will not germinate in water.