Diseases Black Spot - Fungus will cause a rose bush to defoliate. Shows up as black spots that quickly turn leaves yellow and cause them to fall off. What do I do? Downey Mildew - Characterized by purple splotches on leaves and stems. Will cause bush to defoliate. What do I do? Powdery Mildew - Characterized by a white powdery substance that causes leaves to curl and growth to become deformed. What do I do? Rose Mosaic - Otherwise healthy leaves have a marbled effect of light and dark green. What do I do? (ARTICLE COMING SOON!) Rose Rosette - Deformed bright red canes covered in spongy thorns. What do I do? (ARTICLE COMING SOON!) Anthracnose - Characterized by dark sunken lesions in leaves that will eventually cause the bush to defoliate. What do I do?