Conidia, the infecting spores, will not germinate in water. The fungus overwinters in rudimentary leaves in buds or in the inner bud scales. Fungicide sprays provide the best control. There are several leaf spot diseases of roses, including those caused by Alternaria, Cercospora, Colletotrichum, and Sphaceloma rosarum. 12 Spot anthracnose is caused by Sphaceloma rosarum.
Young spots are red, occasionally brown or purple, and occur on upper leaf surfaces.
The center of spots turn gray or white. The center of the spot often falls out, leaving a shot-hole appearance. Heavy infections of spot anthracnose can result in yellowing of infected leaflets and will cause defoliation. Conidia are formed in spring and continue to form on rainy days through the growing season. Spores are spread by splashing water.