Fungal spores which were formed on infected fallen leaves or on canes are splashed to the new lower leaves of roses in the spring by rain or sprinkler irrigation. The spores must remain wet for several hours in order for the spore to germinate and invade the leaf. If temperatures are warm following infection, symptoms may become visible in 3 days. New spores can form in as few as 10 days after infection. Thus under some conditions the fungus can infect a plant, begin producing new spores, and spread to other leaves within two weeks.
Spots are smaller with more distinct edges in other cultivars.
These spots may have a yellow halo surrounding them. Infected leaves yellow and fall. Canes - small purplish spots form on the current years growth. Avoid dense plantings. Maintain good weed control so that air circulation around the plants is good.