Red spot (Measles) of Peony
Pathogen: Cladosporium paeoniae
Symptoms: Initial symptoms typically develop after bloom. Small reddish circular spots form on the foliage, lesions may coalesce making blotchy patches. The upper surfaces of infected leaves become a dark purple color, while the lower surface is brown. Foliage that is infected before it is fully expanded may become slightly distorted. Infection of stems causes reddish streaks.
Spread: This fungus survives on dead plant debris and in soil. Spores are spread by rain or wind.
Management: Remove and destroy infected plant material. Avoid overhead irrigation or carefully time irrigation such that the duration of leaf wetness is minimized.