In resistant cultivars, or during dry weather, only small spots may form without yellowing and defoliation. Lesions on canes rarely kill branches, but are very important to the pathogen's survival over the winter. Infection of petioles, stipules, peduncles, fruit and sepals may result in inconspicuous, reddish-purple spots. Yellowing of leaves due to severe black spot. Black Spot Optimum conditions for infection and disease development are 75 F to 85 F, relative humidity above 85 percent, and six or more hours of leaf wetness.
Leaves are most vulnerable to infection while still expanding.
The black spot fungus survives on the host throughout the year as mycelia, in fallen leaves or in infected canes. During the growing season, spores produced on leaves and young canes are dispersed in rain water or dew and are disseminated primarily by splashing water. Infected fallen leaves blown by wind disperse the black spot fungus locally within the garden. Plant architecture influences disease development. Roses that have a more compact form or that develop leaves near the ground are more subject to infection.