Not surprisingly, the disease black spot
(Diplocarpon rosae) produces black spots on the leaves of infected plants. These spots are surrounded with yellow. Eventually, leaves turn yellowish pink and fall off. In severe cases, this disease can defoliate a rose bush by midsummer.
Black spot prefers roses with light-colored blossoms; red roses are less susceptible. It thrives on moist environments and is most common in northeastern and southeastern states, and in some Midwestern states where summers are warm and humid. Black spot begins to appear when the air temperatures approach 65°F and rain is abundant or humidity high. Infection begins low to the ground. Young leaves, 6 to 14 days old, are the first to go.
To control this disease, prune and destroy all the affected leaves immediately and begin a weekly application of a flowable sulfur spray, continuing applications throughout the season.
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