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Answer

Roses commonly drop leaves when they become infected with black spot. Black spot is a fungus (Diplocarpon rosae) which infects rose leaves causing black spots sometimes with yellow rings around the spots. Infected leaves will with drop off often completely defoliating the rose bush with the exception of a few new leaves on the tips of the canes.

The best defense against black spot is planting disease resistant varieties. The fungus will survive over winter on fallen leaves, raking leaves will provide some control. There are other cultural practices that can reduce the severity of black spot include avoiding irrigation that sprays the leaves, plant roses in a full-sun location with good air circulation, water in the morning rather than evenings. Fungicide sprays and systemic treatments are also affective.

Thanks for the rose question!
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