Round black or dark brown spots and yellowing leaf tissue are symptoms of black spot, a fungal disease that also can cause premature leaf drop, poor flowering, and dark blisters on young canes. If left untreated, recurring infections can kill the plant. Black spot frequently occurs in places with high rainfall or high humidity in spring and summer. There are several ways to deal with the problem. Water your roses at the base of the plant, because overhead irrigation spreads the disease. Mulch roses to prevent soil from splashing up on the foliage. Prune affected stems from the plant. Remove diseased leaves from the stems. Rake diseased leaves from the ground in fall. Destroy infected debris rather than composting it. Spray with a fungicide to keep the disease in check. Plant resistant varieties.