You have black spot. Almost all mini's will get it in humid weather. Go to Lowe's and buy Bayers, Desease Control for roses & shrubs. Nothing else. Follow directions and spray you're roses every two weeks. Any infected leaves will fall off, so don't get excited. The roses will start to put out new growth fairly quickly. Healthy roses are happy roses. Happy rose bushs put out a lot of blooms.

Thanks. Should I spray all of them or just the infected bush?

Spray all you're roses (Not Rugosa's). That way nothing can travel from bush to bush. You want to prevent black spot. Not fight with it once it is going crazy in you're garden.