What should I do about black spots on my rose leaves?

Your roses are likely infected with black spot, which is caused by a fungus. Click here to read more about this fungal pathogen.

Black spot can be treated using a fungicide (anti-fungal) spray or an organic option, such as neem oil. Spray on the tops and bottoms of leaves and repeat every 7-14 days. These products can also be used preventatively to guard against infection before it breaks out by applying every 1-2 weeks during the growing season (spring-fall). For the best effect, you can get two products and alternate (e.g. apply one, wait a week, apply the other, wait a week, and repeat the cycle). 

To keep the disease from coming back, you can minimize dampness on the leaves, which can encourage fungal growth, by watering early in the day rather than in the evening and watering the soil directly rather than showering from overhead. You should also ensure that any infected material (fallen twigs and leaves) is removed, and that any garden tools such as pruners are properly cleaned and sterilized.