There are several different kinds of fungi that cause foliage discoloration on rosebushes. Although Knockout roses are considered a disease resistant bush, proper care and maintenance are required to ensure a healthy shrub.
Downy mildew causes black spots on rosebush leaves, as does the aptly named black spot disease. Diplocarpon rosae is the fungus responsible for these eyesores. Brown spots are caused by spot anthracnose, another fungal disease. Still other brown spots with yellow halos are caused by cankers. These diseases appear in early spring, with the exception of black spot, which occurs in late spring.
Foliage holes and tears are caused by a variety of pests from deer to aphids. Sawfly wasp larvae are called rose slugs and attack foliage, leaving small holes. They are found on the underside of leaves. Leafcutter bees leave holes on leaf edges. Moth and butterfly larvae leave tiny holes throughout the leaf surface.
Pests are removed from foliage through systemic insecticide sprays and soaps. Take care to spray the undersides of leaves as well as the tops. Some gardeners use soapy water. Caterpillars can be controlled by spraying Bt, a natural insecticide. Fungal infestations are preventable but not curable. Avoid watering your plant at night and overwatering your rosebush in general. Fungi thrive in wet environments.