Roses are prone to several fungal diseases, black spot and powdery mildew being the most common. To help prevent your plants from getting these diseases, sprinkle them with sulfur powder in May and June. This will stop the fungus spores from germinating. Also make sure your plants have adequate air circulation around them. By periodically removing some of the inner branches, air will be able to pass through the plant, reducing the likelihood of fungal diseases.
Roses are also prone to insect damage.
They are definitely a favorite food of beetles. Insecticides may help somewhat, but you may need to spray your plants virtually every week. During years when the Japanese beetle population is large it is a loosing battle. Other insects that are attracted to roses are aphids, thrips, leafhopper, rose-slug and borers. During the cool spring and fall weather spray your roses with dormant oil.