Blackspot – Appropriately named for the dark-colored spots that appear on leaves.  As the disease progresses the leaves turn yellow and fall off the plant.  Blackspot often appears in humid areas or later in the summer in area know for July and August thunder showers. 

Leaves exhibiting evidence of blackspot should be removed (remember to clean up any leaves that fall around the base of the plan).  You may need to cut back areas of the plant. 

Mildew – Powdery mildew appears as a whitish powder on the leaves and stems of the plant.  Left untreated it will spread along the canes.  Powdery mildew is common to areas with warm summer days but cool and damp nights. 

Mildew can be treated with a fungicide spray.  Thin out congested areas of the plant to increase air circulation.  Clean around the base of the affected plants as well.

Rust – Another aptly named disease, rust shows up as orange spots on the underside of leaves.  Rust can spread throughout a plant, defoliating the leaves and possibly killing the rose. 

Although rust can be treated by some sprays, not all will be effective.  Remove effected leaves and areas of the plant.  Thin canes as well to promote better air circulation within the plant.  Rust appears on plants in humid or damp areas so if you are watering with sprinklers you may want to check into other method that doesn’t leave the upper part of the bush wet.

Overall prevention
You may have noticed a pattern to help prevent many diseases common to roses.  Poor air circulation mixed with damp foliage affects both the onset and spread of disease.  If you are constantly fighting disease in your rose garden look at the spacing between the plants and see if there is any way to increase the air circulation in the area.  Sometimes fences or think plants bordering the garden can be opened up to promote more air movement.

Although you can’t do much about a damp climate you can check to see if you are watering the leaves instead of the roots.  Although roses like water they need soaking at the base not in the areas prone to disease.

Natural Disease Resistance
Many roses are more susceptible to certain types of disease.  The good news is that there area also many new roses which are naturally disease resistant.  Weeks Roses has several varieties that are not only beautiful, heavenly scented bloomers but they are also easy to care for because of their inherent ability to fight off these rose maladies.

Recent All-America Rose Selections (AARS) winners Julia Child, Strike it Rich, Memorial Day, Livin' Easy, Hot Cocoa and About Face are all examples of stunningly beautiful roses with a natural resistance to disease.

If you have one or more older roses that repeat the disease cycle each year you may want to consider some “shovel pruning” and replacing them with new, more disease resistant varieties such as Home Run.

(taken from Weeks Roses website)