For most diseases you can use a fungicide to solve or correct the issue, unless cutting back is the only solution. If you need to cut back your plants, clean your tool blade with rubbing alcohol to avoid the spreading of the disease.
Powdery Mildew is from cool nights, warm days and high humidity. The leaves will distort at mid-rib and are covered with a whitish gray mold.
Black Spot is a fungus brought on by rainy weather or improper watering. There are round dark spots on leaves while half of the leaf yellows or drops from bush.
Rust is present in the soil and may cause problems once the temperature reaches 55-65ºF. There are orange spots which turn black on the undersides of leaves, and new shoots will turn reddish and shrivel. Usually emerges due to a cold spring following a dry summer and a hard winter.
Canker is a brown and sunken area close to the base of the stem caused by insects, disease or mechanical damage. It can encircle the whole stem and cause it to fall off. Cut off and get rid of the wood.
Die-Back can be caused by frost damage, canker, waterlogging, mildew or black spot. Fertilize in spring if die-back is a problem and cut off affected shoot at a bud below the dead area.
Downy mildew is an airborne, cool season fungus that is always fatal if left untreated. There are violet-purple blotches on canes at temperatures below 45ºF. Cut away affected areas, disinfect pruners between cuts, use Daconil (Fertilome Liquid Fungicide) as a preventative spray, which is in Fertilome Rose Spray and Triple Action.
CULTURAL PROBLEMS
Slow starting roses are possibly caused by dry canes or roots. You should water heavily for the first 3 weeks and spray the canes daily. If the rose shows no new growth within 3 weeks, sweat the plant by placing wet sphagnum moss in the center of the crown and then cover the canes with burlap. Remove burlap when new buds appear.
Sucker growth on grafted roses is the rootstock trying to grow out and around the graft and the growth has no lower leaf buds. Wearing gloves, grasp firmly and snap or cut below ground level at the root.