The first rule for controlling rose diseases is prevention. You prevent diseases by ensuring your rose has its Three Basic Needs — plenty of sun, plenty of air, and plenty of nutrients which include water, the proper fertilizer for your environment and soil conditions and deep, organically improved, well-drained soil. By providing these needs, you reduce the stress on your roses, their susceptibility to diseases, and ultimately your need for potentially toxic pesticides in the garden.
The next rule for controlling diseases is to select resistant cultivars. Although many hybridizers are beginning to produce less disease-prone roses, others are still more concerned about "show form" than their rose's ability to withstand the onslaught of fungi and viruses. I refer to these latter roses as "chemically addicted" to pesticides.
Various rose texts can offer suggestions for disease resistant cultivars, but much of this determination is a matter of experimentation on your part. Find out which roses do best in your garden. Here's where you local rose society and their consulting rosarians can help. And ask your neighbors or fellow "cyber-rosarians" which roses they prefer for disease resistance.
Despite all of your efforts, however, sometimes nature plays tricks on your garden and your roses just get sick. Identifying which disease they have will go a long way in determining the cure. To help in that regard, the Texas A & M University Agricultural Extension Service's Plant Answers database will definitely help. There, you will find that rose diseases generally fall into three groups — bacterial, viral and fungal.
Bacterial diseases (like crown gall) and viral diseases (like mosaic, rosette, and strawberry latent ringspot) are pretty rare for the average rosarian. Transmission of these diseases is usually spread via grafting irregularities or by the lack of clean horticultural practices when pruning. If these diseases have found a way into your garden, most likely they came in with the plant. And although some extraordinary measures can be taken to cure some of them, your best solution is to "shovel prune" the offender and try again.
Fungal diseases, on the other hand, are the most common for rosarians. They exist because environmental conditions favorable to their reproduction have appeared. The more common ones are powdery mildew, blackspot, downy mildew and rust. The less common diseases are botrytis, anthracnose, and canker.
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