There are several type diseases that rosarians will need to be familiar with. Just like the symptoms of the common cold, these can be easily diagnosed, and most treated to maintain heathy plants. As a general rule diseases that affect roses are not unlike those that affect humans. They can also be classed usually into either a viral, fungal, or bacterial. Listed below are some examples of some types that I have seen while growing roses. Keep in mind this is not an exhaustive list, however, it will get you started in the right direction.
As you can see my experience is with more fungus type diseases than viral or bacterial. That is a good thing because as a general rule there is no cure for either of the listed viral or bacterial diseases. Since we can't do a lot with those let's talk about them first.
Mosaic is a viral disease that causes the leaves to become streaked or spotted with yellow or light green. These leaves may curl and the plants are sometimes stunted. This condition is usually seen in low humidity climates, usually along the West coast. Unfortunately there is no cure for this disease. If the condition is limited there are chances the plant will grow out of the condition. If the condition is severe, the plants should be destroyed. If not destroyed the plants could spread the disease to other plants next to them.
Crown Gall is a bacterial disease that appears usually at the base of the plant around the bud union, or on the trunk. It appears to be a corky substance that will turn brown and harden as it ages. Crown gall can often times be found also on root systems, however, these growths are usually caught by the grower and the plant is not allowed to be shipped. Like viral diseases, crown gall is un-treatable. The growth can be removed, however, chances are it will eventually return. Plants with this disease will usually be stunted in their growth habits. If the growth is removed care should be taken to sterilize equipment to prevent spreading to other plants.
Now that we have looked at a couple of diseases that we have little control over lets look into some we can control. Just like humans plants have cells that need water and nutrients. Some people say plants need love also. I usually see too many plants to walk around talking to all of them but if you want fantastic roses, I don't see where it would hurt. Back to the physical stuff. When plants are healthy it stands to reason they are less susceptible to contracting diseases. Just as we are less susceptible to catch a cold if our immune system is at its best . Roses need plenty of water, fertilize, and sunshine to make them healthy. That, in the nutshell is the key to great roses. A well balanced diet and spray program will eliminate most of the following diseases. Roses don't take a lot of time, they just take a little time more frequently than some other plants do.
Black Spot is a fungus that is perhaps one of the worst "thorns in the flesh" that we see here in the south. This disease appears as black spots on the leaves usually followed by a yellow halo around it. The spots will get larger and eventually begin to grow together. Eventually the leaf will fall off. This will continue until the plant is completely defoliated. This condition is more prevalent in areas where the humidity is high. Roses need a lot of water, however, they don't like it to stay on their leaves.
The best way to prevent and treat this disease is to conduct a regular spray program. Once the leaves are spotted they cannot be reversed. Infected leaves should be removed and discarded so nearby plants will not be infected. The disease can be stopped with a good rose fungicide. I recommend finding at least two different fungicides and spray the plants at 7 to 10 day intervals to eradicate the disease. Once under control the plants should be re treated at least every 14 days throughout the growing season.
Botrytis Blight will cause the leaves, stems, and new growth to turn black. The buds will cause the buds to turn black and not open. One way to identify botrytis noticing a gray fuzzy growth up and down the canes. This disease usually appears during rainy cool weather. It is also triggered if plants are watered too late in the evening and allowed to be wet during cool nights. The best way to control botrytis is by getting the plants dry. Remember, roses like plenty of water, just not on their leaves, and most especially not during cool nights.
Canker is a fungal disease that appears as a red, brown, or black spots that are sunken and elongated up and down the canes. This disease usually appears around early to mid spring. Cracks may appear in the spots and the spots may eventually enlarge to encircle the canes, killing them. This is a widespread disease, usually caused by mechanical injury to the canes. There is no chemical preventatives or controls for this disease. Simply remove the infected canes.
Downey Mildew is a nightmare in greenhouse operations. This because the very thing that is needed can not be provided. Plenty of air circulation and warm dry weather. This is another disease similar to black spot. At first, the plants will display the same black spots as symptoms. If conditions are right, I have seen a whole greenhouse loose its leaves in two days. Luckily the average gardener does not have to deal with this disease. The time to look for downey mildew is in the spring when the nights are cool and we are seeing damp weather. If infected spray with a good fungicide and prune infected canes to prevent spreading.
Powdery Mildew is a fugal disease that appears as a dusty white or gray powder that coats the leaves and canes. Powdery mildew will cause the leaves to curl and hinder the buds from opening properly. Powdery mildew is most commonly seen when the days are warm and the nights are cool. The best treatment for this disease is a good fungicide spray program. A dormant spray of lime-sulfur in early spring will help to prevent powdery mildew before the leaves appear. Irrigating early in the morning will help to prevent this disease also by allowing the plants to dry before nightfall. If the disease does occur, spray fungicides containing sulfur every 7 to 14 days and prune infected areas.
Rust is a fungal disease that causes the tops of the leaves to turn pale green or yellow while the underside will get spots of orange powder. Infected leaves will wilt or curl. This disease is most prevalent in cool humid climates. The best treatment again is a good fungicide spray program, while watering early in the day to prevent damp canes in the evening hours.
or spotted with yellow or light green. These leaves may curl and the plants are sometimes stunted. This condition is usually seen in low humidity climates, usually along the West coast. Unfortunately there is no cure for this disease. If the condition is limited there are chances the plant will grow out of the condition. If the condition is severe, the plants should be destroyed. If not destroyed the plants could spread the disease to other plants next to them.
can be removed, however, chances are it will eventually return. Plants with this disease will usually be stunted in their growth habits. If the growth is removed care should be taken to sterilize equipment to prevent spreading to other plants.
Eventually the leaf will fall off. This will continue until the plant is completely defoliated. This condition is more prevalent in areas where the humidity is high. Roses need a lot of water, however, they don't like it to stay on their leaves. 
s the tops of the leaves to turn pale green or yellow while the underside will get spots of orange powder. Infected leaves will wilt or curl. This disease is most prevalent in cool humid climates. The best treatment again is a good fungicide spray program, while watering early in the day to prevent damp canes in the evening hours.