Some roses are extremely susceptible; others less so.

Depends up your prevailing climate -- here in the PNW, perfect place for black spot.

Also depends upon where it's planted at your place. Best is *full* sun and lots of *freely* flowing air.

It amazes me when I see gardens thick with roses free of any BT. How is that done!

Do you use the cider vinegar/water spray after every rain and once a week or so? I use that on my vegetables and roses in the early morning about once a week. You also could spray with full strength peroxide once a week or if it is rainy, after a rain or twice a week. The Hydrogen peroxide prevents bacterial and fungal problems. Another solution is using 1 part milk to 7 parts water or 1 part milk to 1 part water. It is suppose to be a general fungicide. I use this on tomatoe plants. Using bleach diluted with water is also something to clean an entire plant, even its roots. Some are keen on measurements, but I just figure about 1 part bleach to ten parts water. I've tried using baking soda and water with no success. You might also remove the soil under and around the bush and replace it with some clean dirt. Silica sand is a medium that is used to propagate with cuttings. That's because fungus/spores have a hard time living in it. Playsand or beachsand will not work the same. If you use Silica sand, wet it down before you work with it. It is very dangerous to be around because it easily become airborn and breathed in will damage your lungs nonstop forever because it does not break down in the lungs. I worked with Silica sand back when I refinished some furniture and the stuff gets airborn too easily to be safe. You might even try covering the soil with some heavy paper (brown grocery bags) to catch leaves that fall and replace the paper often (if leaves are falling).

I know this sounds very drastic, but I don't know of any other way to get things under control once BT has moved in, but I would take the bush out and soak it in the bleach solution, clean the area up, put new soil down, and spray clean the surrounding area -- plants, soil and everything.

I cover my roses at nite because of the deer, but I think the sheets are also a shield against the high humidity and dew. The sheets give protection from damaging winds too.

Black Spot on Roses, which is what you do have, is a fungal disease that needs fairly specific temperature and humidity conditions for the spores to infect plants. The best defense against this of plants growing in a good, healthy soil well endowed with organic matter that is evenly moist but well drained. The spores need plant tissue to survive and do not last long in soil. To keep spores from becoming active dry off your roses folige as soon as possible after it gets wet, which is why growing in full sun is a requisite for roses.
Infected roses can be spayed with a solution of 1 teaspoon of Baking Soda in 1 quart of water, although others have somewhat different formulas, with a drop or 4 of some oil or soap (which aids the Baking Soda in spreading and sticking to the foliage, every 3 to 7 days.
Be sure all infected plant tissue is removed from the area around the roses.