You might want to check with the Rose forum folk on this. Often, black spot is something that shows up when the soil is a bit out of balance. One thing that can help is a side dressing of wood ash or sulphate of potash. That tends to strengthen the leaves and protect against black spot. It can take several years of regular compost-adding to reduce the outbreaks of disease but it can definitely happen.

Around about this time it's useful to give a summer pruning to let some new growth come through for a repeat flowering. It may not be as full-on as the spring show - but it's worth putting down some blood and bone and a serving of a slow-release fertiliser to get that second flush.

Black spot is a bit like catching a cold - not everyone is susceptible at the same time (thank goodness!) so most of your other roses are probably safe - particularly if you have a spray programme running while they settle into being fully established and productive (which can take a number of years to happen.)

Black spot can be controlled by drenching (saturating both top- and bottom-sides of all leaves) with a milk mixture or a baking soda mixture. Either method will stop the spots from spreading but *neither* mixture will remove spots already present. Use 3 tablespoons of dried milk or 2 tablespoons baking soda (not washing soda) plus 1/2 teaspoon canola oil per gallon of water and spray thoroughly now, and again after each rain or weekly - whichever occurs first. Adding 1 tablespoon of garlic juice is optional but does help keep mosquitoes away from the area. In my personal use, the mini-roses seem to be better with the baking soda while the shrub roses recover fastest with the milk drench.

There have been experiments in using corn meal sprinkled at the base of plants to prevent the occurance of blackspot; empirical evidence seems to show effectiveness.