It sounds like your mini rose has powdery mildew a common infection on the darling but oh so picky mini roses. You are doing the right thing spraying a fungicide on the leaves but you are still going to see some leaf drop as the oldest leaves die off.  You need to spray as often as the instructions recommend on the label or about every 3 to 4 weeks. To avoid powdery mildew do not let the roses dry out, do not get the foliage wet (I know, impossible if it rains and they are planted out in the open) and try to give the roses lots of air circulation by keeping the space around them light and open. No crowding other plants nearby. You can pull off any infected leaves before they fall and do not compost the brown foliage. Mulch the soil at the base of your rose plants with marbles, corks or rocks to keep the mildew spores in the soil from splashing back up to the plant. A good mulch will cover the spores already in the soil. Finally, mini roses can be cut down to within a few inches from the ground, fertilize and then fresh new growth will emerge that is most likely going to be mildew free...…but still mildew prone.  Keep growing, Marianne Binetti