leaf problem on lady of shalott rose

Hi all! I have recent bought the above rose and been looking after it keenly. It's in a container which is east facing. I have filled the pot box with compost and sprayed with Rose Clear to keep pests at bay. However the last few weeks I have noticed a kind of white mould developing on some leaves. Despite spraying with rose clear it is still present. I have now noticed that some of the small rose buds are turning black. Does anyone have any suggestions as the other 2 varieties don't have this problem?