In case you see dark spots on the leaves, and they start falling off, it might be a signal that your roses suffer from black spot. It is a fungal disease which is very damaging. These spots can be black or purple, and later they can have a yellow perimeter. If you ignore the problem, it will spread to other leaves, and they will fall off and die.  To prevent this disease, spray Biobizz Leaf Coat in advance on the plant and the topsoil.