The fungal spores of diseases such as black spot can overwinter on fallen rose leaves right through winter, ready to re-infect your plants in spring as soon as they start into growth again. To improve your chances of keeping your rose bushes healthy, pick up all fallen leaves lying around the base of plants, and remove any leaves that still remain on the bushes. Rather than risk composting diseased leaves, put old rose foliage in your council green waste bin. Spread a fresh mulch of bark chips or leaf mould around the base of the plants to keep any remaining spores away from the roses.