Fungus can take up residence on your plants during the growing months, but you can combat it effectively during the winter. Be sure to clear fallen leaves from plants that are particularly sensitive to fungus, such as roses and fruit trees. This will help prevent the fungal spores from spreading to new growth in the spring. After all leaves have fallen from the plant and you have pruned, spray a combination of copper fungicide and horticultural oil to lightly (but thoroughly) coat the branches. Once the mixture has dried, it will guard your plants against harmful fungus and other pathogens.