Roses are among the more difficult flowering bushes to grow, primarily because they are vulnerable to a wide range of plant diseases. Brown spots on rose bush leaves are among the most common problems encountered by gardeners. Those who wish to garden organically don't have the wide arsenal of chemical sprays to turn to that solve all your rose problems in one spray but may have deleterious effects on the environment. Fortunately, organic methods of getting rid of brown spots on rose leaves are available.
Prevent brown spots before they become a problem by practicing careful watering and pruning. Brown spots on roses are nearly always the result of a fungus; hence, good moisture control is essential to keeping them at bay. Do not water the leaves of a rose bush, only the root area. Avoid allowing plants to become crowded, and don't plant roses in a high-humidity area of your garden, such as an area that receives less sun or where mold problems have occurred before. Prune rose bushes well to promote healthy growth and clean your pruning shears and any other garden implements that touch roses well, to prevent spreading mold from one plant to another.
Inspect plants to verify that the problem is, in fact, fungal. True brown spots always are; however, brown curled edges of leaves are a sign of scorching, and small yellowish spots can be a sign of insect infestation. Observe whether the spots are evenly distributed on the leaf or whether the browning is at the edges. If it is primarily at the edges, and the leaves are curled as well as discolored, excessive sun exposure is the culprit, and moving the plant to a shadier area after the last flowering is the best option. If the spots are yellowish, rather than brown, insects may be the cause. Look for bugs on the leaves to confirm. Organic insecticidal soap is the best bet for dealing with infestations.
Spray rose leaves with organic fungicide if a fungal problem has been established. If using baking soda instead of purchased spray, mix one part baking soda with 15 parts water, and place in a clean laundry spray bottle. Re-apply fungal spray once a week and after any rainfall. If the infestation is small, remove diseased leaves and discard them in the garbage (do not discard them outdoors, since this can lead to the fungus spreading).