You can make a natural fungicide for rose bushes using simple ingredients such as baking soda and dish soap. Put the mixture in a clean plastic spray bottle for easy application. For larger rose gardens, apply the fungicide with a clean garden pressure sprayer.
Mix the fungicide
Mix 1 tablespoon of baking soda, 1 tablespoon of bleach-free dish soap and 1 quart of water. The baking soda lowers the pH on the surface of the leaves to control the fungus. The dish soap helps the solution to stick to the leaves and prevents aphids. While adding 1 tablespoon of horticultural or Neem oil to the mixture is sometimes recommended, overuse of these oils affects the populations of helpful insects.
Apply the fungicide
Spray the mixture on both sides of the leaves. Spray early in the morning so the water dries before sunset. Spray weekly during the rainy season and after any heavy rainfall.
Prevent future fungus
Remove any yellow or diseased leaves and dispose of them away from the bush. Remove any fallen leaves from the base of the plant. When watering roses, use a soaker hose or drip irrigation system to keep water off the leaves. Avoid watering late in the day so leaves are thoroughly dry before night.