Blackspot, "Diplocarpon rosae", is a nasty fungus that manifests itself on rose bushes as black spots on leaves progressing to black spots fringed with yellow rings on both sides of the leaves. As they develop the spots enlarge. Eventually, as the disease spreads, the entire leaves will go from green to yellow and then drop to the ground. With time the entire rose bush may become defoliated. Leaves less than two weeks old are the most susceptible to this disease.

Once you have discovered that your rose bushes are infected it is best to prune off the damaged parts of the plant and gather the diseased foliage. Dispose of this diseased material in bags or burn it. Do not add to the composter, as the fungus shall only return to haunt you when you recycle the soil back into the garden. It is vital to do an end of season cleanup so the spores will have no where to hide over winter.

After having removed the diseased parts from your rose bushes it is necessary to apply a preventative formula to minimize further attack. Using fungicidal soap or sulphur several times over the course of summer is one solution (especially after rain as these products tend to wash off). There are a few home remedies that have met with some success and are worth trying, especially for those that really do prefer organic garden methods. One is a solution made with baking soda: dissolve 1 teaspoon baking soda in a quart of water, add a few drops of liquid soap to the mix to help it cling better to the foliage, spray infected plants thoroughly. Another unusual remedy for fighting fungal diseases is manure tea. This formulation fights blackspot, as well as mildew and rust, while providing foliar nutrition. Place one gallon of well-composted manure in a 5-gallon bucket and fill with water. Stir the mixture well and let sit in a warm place for three days. Strain the mixture through a cheesecloth or mesh and use the resulting tea to spray disease affected plants (the solids left behind can be applied around the base of the plants as added fertilizer).