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A common rose disease is known as black spot ( Diplocarpon rosae ). The name is very appropriate, as this fungal disease forms black spots all over the foliage of rose bushes. If left unchecked, it can cause a rose bush to totally defoliate. Let’s look at what causes black spots on rose bush leaves and steps for treating black spot roses. What Causes Black Spots on Rose Bush Leaves? Many frustrated gardeners wonder, “What causes black spots on rose bush leaves?” Black spot and roses usually go hand in hand. In fact, many roses get a little black spot, which can even be tolerated to some degree without any harm to plants. However, heavy infections can seriously defoliate plants. Rose black spot is caused by fungus. Dark-brown to black leaf spots develop on the upper leaves, which eventually become yellow and drop. Black spot can be distinguished from other leaf spot diseases by its fringed edges and dark black color. Raised, reddish-purple spots may also appear on rose canes. Wa