Rescuing Roses: Identifying and Preventing Black Spot Fungus
Diplocarpon rosae, known informally as Black Spot, is the sworn enemy of many rose enthusiasts, especially those in the South-east and other particularly humid climates.
Black Spot is a fungus that attacks roses, first by appearing on the leaves as rounded purple and/or black colored spots, with indistinct, fuzzy edges. Once it has established itself on the leaves it attacks the canes, where it may be black or reddish in color. Black Spot usually appears in the middle of summer.
Prevention is the first and best defense against this rose-killing disease. There are a variety of commercial fungicides available, but it is best to use these as a last resort. This is in part due to their toxicity, and in part due to the fact that overuse will actually produce a strain of Black Spot resistant to the chemical. If using a commercial fungicide, alternate between two different types to avoid creating resistant strains. A preferable option, and an inexpensive one, is using a solution of one tablespoon of baking soda to one gallon of water, with a few drops of dish detergent (or two tablespoons horticultural oil) added. The dish detergent (or oil) will help the mixture adhere to the leaves.
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