Black spots on plants are caused by a mixture of fungus and bacteria that can spread and kill a plant. Most commonly the fungal disease appears on the leaves of miniature roses, where the black spots turn yellow and the leaves fall off. The black spots can also spread and create purple spot on the stems of the flowers, causing the entire flower to die. Curing the black spots depends on how far the disease has spread on the leaf. However, it's still important to treat the leaf and entire flower to prevent the disease from spreading further.
Add 3 Tbsp cider vinegar and 1 gallon of water into a bucket and stir thoroughly. Pour the mixture into a spray bottle.
Spray the mixture onto the black spots. Make sure that you only spray the leaf and not other parts of the plant. Let the mixture sit for at least five minutes before continuing.
Wipe the entire leaf with a paper towel. Make sure that you wear latex gloves when doing this. Wipe two or three times and see if the black spot still remains. Apply the mixture again to the leaf if the black spot is still there, and then wipe the leaf again after five minutes.
Cut of the leaf with a sharp knife or gardening scissors if the black spot cannot be removed using the vinegar and water mixture. Dispose of the leaf and rake up any other fallen leaves.
Continue using the solution to remove any more black spots that appear and remove any of the diseased leaves if necessary. It's best to act immediately so that the disease doesn't spread throughout the plant.