I am sorry for this delayed reply to your gardening question as we are transitioning to a new Q&A system. I hope this reply is not too late to be of help.
Black spot of roses is caused by a fungus. It is a major problem on roses across the country. The best way to manage it is to plant resistant varieties. There are several fungicides registered for use on roses that will control black spot, but regular spraying is required to maintain control on susceptible varieties.
Another cause of black leaves is sooty mold. When aphids infest a rose bush, they excrete a sugary liquid known as honeydew. This falls and settles on the surface of leaves the below. The sooty mold is a fungus that grows on the honeydew. A strong spray of water will dislodge the aphids and can help remove the sooty mold too. Insecticidal soaps also help control aphids as do products containing neem extracts. Avoid mixing soap sprays too strong or using them in the heat of the day or they can burn the plants.
Black spot will not spread to your geraniums or other plant species.
Give your roser light frequent feedings using a soluble fertilizer such as Miracle Gro or Peter's Plant Food at the label rate. This will help it to regrow and recover from the disease and loss of foliage.
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