I am sorry for this delayed reply to your gardening question. The spring rush has brought a deluge of questions and we are working hard to catch up!
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.
Thanks for the question. Best wishes for a wonderful gardening season. Please stop in again soon!
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