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. When left untreated the bush loses vigor due to a lack of adequate foliage to make carbohydrates. Thus you end up with a weaker bush with less blooms.
Your Home Depot garden center carries a number of products to combat black spot on roses. A garden center manager can direct you to an appropriate product that they carry.
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