Flowers

Question

What could be the cause of black rings on leaves?

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Answer 1

Although some plants do this, naturally, it could be a fungal infection. Did this recently start, or has it been like this since the beginning?

As a preventative and treatment I recommend using a small amount of wettable sulfur, and dolomitic lime. These will help take care of the issue. If it does not come back on the new leaves, then the problem is solved. If it does, then it could be that the plant will naturally have these.