sounds like a fungus problem. spray with serenade, try to keep the leaves dry. one or two aspirin disolved in a gallon water and sprayed on might work.

Nothing will kill old fungus but you can stop new fungus from growing. the old black leaves will die. the new green ones can be kept healthy.

You can also try hydrogen peroxide or a stronger form which is Zero Tolerance.

All the above are very safe Organic solutions and are very safe for humans. They work against all forms of fungus and probably most or all bacteria etc. They are sterilizers.

Here is a link that might be useful: a note on serenade