Marylynne,
   There are a number of things that can cause leaf tips to turn black and the most common at this time of year is leaf scorch. When heat is excessive and water less available, it's the leaf tips that show the stress most often. The best treatment for this situation is deep, soaking watering and mulching to lessen moisture loss.
    If you feel what you are seeing is a disease issue and not water and heat related, I would recommend you take a cutting off one of the plants, place it in a plastic bag and take it down to your local, year-round garden center. When you take your sample, make sure you collect enough of the plant where they can see the damaged leaf and healthy looking part of the plant as well.  They will be most happy to assist you with the problem.
   Thank you for your question and I hope you have a great summer.