That sounds like their soil is either too dry for a period of time or too soggy wet. Both conditions can kill roots and cause such symptoms. Also if you haven't fertilized them recently try applying a light dose of a complete product such as Miracle Gro. Finally I suggest you take a sample of the leaves with the black spots to your County Extension Office and see if they can identify if the problem is due to a fungus or bacteria and if necessary prescribe an appropriate remedy.
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