Got Black leaves!any ideas

Hello,
I'm having a problem trying to work out what is going on in my garden. Normal garden books dont seem to help. My Jasmine plant has leaves which are going black from the inside stem outwards. Last year this also happened to my Astilbe, Clematis and Weigela. It didnt appear to affect new growth to start with but my Clemetis just went all shriveled. Its growing OK right now though. Anyone know why this is happening and what should I do about it?