It is the natural cycle of the plant,Sheena,as it is a perennial,and so will grow back every year.,and this is what most of them do...there are some which are usually bought in pots in early Autumn,but are usually only for one season,for late colour...

You could try feeding them in a few weeks time with a tomato type fertiliser to encourage flowers. Then later on when they have their leaves proper make sure they get plenty of water and spray on a regular basis with a fungicide.

Thank you so much for your advice and information I will try what you suggest. I like Michaelmas Daises and would love to have a success with them.

You are welcome,Sheena..I hope you are successful...

After having a bit more time to ponder. I would also suggest that you keep a careful eye out for greenfly on the plants. the black sounds like Sooty mould which grows on greenfly droppings.

Even more pondering (and pond dipping too, but that is another story). there is a problem called Aster wilt. Never seen it myself so only going on the books now. The leaves on plants affected wither from the base of the stem upwards, turning yellow first then going black. The main diagnostic is to cut a wilting stem about halfway up, then slice it vertically. If you see a purple stain inside the stem then this MAY be what you have. Sadly no cure.

Wow - you all sound so knowledgable. What I have sounds like what was described as Aster Wilt. However it has happened to both pieces and they are in different parts of the garden. It is a sunny garden mostly all day but where one piece is it is inclined to be wet I don't think the other part of the garden is as wet.
As I said previously I will take on board all your comments and systematically go through them. I will let you know what my findings are.
I am so glad I found this website.
Thank you. Sheena