Most of my buzzy lizzies are dying with something that looks similar
to the dampng off disease you can get in young plants.

The plants are from several different sources and are spread around
the garden (which is a decent size front and back) and also in the
consevatory.

From fist signs it takes 1 to 2 weeks for the plants to rot back to
the ground.

Any idea what this is and how it may be treated or, just in case,
prevented next year?
About 150 out of around 300 plants are affected at present but the
disease seems to progress steadily everywhere.
I do not know if it is that significant but the white plants (so far)
have remianed unaffected - but htere are only about 20 white ones in
total.

Other bedding plants - french marigolds, alyssum, lobelia, begonias
petunias, are (so far) unaffected.

Any help greatly appreciated