How old are the blooms? The petals will drop off an old bloom just as you describe above, when they have aged beyond usefulness. I don't know of any illness or bug that would cause such a phenomenon, but I don't leave mine on the plant any longer than nearly ripe or ripe stage.
Could it just be age of the bloom?

Hi,

I dont think age is a problem as the oldest bloom isnt necessarily the one affected, but perfect blooms are. I am guessing that it happens when the bloom is about 10/14 days old (but I will start keeping a record of this).

I have four varieties of dahlias and the plants themselves are no more than 3/4 seasons old (tubers having been taken from older plants)and all are affected sporadically. It almost happens in front of my eyes. I have a beautiful cream dinner plate dahlia and it is heartbreaking to see one of the flowers end up as a pile of petals on the ground in the space of a few hours.

Thanks for you thoughts, much appreciated.

10-14 days is a long time to last on a plant. I have no other guesses other than age of bloom, but do commiserate with your pile o' petals falling before your eyes.

MIne drop off also, after 4 or 5 days. This is perfectly normal. If yours are lasting 10-14 days, that is amazing. I agree with poochella on this one.

I am now having the same dahlia problem. Perfectly young bloom, then all petals setting at the base of the plant the next morning. It has to be an animal or insect. Any ideas?