Several of my two-dozen plants, Wheels from Swan comes to mind, are very fragile where the stem reaches the blossom. I can sometimes knock a blossom off by bumping the vase.

My plants get afternoon sun. Trees shade them in the AM. It has been a rainy summer in N.VA, but I see no sign of fungus.

Anyone else have glass-necks on their blossoms?

In my particular case--and that's not going to be universal by any means--it was a shortage of phosphorus and boron in the soil. Cells didn't develop properly, leading to weaker, easily broken growth.

Assuming it's not some strange disease, a soil test might help. While adding phosphorus in modest amounts is pretty harmless, you can't do that with boron.

and a rainy summer probably results in lanky plants, making them more fragile.

The other could be genetics, some cultivars have sturdier stems than others. Do all your plants have that fragile stem/ neck or only certain cultivars?

I grow my Dahlias on two plots, one with full sun, the other with some hours of shade. Some cultivars took the part shade not well, producing weak flower stems.

I planted those in full sun in the following season which helped.

About soil and fertilizer: K promotes healthy cells and probably good tissue. Proper soil testing service should include an explanation of the results and recommendation how to feed properly for your purpose, that is, Dahlias.

Good luck, bye, Lin