not another one💀

I have no idea what's going on this year, i had those sarrs since last year's Winter, and they did well, last spring and Summer, this summer i lost a purpurea and 2 small hybrids, and now this!
Should i unpot this plant and remove the dying Apex?and how do i stop the infection?i tried to save the previous ones but they still die, not even fungicides seem to do anything🫠.
Should i do something to the other plants as well?