I have been growing dahlias for 4 years now and have not had a problem until this year. I started my plants indoors and then transplanted outdoors 1.5 months ago. My plants have been droopy, then the bottom leaves started turning brown and dying, and moving up the stem until only about the top 1/4 of the plant has green foliage. I have 25 plants and this has happened to almost every one. Most of my stems have all turned a dark purple color. The first bloom I had did the same thing, the underneath of the bloom wilted and dried out. We had clay before and they grew fine. This year we dug the clay out and replaced with a topsoil. I am wondering if maybe they are missing a nutrient? I did fertilize with a Miracle Gro liquid hooked onto the hose. I have checked for spider mites and no evidence. I have been watering the same as always, and everything else in the garden around them seem to be doing fine. Any suggestions? THANK YOU!