I'm not an expert in the garden but I had a similar problem last summer with my sunflowers. I came to the conclusion it is a fungal problem in the root system. We don't get much sun in the summer and sometimes the ground just doesn't warm up and dry out. I got a powder called Actinovate and fed the plants with it and it really helped. It definitely helps with fungal issues. I just treated all my plants today even though I haven't noticed a problem. The powder adds beneficial microbes that help root systems. My local hydroponics store also had another brand of microbe that also helps roots out but don't remember the name. Maybe go into your local stores and ask them. I found nothing at Home Depot and more commercial like nurseries. These items were carried in more organic gardening type places. I hope this helps.

You might also try a tablespoon of Epson Salt at the base of each plant, watered in well. It is amazing how the Magnesium helps dahlia plants!

Good luck, and please update us on what you've tried, and it's result!

CC