I have read in this forum that if a dahlia has a disease, it has to be distroyed. Then I read that dahlia viruses

(disease?) is spread by insects. Hmmm. I thought that most diseases are fungal, spread by contact,wind, water, or contaminated tools. Anyway, I've had a hell of a time this summer with sick hahlias. They are potted on my deck, in full sun, good soil, never neglected with water or food, and no, they are not over watered. Their leaves have a number of things going on. Fuzzy mold spots(mildew)which suprisingly responds minimally to a fungicide called Green Cure, mottled yellowing leaves which are limp and wilted, black or dark brown streaks through the yellowing, black streaks on the stems, and no sign of any insects. I use Bonide Orchard Sray that is usually terrific for insects as well as disease, (and sometimes I add Green cure in minimal amounts with the orchard spray) but to very little effect on these dahlias. I have cut the uck stuff away from them, although I'm losing ground and so not vigilant anymore. I want to know what it is they have, and if I can regenerate the plants by cutting them down to a short stem, prompting them to new growth....? I'm not new to dahlias, but this has me stumped.