Rosemary woe

Rosemary woe

When I lived and gardened with my dad in southern California, we grew a rosemary hedge that to this day is bigger, greener, and glossier than anything I've seen this side of Italy. Well, now I live and garden in Colorado and I have killed every single rosemary plant I've purchased. I keep them in their planters because our zone is too harsh for the plant (I'm in zone 5b)...but every single time, transplanted or not, let to dry out or not between waterings, talked to or not, fertilized or not, pinched on or not, they die. Always the same way: part of the plant starts to get pale and withered, then turns a dark charcoal grey and totally dries out. It seems to be some sort of fungus or other attack, but each plant's variables are so different I'm finding that hard to believe. I've had tiny little seedlings up to big trees do the same thing. My most recent plant was purchased back in mid-October, a lovely topiary tree I even decorated for Yule with teensy coloured glass balls, and just today I turned it to discover almost one whole side suddenly consumed in this fashion. Please help! I love rosemary to abandon, and so far none of the books I've read have offered me any assistance: no matter what is recommended I do, the plants die. The longest I've had a plant has been about four months before being suddenly consumed. I've had plants outdoors and in, so I'm loathe to think it's some sort of indoor parasite/fungus. I've tried pruning off the affected areas hoping that would help, but that just seems to accelerate the plant's demise. The plants never smell anything but wonderfully rosemary-y, even the crispy dead bits. The leaves are always affected first, and the stems will remain pliable until they are suddenly as crispy as the leaves. So. Totally baffled. Help?
Fox Red Curly Sedge

Fox Red Curly Sedge

From my terrace…

From my terrace…

Hi again!

Hi again!

Hibiscus identification?

Hibiscus identification?

My place

My place

Dahlias!

Dahlias!