I consider myself a competent gardener- with about 20 years of experience. I have grown dahlias for a long time now and am really discouraged by this year. I lost about half my plants to what I believe is a virus- they were either stunted to begin with and died- or shriveled up and turned yellow- after getting off to a strong start.

For the last 4 seasons I have grown dahlias in full sun each with adequate water and I believe adequate nutrients (bone meal added at planting and reg fertilizer there after)- on the south side of the house in raised beds (about 8 inches high). I've grown them in this same location now for the last 4 seasons- each season has brought more fatalities to my plantings- with this year being the worst! Here are my questions:

-Do I need to rotate planting them (maybe plant in this area every other year?)

-Is it the stock I got to begin with- which then affected other plantings? (Some of the tuberes were bought from a specialty grower in OR- those are the ones that had a 100% failure)

-Did the drought this summer- with its intense heat- weaken everything to the point of it getting sick? ..and maybe this was just a bad year for dahlias?

-Do I need to truck in some horse manure to amend this location's soil-is it a Nitrogen issue/deficiency?

Please help!

Thank you!