Answer: Normally you would leave the foliage alone on the dahlia so the leaves can manufacture and store energy for next year's growth. But given its diseased condition, I'd recommend cutting all the affected plant parts off - to ground level if necessary. Then dig and move the tubers. I wouldn't dig the aster - it may fail to bloom if you do - but do cut off any affected plant parts. I think just removing the dahlia from the bed will open your aster up to better sunlight penetration and better air circulation, which may be all that it needs to grow and bloom. Plant your dahlia tubers in a site with lots of sunshine and good air circulation all around. This should keep next year's foliage problem free.
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