Hi dandth,
A suggestion - either transplant (in the fall) or trash the sickly August Beauty and replace with another variety, like "Mystery" or "First Love". I have two HUGE August Beauties - both have produced so many blooms in the past two weeks that I've cut hundreds for inside the house and given away at least that many more and still pick off dozens of yellowed ones every morning and throw into the woods. I also have two "Mystery" bushes that are as tall as the AB's but much narrower due to their more upright habit. They just started blooming a few days ago and won't be nearly as prolific as the AB's, nor do they rebloom, but oh my, they are so much more fragrant. So, I've decided to transplant one of the AB's (if the root ball isn't too massive - if it is, it still goes), which will give the lilies and Phlox much-needed room to spread out next year and maybe a little space left over for something else fragrant. So, you might think about either transplanting or trashing the unhealthy AB and planting a Mystery or other type of larger-bloom gardenia in its place in the fall.