Spring is the recommended time for fertilizing both camellias and gardenias.  Fertilizing now could cause your plants to put out a flush of new growth that may not harden off before our first frost in the fall.  Regarding your gardenia, there are several possible causes for the partial demise you are seeing in your shrub.  Voles can tunnel under mulch and will gnaw the bark off branches that are under the mulch causing that section to die.  Another cause could be a root rot which is a soil borne fungus.  This organism slowly kills a plant one section at a time as it attacks those roots that support that area.  For now I would just try to minimize stress on your gardenia by insuring adequate moisture and proper mulching (keep mulch at least 6 inches away from stems and branches that make contact with the ground).

Gardenia | Home & Garden Information Center (clemson.edu)