Both gardenias and ixoras seem to attract pests in droves. Sooty mold, a black sooty covering on parts of the plant, is a fungus disease. It lives on honeydew secreted by scale insects, mealybugs, aphids and whiteflies. If left on the plant it interferes with photosynthesis, the process by which green plants trap energy from light and use it for chemical reactions such as the formation of sugar. So the cure lies in getting rid of the bugs that secrete the honeydew.
Mealybugs leave cottony masses in stem crotches or on the underside of leaves.
If you have lots of time and patience, you can remove the stuff with a cotton swab saturated with alcohol. Or you can spray with Malathion, Sevin or Diazanon. Aphids are small soft insects that cluster on stems, leaves, buds and flowers. They suck the sap, which deforms buds and discolors and curls foliage. Spray all parts of the plants in two treatments, two weeks apart, with Malathion or Diazanon.