When sooty mold coats gardenia leaves in summer, it is usually due to an infestation of whiteflies. These sucking insects look like bits of cotton. While eating, they excrete excess moisture in the form of honeydew and this, in turn, supports the growth of a black fungus. Similar in habit are small gray aphids, which cling to leaf undersides. In sandy soil, nematodes can cause gardenias to be stunted or even die.
Nematodes are microscopic worms, which live in the soil and feed on plant roots.
Root rots caused by several different fungi can also be a problem, especially in poorly drained soils. Flower buds abort and drop off just before they open. Mystery' is the best-known selection. Radicans' grows to only 12 inches tall and spreads 2 to 3 feet, with small, dark green leaves and inch-wide double flowers in summer. Daisy' is a more cold hardy variety recommended for the Upstate of South Carolina.