Powdery mildew of gardenia primarily affects young leaves and shoots, and symptoms of infection typically include deformed leaves and buds, leaf yellowing, and leaf drop. Control: Properly space plants to allow for good air circulation which reduces the humidity level needed by the fungus to infect.
http://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2058.html
Iron Chlorosis: Gardenias are acid-loving plants that grow best in a soil with a pH of 5.0 to 6.0. Iron is an essential nutrient that plants use to produce the green pigment, chlorophyll. While iron is rarely deficient in soil, if the soil pH is above 7.0, the iron may be in a form that is not available to the plant.
http://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2058.html
If your Gardenia is established and growing big, and the temperature is hot, then you can water heavily. 5. When healthy, Gardenia needs a lot of water, and grows very fast into a fat shrub. The leaves, shiny like the surface of a mirror, glow green vigor and the waxy flowers radiate brilliant perfume.
http://forums2.gardenweb.com/discussions/1595946/gardenias-blooms-are-yellowing-and-dropping
Yellow leaves on your gardenia could be caused by a couple of different things. It sounds like you've got the water thing under control; gardenias should be kept evenly moist but not soggy or standing in water. Raise the humidity around your plant if you can. Next, I'd recommend fertilizing with a water-soluble fertilizer mixed at half strength every two weeks.
http://www.bhg.com/gardening/trees-shrubs-vines/shrubs/yellow-leaves-on-gardenia-bush/
Printer Friendly Version (PDF) With their wonderfully fragrant blossoms and lustrous, dark green leaves, gardenias (Gardenia angusta, previously known as G. jasminoides) are popular shrubs with many southern gardeners.
http://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2058.html
If you can do it every day, great. if no, then try as often as you can. Just the leaves, try not to wet the soil below much. If you've got a few inches of mulch under the plant, don't worry, the spray won't soak thru to the roots. Remember, we want those roots warm and dry when the sun goes down. If you can, compair the color of your leaves with some other object of the same shade of yellow. An old rag, shirt what ever. You can use that as your guide to compair the color change in the leaves.
https://answers.yahoo.com/question/index?qid=20060719135931AAvXM21
I have a gardenia in a pot that is outside in spring, summer, and fall and inside in a sunny window (floor to ceiling glass) for the winter. I find it is very easy. It is large now- have had it since it was about 10 tall. The blooms always yellow as they fade, so don't worry. Also, don't expect flowers in the summer.
http://forums2.gardenweb.com/discussions/1595946/gardenias-blooms-are-yellowing-and-dropping
Gardenia's blooms are yellowing and dropping... Hi all! I purchased a beautiful gardenia shrub from a farmer's market a couple of weeks ago -- 3 gallon pot. Immediately repotted into a very large (2-1/2 foot diameter) pot to go on the patio. Pot was filled with an organic, tried and true potting soil.
http://forums2.gardenweb.com/discussions/1595946/gardenias-blooms-are-yellowing-and-dropping
In addition to problems resulting from improper growing conditions, gardenias are also susceptible to several diseases, insect pests and other problems. For information on insects and related pests that attack gardenias, see HGIC 2059, Gardenia Insects & Related Pests.
http://www.clemson.edu/extension/hgic/pests/plant_pests/shrubs/hgic2058.html
The other 9% by too much fertilizer or soil ammendments (manifested as browning of leaf tips) and 1% by other causes (insects, pests, underwatering or not enough sunlight). Try to stop watering until the soil dries out and water only when the leaves become droopy (sign of dehydration).
http://forums.gardenweb.com/discussions/1913284/yellow-and-brown-leaves-on-my-august-beauty-gardenias
causes for gardenia leaves turning yellow
The cause for gardenia leaves turning yellow are the water thing under control gardenias should be kept evenly moist but not soggy or standing in water.