It sounds like your gardenia has a couple of issues going on here. The first problem sounds like powdery midlew, which is a common problem, and is called by a number of types of fungi. To rid your plant of the mildew, thoroughly spray all of the above-ground parts of the plant, including upper and underside leaf surfaces, with fungicide which you can purchase at most greenhouses and garden shops. Ideally, you should start the spraying when the mildew is first seen, and usually sprays are required at 7- to 14-day intervals to keep young, susceptible growth adequately covered. Avoid as much as possible spreading spores that by splashing water when watering, and do not mist your plant. Mycelium and conidia of powdery fungi are waxy (or oily) --so the addition of a small amount of a household detergent or dishsoap (just 2-3 drops per quart) will make the spray more effective.
The second problems sounds like spider mites or aphids. Look closely for sign of insects--to identify which. In addition to sucking and chewing on leaf growth and buds, these insects can also help continue to spread the mildew, so getting rid of them will also be important.
After you ID which bugs, (spider mites are quite tiny and look like tiny spiders, black or red usually-- and aphids are kind of pear-shaped, yellow, brown, or white usually), use the suggested insecticide or insecticidal soap (again, check labels on products for indoor plants at garden center to determine). Some products may contain chemicals that can control both the mildew and insects. Normally I would suggest the repeated dunk/wash/rinse method to rid your plant, but with powdery mildew, your gardenia shouldn't sit around with damp or wet leaves.
GOod luck- gardenias are beautiful plants!
The second problems sounds like spider mites or aphids. Look closely for sign of insects--to identify which. In addition to sucking and chewing on leaf growth and buds, these insects can also help continue to spread the mildew, so getting rid of them will also be important.
After you ID which bugs, (spider mites are quite tiny and look like tiny spiders, black or red usually-- and aphids are kind of pear-shaped, yellow, brown, or white usually), use the suggested insecticide or insecticidal soap (again, check labels on products for indoor plants at garden center to determine). Some products may contain chemicals that can control both the mildew and insects. Normally I would suggest the repeated dunk/wash/rinse method to rid your plant, but with powdery mildew, your gardenia shouldn't sit around with damp or wet leaves.
GOod luck- gardenias are beautiful plants!