Gardenia plants are not easy to grow, however, they are very beautiful. Rapid leaf fall, without a period of time where the plant was wilted or discolored, is due to a shock to the plant. There may have been a sudden change in the amount or type of light, or cold draughts.
The tips of leaves can turn brown due to moisture stress (usually too much), fertilizer deficiencies, or excess soluble salts. Straw colored spots or patches on the leaves is likely caused by watering with cold water.
I would remove the affected branches and carry on with the care requirements...and make sure tepid water is used.
Gardenia plants like average warmth (minimum 60 F in winter). For flower buds to form, a night temperature of 60 - 65 F is required. An even temperature and careful watering are needed to prevent bud drop.
Gardenias like bright light but not direct light. Keep the soil moist at all times but reduce the amount during the winter. Bottom watering with tepid water is best. The plant will soak up what is needed and the rest can be emptied from the drainage tray. Mist the leaves frequently.
Good luck
Barb
barbsblooms@shaw.ca