It is not unusual for a Camellia shrub to produce more flowers than it can open. Consequently, many buds fail to mature and fall to the grown. Other flowers that make it to full bloom are often brown or show signs of cold damage. However, buds damaged by cold weather usually only show browning around the petal edges.
Another cause of unsightly buds and blooms is far more serious. Camellia Petal Blight is a fungal disease that can move quickly. You’ll know a plant is infected by this if there is browning around the edge of the flower that quickly moves to the center (usually within 24 hours). The entire flower will become brown and leathery and will fall from the plant. A hallmark sign of this disease is dark brown veins within the lighter brown petal. It is most common when conditions have been mild during bloom and humidity has been high due to frequent rain showers.
Once a plant is infected with this disease it is difficult to control. The best bet is to remove and destroy all leaves, buds and petals from that have fallen to the ground. Be sure to follow up with a fresh layer of mulch, 2-3 inches thick. This helps create a barrier that will inhibit the spread of the spores that cause the fungus. The fungus that causes the petal blight persists in the soil so it is important that you repeat this process every year in early Spring.
As an added measure of control, a fungicide soil drench and foliar sprays can be used but only when in addition to the sanitation measures listed above. When applying the soil drench be sure to soak the entire fall line and beyond. This can be 10 feet or more in some cases. Start applying the drench several weeks before the buds begin to open, and repeat the process every three weeks until all buds have fallen. Fortunately the blight only attacks the flowers, so it is not necessary to treat beyond the removal of the last debris. However, you should repeat these control methods for five years to ensure the problem has been eliminated.