We planted this Camellia more than ten years. The tree still in good shape. In recent 3~4 years, its leaves covered with black dusts which can be wiped off. Some leaves underside have white spots. Some flower buds didn't bloom recently. My house faces south. This Camellia is planted near the east fence in our backyard between house and garage. It is under sunlight in spring & summer, but in the shadow in fall & winter. The soil is shallow, most organic soil, and wetter. Our house is located near Queen Elizabeth Park. No air or dust pollution nearby. Other plant leaves are clean as usual.
Scale insects plus sooty mold fungus. The white things are pooping sugar on the leaves beneath which the black fungus grows on. There is more sugar than they can use in the plant sap they are sucking so they pass most of it.
