Question
Hi Jim, I have a camellia hedge that needs help.  The leaves branches and trunk are covered with black mildew I have sprayed it with white oil and the mildew comes off if sprayed hard with water, but it comes back. Also its a hedge so many plants are affected.  Please can you help. Deb

Answer
This is called sooty mold.
Sucking insects are the primary cause of sooty mold growth. Many plant sap-sucking insects feed on leaves and stems of trees and shrubs. These sucking insects often produce excessive, watery excrement rich in sugars. This excrement is called honeydew. Excreted honeydew often falls on leaves or needles, branches, fruits or anything else immediately underneath the infested area of the plant. It is on this excretion that the sooty mold fungi grow. Sometimes plants not actually infested by insects may be affected if a tree above them is being attacked by a honeydew producing insect and the honeydew drops onto them.

I would guess the shrub  has a scale problem. Scales are insects that live under a shell like structure and suck the plant juices. They are difficult to control with regular insecticides since they are protected with the shell. I would use a systemic insecticide called Bayer Advances Tree and Shrub insect control. This is applied to the soil around the plant the the roots absorb the insecticide and transport it to the twigs and leaves and when the scale sucks the juices they are killed. When the scales are gone the sooty mold will not have anything to grow on and will go away.

Check with your local nursery for this insecticide.