Question
My camellia, jurys yellow, has developed a sooty black deposit on the upper side of some of the lower leaves and a few buds. Some of it will wipe off leaving me with a black finger and a shiny black leaf. Please can you help?It is a lovely plant,bushy, with masses of buds and I would hate to lose it. Thank you in anticipation.   Margaret

Answer
I suspect your camellia has some type of insect problem.  Several types of insects (Aphids, Whiteflies, Scales, Leafhoppers) feed on leaves and exude a sticky substance called honeydew.  The black deposits you are observing are the result of sooty mold, a fungus that feeds on the honeydew produced by the insects.  The mold itself will not kill a camellia, but it can cause stunting and reduce vigor since it interferes with photosynthesis.

Sooty mold does not generally penetrate the plant, so you may be able to wash the mold off or wipe it off the leaf surface.  To completely solve the problem, you need to solve the insect problem. Start by washing the leaves including the undersides to remove/dislodge insects.  You may need to treat with a horticultural oil or insecticidal soap to completely eliminate the insects.  You should be able to buy these products at your local garden supply store.