Question
Sorry John,
I put the question in your rating. My follow up question is that when I told you my lemon bush was big, I forgot to tell you it's approximate volume which is 8ftx8ftx8ft. What is the best way to treat this large bush?

-----Question-----
Dear John,
I have a very large meyer lemon bush. The bush is very productive but I have had two problems in the past year. The first was an infestation of tiny white flys. I used a mild detergent solution and sprayed the bush to get rid of most of them. I have also had a black residue on the lemons that I suspect is a fungus. It only is on the surface and washes off easily. How can I treat the bush to get rid of this fungus?
-----Answer-----
Hi Tom,

Given the pairing of prior white fly infestation and the black residue, it strongly suggests black soot mold, a fungus that feeds on the honeydew deposited by insects such as whitefly, scale insects, and mealybugs.  

Examine the plant very carefully for scale insects.  Since you don't mention whitefly infestation, I am assuming you do not see any visible insects.  Scale insects can be very difficult to see.  They are small (5mm), circular to oval, flat insects that are mobile only in the juvenile stage.  Mealybugs are often seen at the crotch of the stem where the leaf petiole attaches.  They aggregate in clusters of white fuzzy fluff.  The juvenile scale insects and the mealybugs can be controlled by a 70% isopropyl alcohol spray that drenches both sides of the leaves.  Adult scale insects are more difficult to dislodge.  Take a soft rag saturated with insecticidal soap or isopropyl alcohol and rub gently.  The scales will eventually dislodge.  This treatment may need to be repeated at two (mealybug) to four (scale insect) week intervals.

If you don't see any insects, let me know.

Answer
Hi Tom,

Yes, the size of this tree will make a difference in how you treat for insects. I assume that this tree is outdoors given the dimensions.  First, though, you need to see if you have scale insect infestation. If confirmed, there are insecticidal soaps that can be sprayed on the tree to control insects. There are also contact pesticides for treatment. A common treatment is to apply a good quality horticultural oil to control scale infestations.  These oils should be used very, very carefully according to manufacturer's instructions as they are temperature sensitive and can cause damage to the tree if applied incorrectly.  Your local garden center will have more information about the application of either pesticides or dormant oils (Volck oil, horticultural oil, etc.).  These oils cover the scale insects, and the insects are smothered.  If the infestation is severe several applications over a 4 to 8 week period may be necessary.

Plant disease diagnosis is difficult long distance and without any visual clues, so we have to rule out the most common problems first before proceeding.

Good Luck and Good Gardening.