Question
We live in South Devonshire, England and have a lemon tree/plant in a pot.  The plant is about 2 ft tall and has produced lemons over the two past years.  The leaves are spotted in a colourless, gluey substance.  There is also a black sticky dust on the floor under the plant which cannot have come from anywhere else.   Everything else seems normal.  It has flowered and is now producing fruit.  Can you tell me what the stickiness and black dust are,please and how to remove it if that is necessary ?

thanks you

Answer
Hello Ronald,

Greetings.  What you see on your lemon tree is sooty mold which caused by a fungal infestation.  This particular fungus feeds on the sticky sweet "honeydew" which is produced by either scale insects or mealybugs. These are both very common indoor pests.  

Examine the underside of the leaves carefully.  If you see any small circular/ovular (5mm) "spots", these are mature scale insects which produce the honeydew on which the fungus feeds.  The juvenile scale insect is mobile and appears very small and fuzzy.  If you see small cotton-like tufts at the juncture of the leaf petiole and stem, you have an infestation of mealybug.

To treat juvenile scale insects and mealybugs, applications of insecticidal soap are effective.  The adult scale insects are very tenacious and can be removed with light pressure using a sponge saturated with insecticidal soap.

This treatment will also remove the black soot.  It should be noted that the soot mold does not affect the plant directly, but with large infestations, light can be excluded and photosynthesis reduced.

I am not familiar with what insecticidal soaps may be available to you in England, but you can inquire at your local garden centre.

Good Luck and Good Gardening!