Question
I have a 6 foot meyer lemon tree that lives in a pot  it has black dust on top of the leaves and specs of bluish white on the bottom of the leaf. I also noticed the bottom leaves are turning yellowish.  What should I do??  

Answer
Hi Richard,

This is most likely a scale insect infestation.  The black dust is probably sooty mold which is a fungus that feeds on the sticky honeydew residue provided by the scale insects.  

The sooty mold is not harmful to the plant except in extreme cases.  Once you remove the cause (i.e. the scale insects), the sooty mold with go away.

Scale insects can be controlled by a number of horticultural oils, but be cautious to follow the instructions to the letter.  The oils can cause the plants to "burn" if not applied properly.

Another way to rid the plant of these insects is to manually scrub them away with a toothbrush and soapy water.  Depending on the size of your trees, that may be a very tedious undertaking.  Juvenile scale insects appear white and fluffy, and can be controlled by applications of rubbing alcohol.  Once you have treated the plants for scale insects, place them outside if you have any good place for them.  In a garden setting, the natural predators of the scale insects keep the populations in control.