Question
Dear Mr. Gulliksen:

Last summer my husband and I transplanted three healthy holly bushes from an area in our yard to a new bed near our porch.  We planted them in soil and filled the bed with a chunk bark mulch.  We noticed last summer that the bushes became infested with black flies.  They would swarm everytime we came near the bushes.  Now, with spring just a week away, I noticed that the bushes are coated from top to bottom with a dark, black, powdery substance.  Do you have any idea what it could be?  I'm thinking it may be a mold but with the fly problem, maybe it's from fly droppings.  These are beautiful bushes and I hate to lose them.  Were we wrong in putting the bark around the bottom of the bushes?  Do you have any ideas?

Sincerely,
Lorrie Williams

Answer
Lorrie,
The problem with the hollies is scale. The little black flies that you saw last summer were the adults flying around, mating and laying their eggs. The dark black powdery substance that is covering the hollies is called sooty mold. Sooty mold is a mold that grows on the honeydew or excrement from the immature scales that are feeding on the hollies.
I would recommend spraying them this spring with horticultural oil to minimize the scale population. When you spray, make sure to completely coat the interior of the plants, since this is where the scale feed and their eggs can be found.
Thanks for the question.
Ed Gulliksen