Question
I have several acers and just recently I noticed white marks on the trunks.  On further investigation the white marks were soft and had the consistency of soft marshmallow and what appeared to be a blackish/brownish scale in the centre.  I was told it was mealy bug but could not find any corresponding photos of this condition.  Could you please advise me on what it could be and how it should be dealt with without harming the trees.

Answer
Sounds like a scale insect. Scales attach themselves to the branches and will not move it touched--mealy bugs will move if touched. I would treat the tree with an insecticide called Bayer Advanced Tree and Shrub Inset Control. This is applied to the soil around the tree and the roots absorb the insecticide and transport it to the branches and when the scale sucks the plant juices it the killed. Check with your local nursery for this product. here is a web link to more information on this product.

http://www.bayeradvanced.com/product/Tree-Shrub-Insect-Control/concentrate.html