Question
Our 3 year old plum tree is showing signs of being infested with something.  The bark on about 3 branches is raised up, broken and looks like some insect may be bored into it.  One of the infested branches has no leaves.  I'm not sure what it is so I don't know how to stop it?  Should I cut a piece off the tree and take it to a nursery for advice?  I don't have an image available right now but I could send one when I'm next at home.

Answer
Sounds like a disease called black knot. Black knot can be managed by pruning out and destroying all galls infecting the tree. Prune trees 2-3 inches below the gall, preferably when the tree is dormant. Sanitation measures usually control the disease adequately, but fungicides such as lime sulfur or tribasic copper sulfate may aid in the control of black knot. Several applications need to be made beginning before bloom, and continuing until after fruit set. Fungicide use alone will NOT control the disease.

Since the knots are localized, the disease can be controlled by cutting off twigs and branches several inches below the last visible signs of the knot. This should be done during the dormant period (winter).

On large, main branches and trunks, knots should be cut out with a knife or chisel. One inch of healthy bark around the knot should be included in these cuts. Taper the cuts to a point at each end to promote healing.

Any nearby wild plum and cherry trees harboring the disease should be destroyed.

BURN ALL PRUNINGS OF KNOTTED PARTS BEFORE THE TREES BREAK DORMANCY IN THE SPRING. Spores can develop ad spread from knots left on the ground or in brush piles.

Chemical control procedures are as follows: ?Tablespoon Benlate plus 1 ?Tablespoons Captan per gallon of water. Apply the first spray at the early bloom stage (green cluster) and repeat at 7-10 day intervals until mid-June.  

Here is a web link to different stages of black knot to compare yours with. A nursery should be able to ID this disease easily. They should also have the fungicide.

 http://www.nysaes.cornell.edu/pp/extension/tfabp/blknstf.shtml