Question
I have a plum tree that has numerous clumps of black stuff on the branches,  
I live in the lake sunapee region of nh.  the tree is about 8 years old and has
never had this before.  someone locally said that the tree is doomed, but i
wanted to ask you.

Answer
Black knot of plums and cherries is a widespread and serious disease throughout the United States. Black knot is a common disease in Ohio on wild plums and cherries and in home orchards where pruning and spraying are not regularly practiced.

The disease becomes progressively worse during each growing season and unless effective control measures are taken, it can stunt or kill the tree.

The fungus over-winters in the stem of the infected host and erupts during the spring. These early infections produce asexual spores (conidia) which may infect the host. As the knot darkens through the summer and the following winter, sexual spores (ascospores) are produced and it is these spores which cause most infections. If the knot has girdled the stem sufficiently to cause its death the infection will stop. Otherwise the knot will continue to expand and produce new spores in successive years.

Control

Black knot can be controlled using a combination of prevention and sanitation. 1) Remove all knots and swellings by pruning 3-4" below the knot during the dormant season before April 1. Where infections occur on larger branches which should be saved, cut the infection out down to the wood and 2-3" from its edge. 2) Burn, bury, or otherwise remove prunings from the area because they may still be an active source of spores. 3) Severely infected trees should be removed entirely. 4) Cut and remove wild hosts of the disease. 5) Use resistant varieties if disease pressure is high.  . 6) Preventative sprays may be necessary if nearby disease sources cannot be eliminated or when bringing a heavily infected tree back to health. A dormant spray of lime sulfur may be helpful when pruning heavily infected trees. Fungicides which have been effective against black knot include:  captan, copper hydroxide, thiophanate-methyl, lime-sulfur, and sulfur. Sprays should be applied at budbreak and every week to two weeks, especially before rain, until terminal growth stops.

It all depends on the extent of the knots throughout the branches and you pruning the knots and applying a fungicide spray program.
Here are a couple web links to  fact sheets on Black Knot:

http://plantclinic.cornell.edu/FactSheets/black_knot/blacknot.htm

http://ohioline.osu.edu/hyg-fact/3000/3011.html