I have young mayday trees 11 ft tall that appear to have a black fungus growing on them and they look sickly what will happen if I trim them right down. Will this get rid of the fungus.
First we need to confirm that by "mayday trees" you mean a wild cherry known by its Latin name Prunus padus. It is also known as bird cherry and welcomed for its early copious spring flowering. Unfortunately the wild cherries are known to be susceptible to the black knot fungus. If you follow the link there is a picture there which will allow you to confirm that is what you see. This article, and many others discoverable with an internet search will show that this is a difficult problem to handle; by the time it is plainly visible it is troublesome to control. It is possible to control if the disease has not progressed too far, but frequently the solution is to remove the infected trees completely and start again with strict attention to preventive measures.
It is possible that there is a source of infection in your locality, in which case even if you start over it will require constant attention and prevention measures. Also make sure you read this previous related question.