That 'poop' is a fungus, well known among those of us who keep cherry or plum trees. It is known as Black Knot (Apiosporina morbosa) and looks a bit like cat poop on a stick. It is a nasty little pathogen causing tumor-like, charcoal growths on twigs and branches.

 

The first year that the fungus invades it appears as a thickened swelling on a branch. The second year it develops into a blackish growth surrounding the branch.  Eventually larger limbs and trunks may become infected. Cracked and oozing bark are other signs that the tree is badly affected. The infection stresses the entire tree, making it weaken, decline, and possibly die.

 

The fungus overwinters in the knots. About the time of bud break in the spring, the spores explode from the knots following a period of warm, wet weather, creating new infections.

 

Because of the long infection process and disease cycle, this disease is often overlooked by home gardeners. The leaves can mask the symptoms until firmly established infections are in place.  

 

All shoots and branches bearing knots should be pruned out during the winter. Cuts should be made at least 8-inches below the knot. Since the fungus continues to develop in pruned material, this must be destroyed or removed from the site regardless of the time of year the pruning takes place.

 

Trees can live a long time with this disease. Although there is no cure, the tree often has many good years ahead of it. Keeping growth pruned out and a healthy environment will help the tree cope with the disease as well as possible. Give your trees adequate water during droughts, and keep lawnmowers and weed whackers away from the trunk.

 
Often the branch or twig beyond the knot either fails to leaf out or wilts suddenly.

Once established, it is very difficult to manage the disease.

For more information on "Black Knot" fungus.