Pruning cherries and plums can lead to specific problems. They can be pruned, but it is best to do so at or soon after bud burst (i.e. spring, to early summer, so that the rising sap seals and heals the wounds you make). I don't now what your tree is suffering from, but a picture might help. There aren't many diseases or pests which match the description you give. Phil J

Would be useful if you could post a photograph of the tree showing the "white sticky marshmallow goo".

It sounds like the stuff excreted by scale insects, Guest. If you look closely at the 'goo' you'll probably see the scale insects amongst it--they're small, brownish shell-like things and can be about 1/8th. inch long or longer. You can blast them with the hose and scrub the little blighters off, or spray with a systemic insecticide, which can be used for this type of pest.
It might also be woolly aphid, and the same treatment applies. Hope this helps. Look on the net for some pictures of both and then you can decide what it is.....Annie