What is the brownish goop (pic)?

revt

This is a Japanese paste intended for sealing grafts but commonly used for sealing pruning wounds. The question of whether or not to seal wounds is a much debated one. Agricultural studies seem to support no benefit: you are as likely to seal in microbes as to seal them out. This particular brand of paste, which I sometimes use myself, also can seal a wound so tight (it becomes almost like latex) that the wound doesn't really heal over, as if the tree is unaware of the wound. Nevertheless, I will seal over wounds sometimes, for instance on maples, because it slows down or prevents the die-back that happens if you prune too close to a node. Sealants also have a use if you're preparing for a show; this one has a gray color that is nearly indistinguishable from some kinds of bark.