Roofing cement?! :msp_rolleyes:
"Wound sealer" has been mainly phased out, but not quite.
The old "branch sealer" i remember from the 70's was basically creosote -- and if you got some on the cambium, it would die back further from the wound. It was probably phased out for several reasons: plant
and animal toxicity, and lack of long term activity.
Any "sealer" dries out and cracks, at which time it actually provides a better spot for fungi to grow compared to bare wood; at that point, the petroleum-based sealers are no different from latex paint. Under the cracked sealer or paint, mycelia grow from spores in a protected environment. Latex paint is a non-toxic alternative, but you would need to reapply every year; you often see it used in orchards on large cuts.
Also, a fungicide solution first applied to the exposed wood could help; it could also simply be applied alone. One is used on fresh cut live stumps to control infection by annosus root and butt rot (Heterobasidon annosum) when thinning mixed conifer stands, especially on grand fir stumps. The chemistry of the fresh wood no longer allows mycelia to grow from spores that land on the stump.
I looked that treatment up:
Heterobasidion annosum - Wikipedia, the free encyclopedia
they don't say what it is, but I believe it is a borax solution.
Yup
Borax Stump Treatment for Control of Annosus Root Disease in the Eastside Pine Type Forests of Northeastern California