This section is from the book "Commercial Gardening Vol3", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Not infrequently large, bracket-shaped fungi may be seen growing out of the trunk or branches. These fungi are more or less semicircular in outline, and vary from 4 to 8 in. across. The upper surface is brown and coarsely velvety, the under surface greenish and porous. The spawn of the fungus penetrates to the heartwood, which it reduces to powder; consequently the trunk becomes hollow. The fungus attacks all kinds of fruit trees, also many forest trees.
The fungus should be removed at once, otherwise the spores produced are apt to infect adjoining trees through wounds. Cover all wounds with gas tar. Certain other kinds of woody bracket fungi also grow on fruit trees, to their detriment. They should be dealt with as above.