Description and Injury
    Decay here refers to any soft, spongy, unsound, hollow, or open area in the bark of a tree that appears to be rotting away. This part of the tree is dead and the decay is caused by fungi that live on dead wood. A hollow trunk or one that is decaying on the inside is not always evident from the outside.

    Not all stain-producing fungi cause decay, but stain itself reduces the value of the wood.

    Any mechanical- or fire-caused damage to a live tree leaves an open wound, that may be invaded by wood-decay or wood-staining organisms.

Control
    Avoid any injury to black walnut trees that may allow the entrance of decay organisms. Prune trees properly and only during late fall and winter, when the trees are dormant and decay fungi are least prevalent.