Armillaria

Armillaria (common name: honey fungus) is a genus of parasitic fungi that attacks woody plants like trees and shrubs as well as softer stemmed herbaceous perennials. The fungus grows as a thin, white, sheet that wedges between the plant's inner tissues and bark. It spreads underground as it develops, destroying roots and rotting dying wood.

It grows black tendrils, often mistaken for roots, called rhizomorphs.