Walnut

Juglone and the absence of mycorrhiza

The walnut's main peculiarity is allelopathy — releasing chemicals into the environment that suppress the growth of other plants and microorganisms. M. Ya. Zerova notes in her works that walnut leaves, roots and fruit husks contain a toxic substance — juglone.

David Arora confirms in «Mushrooms Demystified» that most classic ectomycorrhizal fungi (boletes, amanitas, brittlegills) can't survive in soil saturated with juglone. That's why you'll never find porcini, a brown birch bolete or a chanterelle under a walnut. The walnut doesn't form mycorrhiza with them and physically suppresses their mycelium.

Saprotrophic mushrooms on walnut wood

Although the ground under a walnut is barren for forest mushrooms, its wood (living or dead) can host a limited number of saprotrophic and parasitic fungi that have evolved to cope with its chemistry.

Roger Phillips gives examples of species able to break down hard walnut wood:

1. Dryad's saddle, also called pheasant's back (Cerioporus squamosus / Polyporus squamosus): This large, fleshy mushroom with its characteristic cucumber or watermelon smell often grows in spring on living but old or damaged walnut trunks. It's edible when young.

2. Oyster mushroom (Pleurotus ostreatus): On fallen trunks or thick dead walnut branches in autumn you can find the classic "bouquets" of oyster mushrooms.

3. Velvet shank (Flammulina velutipes): In mild winters these mushrooms often colonize old walnut stumps.

For a forager, mushrooms on a living walnut are always a bad sign for the tree itself. The appearance of dryad's saddle or honey fungus points to deep heart rot and the tree's rapid death.