Horse chestnut

Why horse chestnut doesn't get along with mycorrhiza

In the «Key to the Fungi of Ukraine», M. Ya. Zerova points out that horse chestnut isn't a typical forest-forming tree of our latitudes. As an introduced species and mostly an urban park tree, it has no historically established ectomycorrhizal network with native forest macrofungi.

David Arora notes in «Mushrooms Demystified» that trees which don't form classic ectomycorrhiza often form endomycorrhiza (arbuscular mycorrhiza) with microscopic fungi that produce no large fruiting bodies above ground. That's why under a horse chestnut you'll never find:

- Porcini and brown birch boletes

- Chanterelles

- Amanitas and death caps

- Brittlegills and milk caps

Saprotrophs and parasites on chestnut wood

Since there's no mycorrhizal flora, the only mushrooms you can find in connection with horse chestnut are saprotrophs (which break down dead wood) and parasites (which attack living trees through wounds).

Roger Phillips describes several species that can colonize horse chestnut:

1. Dryad's saddle (Polyporus squamosus): City chestnuts often suffer from pruning cuts and frost cracks. Spores of dryad's saddle get into these wounds. In spring, large "plates" of this cucumber-scented mushroom appear on them.

2. Velvet shank (Flammulina velutipes): On old stumps of felled chestnuts in the winter months you can come across clusters of this honey-yellow edible mushroom.

3. Oyster mushroom (Pleurotus ostreatus): Occasionally colonizes weakened or fallen chestnut trunks in autumn.

For a forager, finding these mushrooms on a living chestnut is always a sign that the tree is sick, not a reason to go foraging — besides, picking mushrooms in city parks or near roads (where chestnuts usually grow) is strictly off-limits because they accumulate heavy metals.