Here in California, one of the more common white rot fungi you will find on Interior Live Oak and Coast Live Oak, and on other oaks and hardwoods, is a rather unique one. Omphalotus olivascens, the jack o lantern mushroom, is named not only for its dark orange color and its habit of appearing from fall to mid winter, but for a stranger trait: its fruiting bodies are bioluminescent. I am Christopher Hodge, an ISA Board Certified Master Arborist, and this is a fungus worth knowing, both for the wonder of it and for what it can mean for a tree.
Yes, it really glows
Fair warning: the glow is actually fairly faint. You need a fresh fruiting body and a very dark space to see it, and it is best captured with a camera set to a long exposure. But it does indeed glow. As a bonus curiosity, the fruiting bodies can also be used to dye natural fiber, turning wool or silk purple when treated correctly.
Where it fruits, and why it matters for your tree
This mushroom sometimes appears to be fruiting from the ground when it is really coming up from the roots of the host tree, though more commonly it fruits directly on the wood. Anecdotally, it favors the base of trees that often have multiple stems, exactly where a tree needs strength to counter leverage forces. That combination can lead to major stem failures, although there has not been extensive research into the structural impact of this species. If you find it at the base of an oak with multiple stems, it is worth a closer look and, often, a tree risk assessment.
Frequently Asked Questions
Is the jack o lantern mushroom poisonous?
Yes. Omphalotus olivascens is poisonous. It is regularly mistaken for the edible golden chanterelle, and eating it causes severe stomach illness. Enjoy the glow, but never put it on a plate.
Does the jack o lantern mushroom really glow in the dark?
It does, but the glow is faint. You need a fresh fruiting body and a genuinely dark space to notice it, and a camera set to a long exposure captures it best. The light comes from the gills of the mushroom, so a young, fresh specimen glows more than a fading one.
Should I be concerned if it is growing on my oak?
It is worth attention. Omphalotus olivascens is a white rot fungus that decays wood, and it tends to fruit at the base of oaks with multiple stems, right where the tree needs strength to resist the leverage of those stems. It is anecdotally associated with major stem failures, so finding it there is a good reason to have the tree assessed. Call or text (530) 391-6100.