Good evening, friends,
This week’s mushroom is the Parasitic Bolete (Pseudoboletus parasiticus) which I found last Monday in Manitou. We’ve got a two for one special this week because this fungus is an example of mycoparasitism where one fungus is parasitizing another. We’re going to focus mostly on the bolete and why it grows out of the other fungus, the common earthball (Scleroderma citrinum).
There were dozens of earthballs popping on the preserve this past week - they don’t call em the common earthball for nothing - but I saw only two instances where the parasitic bolete was fruiting from an earthball. The only other time I’ve seen these parasitic boletes was at Mycofest in 2021 down in Chambersburg, PA. What an exciting find and a great opportunity to learn a little more (just a little, as you’ll see) about this peculiar fungal relationship.
Fun Facts and Ecology
Surprisingly, P. parasiticus is mycorrhizal - like other boletes, it forms a symbiotic nutrient exchange with tree roots. However, this species allegedly can’t source enough nutrients to reproduce (create a mushroom). The fungus then invades the mycelium of the common earthball, S. citrinum, before leaching the host of enough nutrients to create its own fruiting body (or in the specimen above, two fruiting bodies).
We know that genetically the two species are closely related - both in the Order Boletales - which may come as a surprise since they’re not remotely similar in appearance. That explains why they’re both mycorrhizal, though. P. parasiticus can be found anywhere S. citrinum can be found (eastern North America and Western Europe), and it typically fruits in mid to late summer - like now :)
It’s important to note that the parasitism occurs at the mycelial level and not on the fruiting body. Where I found the first specimen featured, I also saw that tiny earthball above with an even tinier bolete growing out of its base. Theoretically, all the earthballs produced from the Scleroderma mycelium in would be parasitized by P. parasiticus, and that was consistent with the two earthballs in the area. Also, notice how the bolete is growing from the earthball’s point of attachment with the mycelium, not from the actual mushroom itself.
“Theoretically” is the key word above because this relationship is another example of how in mycology we’re often left with more questions than answers. Why does this bolete parasitize another fungus’s mycelium if it’s already able to form a symbiotic relationship with trees? There is even thinking now that the bolete doesn’t harm the earthball at all (Reference 3). The bolete also might grow from the mycelium of a different Scleroderma species, not just S. citrinum. Everything I’ve written about the parasitic relationship could be wrong, but hopefully it’s been a compelling campaign of “misinformation” nonetheless. Sometimes you just tip your cap to Mother Nature and hope for firmer ground next Mushroom Monday.
I’ll be out in Long Island this coming weekend so I should have some cool mushrooms to share next week. The North American Mycological Association annual foray is also taking place in Asheville, NC so if you’re heading that way have fun as well. Same time next week,
Aubrey
References:
1) Kuo, M. (2020, October). Pseudoboletus parasiticus. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/pseudoboletus_parasiticus.html
2) Binder M, Hibbett DS. Molecular systematics and biological diversification of Boletales. Mycologia. 2006 Nov-Dec;98(6):971-81. doi: 10.3852/mycologia.98.6.971. PMID: 17486973.