September 12, 2022
Good evening, friends,
This week’s mushroom is the ash-tree bolete (Boletinellus merulioides). We found this mushroom Saturday during Catskill Fungi’s Friends of Fungi retreat in Phoenicia, NY. Since most boletes are mycorrhizal and form an association with tree roots, we were hoping the recent rains would bring forward those spongy shrooms we’ve been so deprived of this summer. Alas, the one bolete we came across grows in a much more peculiar niche (as if mycorrhizal fungi wrapping around or entering plant roots to form a nutrient exchange isn’t already mind-blowing). We’re going to deviate from the standard template because, as you’ll see, you can’t talk about this mushroom without looking at all the different ecological factors involved. Buckle up folks, this is going to be a long one.
Before we begin our dive into the ash-tree bolete, it’d be helpful if we knew a little bit about ash trees – like how to identify them. In the NYC area, the two most common ash trees we’ll encounter are the white ash (Fraxinus americana) and the green ash (Fraxinus pennsylvanica). Notoriously tough to distinguish, I never really bothered trying to identify between the two and always just assumed I was looking at the more common white ash. Both trees have long vertical ridges in their bark that forms a sort of diamond-like pattern.
Both species have pinnately compound leaves – essentially one long leaf that is comprised of five to nine leaflets. I believe the specimen below is a white ash, but I’ve flipped back and forth on the ID so if you know better please enlighten me. The length of the petioles is another identifying factor, but we’ll just call it an ash for now and if you’re really interested in distinguishing between the two species, check out this site.
All sixteen species of ash trees in North America and Europe are under heavy siege from an invasive insect, the Emerald Ash Borer (Agrilus planipennis) – but none moreso than the two aforementioned species. The beetle is native to Eastern Asia and in 2002 it was first seen in North America (found in Detroit for all my Michigan readers). It lays its eggs in the crevices of ash tree bark and when the eggs hatch the larvae bore into the tree. The larvae begin to devour the tree’s phloem, cambium, and xylem – the three vital layers of cells that are responsible for tree growth and the transport of nutrients and water. The larvae serpentine their way through these cell layers, inflicting enough damage over generations to eventually kill the tree, and leave behind the hauntingly attractive trails seen below.
After having their fill, the larvae then turn into pupae before becoming adults. The whole process from egg to adult can take two years. In the second spring, the pupae molt into adults that eat their way out of the ash tree to mate and promptly repeat the process. They leave behind distinct D-shaped exit wounds. The exit holes in the photo below are a bit worn, as this tree has been dead for a few years, but hopefully you now have an understanding (if you didn’t already) of how this invasive species is threatening our local forests.
There aren’t many mature, healthy ash trees left – relative to where we were prior to 2002. It’s estimated that in North America we’ve already lost hundreds of millions of ash trees with many more trees doomed to this fate as the insect expands its range. The fact that ash trees are in danger also would suggest that the ash-tree bolete is in danger. However, as it tends to be in nature, the relationship is still complex and dependent on more than just two factors.
As previously mentioned, the ash-tray bolete is not mycorrhizal. The mushroom instead grows out of the honeydew produced by the leafcurl ash aphid (Meliarhizophagus fraxinifolii). As if the ash trees weren’t already dealing with enough pests above ground, these aphids pierce their mouths into the roots of the tree to suck the sugary sap. The sap within the roots, and throughout the tree, is pressurized and will shoot right into the aphid’s mouth without the insect having to draw it out. However, sometimes the pressure is so forceful that the sap will shoot all the way through the insect and out the other side. That aphid excrement, so politely named “honeydew”, is what the fungus feeds on.
This fungus isn’t the only one to form an affinity for honeydew. Devout readers of MM will remember Scorias Spongiosa also grows from the sugary substrate, and there are species of ants that protect aphids from predators to preserve that pipeline of precious poo. In a similar display of symbiosis, B. merulioides will actually wrap itself around the aphids, forming sclerotia (hyphal knots of nutrient storage), to protect the insects and secure a direct deposit of honeydew. This relationship was first described by Mark Brundrett and Bryce Kendrick (reference 4). The photo below, taken by Mark himself and sourced from reference 5, actually shows how the sclerotia encapsulates the aphids. What a world we live in.
The aphids cause the leaves on the outer most branches to curl/wilt in the second half of summer – so around now – which can be a tipoff if you’re searching for this mushroom. Regardless, the mushroom can be found as early as June in NYC but it still seems to prefer to fruit during August and September in both North America and Europe. The cap ranges from a drab olive to yellowish brown color which can be hard to see on the forest floor but sticks out like a sore thumb in grass.
The pore surface is yellow with distinct patterning and venation. It bruises brown/blue upon handling. We actually cooked one up since they are edible and found it to have a deep, earthy taste – some have described it to be like mud. The consensus seemed to be that it was pleasant when the mushroom was still hot, but I don’t see these becoming a highly sought-after edible anytime soon.
The species as a whole is considered vulnerable, as it relies on such a specific, threatened niche. The immense die-off of ash trees likely precipitates the domino effect that leads to a great reduction in the numbers of this fungal species. It’s still unknown if the aphid may be able to jump hosts to different trees. It’s likely this fungus that fruits so tremendously, and can produce 50+ mushrooms around one tree when conditions are right, will be seen less and less as we lose more mature ash trees. The quantity and frequency with which we see this mushroom will continue to be monitored by citizen scientists like us. Hopefully as measures to counteract the ash-borer ramp up (the Central Park Conservancy arborists treat the mature ash trees to protect them from the beetle), we’ll be able to preserve this incredible ecological system.
This was fun to write. Thank you for reading,
1) Kuo, M. (2015, May). Boletinellus merulioides. Retrieved from the MushroomExpert.Com Web site: http://www.mushroomexpert.com/boletinellus_merulioides.html