Sci/Why: Read It and Weep: Fungal Guttation

by Jan Thornhill

Guttation on Fomitopsis pinicola bracket fungus

Young Red-Belted Polypore (Fomitopsis pinicola) with guttation drops

Some fungi are prone to exhibiting a curious phenomenon—they exude beads of moisture, called guttation. In several polypores, such as Fomitopsis pinicola, the liquid produced can look so much like tears that you'd swear the fungus was weeping. Or maybe sweating. Other species produce pigmented drops that can look like milk, or tar, or even blood.

Guttation is more well-known in some vascular plants. During the night, when the plant's transpiration system is shut down, pressure from excess moisture in the roots can force beads of sap out of special structures on leaf edges.

Guttation droplets on strawberry leaves (Noah Erhardt/Wikipedia)

In fungi, the guttation mechanism is not so well understood. In many species, however, it's so often observed, particularly during times of rapid growth when temperature and humidity are favourable, that these beads of liquid can be a reliable macroscopic characteristic. Hydnellum peckii, for instance, so frequently "bleeds" pigmented drops in its early stages of growth that it's been given gruesome nicknames, including "Bleeding Tooth Fungus" and "Devil's Tooth." Coincidentally, a 1965 study found a compound in the fruiting body of H. peckii that has anticoagulant properties similar to those of heparin, too much of which can make one bleed to death internally.

bleeding mushroom guttation lisa neighbour

Bleeding Tooth Fungus (Hydnellum peckii) produces red-pigmented
guttation droplets during periods of rapid growth. (Lisa Neighbour)

A couple of years ago, I came across a crop of Inonotus glomeratus on a maple log. I'd found this amazing polypore a few times, once right after it had showered itself, and everything else around it, with millions of sulphur-yellow spores. The one I'd found, though, was very young, and instead of spewing spores, it was weeping globules of "tar" in copious enough amounts that shiny black pools were accumulating on the forest floor. Unlike most guttation drops, which are watery, these exudations were thick and sticky and stained my finger and thumb a deep auburn brown. And kind of glued them together. Oddly, though this unusual guttation has been noted by others, there seems to be no mention of it in the literature. I. glomeratus is so unusual in so many ways, I ended up writing a whole post about it.

Inonotus glomeratus fungus dripping black tar guttation

Fast-growing Inonotus glomeratus produces tarry guttation.

yellow spores of polypore Inonotus glomeratus

This Inonotus glomeratus continued to drip its viscous black exudate
even after it began releasing its yellow spores.

holes made guttation Inonotus glomeratus

The guttation drops on this Inonotus glomeratus were so thick that the fungus grew
around them, producing a pitted appearance after rain washed them away.

Polypores and Hydnoids are not the only fungi to produce guttation. In moist conditions, young Suillus americanus stipes can be heavy with yellow-tinted drops. Guttation is also common enough in the uncommon Rhodotus palmatus that this characteristic is often included in descriptions.

Chicken Fat Suillus (Suillus americanus)

Wrinkled peach (Rhodotus palmatus)

Guttation can happen in incredibly small ways, too. During the Toronto Bioblitz a few years ago, we found some Lachnum subvirgineum that, despite what seemed like dry conditions, were covered in minute guttation droplets, as were most other Lachnum I've since come across.

Lachnum subvirgineum with guttation water droplets

The largest of these Lachnum subvirgineum was less than .5 mm. in

diameter, which makes the guttation droplets impressively small.

Another minute character is so characteristically bejewelled in guttation droplets, it's named for it: Pilobolus crystallinus, which is one of the "Cannon" or "Hat Thrower" fungi found on herbivore dung.

Dung-loving Pilobilus crystallinus, is named for its sparkling
guttation droplets. (See my post about this remarkable,
tiny fungus, also called Hat Thrower, or Cannon Fungus)

Though little is known about guttation in wild fruiting bodies of fungi, it's a common phenomenon of fungal mycelia and hyphae in the lab, and a number of studies have been done to determine what the exudates contain. Penicillin has been found in the guttation droplets produced by Penicillium species in similar concentrations to that found in the culture broth, while gliotoxin, which has immunosuppressive qualities, has been found in guttation droplets of Aspergillus fumigatus. Do these fungi use guttation droplets as reservoirs for metabolic byproducts, or do they simply use them for water storage?

Or have different species evolved to produce guttation droplets for different purposes? The edible bolete, Suillus bovinus, for instance, has been shown in the lab to reabsorb nutrients from its guttation droplets, while leaving behind less useful byproducts, such as oxalic acid. So perhaps guttation has evolved as an efficient method of expelling waste for some fungi.

Is that what's going on with Inonotus glomeratus? Is that viscous, black ooze just a collection of rejected metabolic byproducts? If anyone would like to analyze it and has the means, I have some dehydrated exudate that I'd love to send you!

slime mold Stemonitis flavogenita guttation drops

Even some slime moulds, like this immature Stemonitis
flavogenita, produce guttation droplets. (Ulrike Kullik)

Pink-pored Fomitopsis rosea are even prettier when

decorated with shimmering beads of moisture. I think the
pattern on rim was made by the "teeth" of a grazing slug.

young Punctularia strigosozonata bleeding

Tree Bacon (Punctularia strigosozonata) "bleeds" rust-tinted droplets.

Early nubbins of an unidentified polypore exude milky drops.

teardrop shaped indentations left by guttation on bracket fungus

This Red-belted Polypore (Fomitopsis pinicola) produced guttation
droplets for three months one summer. When it finally stopped,
trompe l'oeil teardrop-shaped indentations were left behind.