by Dr. Sarah Treanor Bois, PhD
Director of Research & Conservation at the Linda Loring Nature Foundation
The zombie apocalypse has been on my mind a lot lately. That may sound dramatic, but if you’re someone who reads and watches fiction like The Last of Us, 28 Days Later, or the sequels: 28 Weeks Later and the new 28 Years Later, you know that the zombie hordes are all around us. For those who don’t know what I’m talking about, a lot of zombie or similarly-themed fiction base their monsters around the idea of humans being infected by something which takes over their brain function. And, like all of the scariest things, that fiction has some basis in fact.
While I don’t actually think there is going to be a zombie apocalypse, the ideas posed in these fictional zombie origin stories have a ring of truth. For example, in the popular HBO TV series T (which is based on a video game), the origin for the zombies (called “The Infected” in the show) came about by regular people being infected with a fungus. Once infected, the fungus controls the actions of the host person. In the beginning of the series, there is an episode which illustrates how this fungus, and the fall of civilization, came to be. The fungal infection is a real-world parasitic fungus that controls the behavior of insects. In the show, the fungus Ophiocordyceps unilateralis mutates to be able to affect people. Ultimately, or so the story goes, the fungus gets into globally-distributed boxes of cereal. Worldwide chaos ensues and, well, you can guess the rest.
So where am I going with this in a science and nature article? My love of zombie fiction and the natural world make it an easy connection in my mind.
The inspiration for the fungus in The Last of Us also was the inspiration for my favorite zombie book of all time, The Girl With All the Gifts by M.R. Carey. In both pieces of fiction, the Ophiocordyceps unilateralis fungus, also known as the “zombie-ant fungus,” evolves to infect humans. In reality, Ophiocordyceps infects carpenter ants. The fungus grows in the ant’s brain, producing metabolites to control the ant’s brain function. The ant’s movements are controlled causing it to leave the ant colony and climb upwards, typically in the nearest tree. The ant dies after death-gripping to a leaf and then the fungal spores protrude from the dead ant body.
The prevailing theory as to why this benefits the fungus is that the fungus needs the ant to get high enough to spread its spores. The fungus can’t travel that high up or that far alone. It needs the ant to ensure the survival of the fungal species.
Ophiocordyceps unilateralis is currently pan-tropical, but also occurs in temperate forests in South Carolina and Florida. We shouldn’t expect to find it here, but a quick look at iNaturalist found at least two records from mainland Massachusetts and one from Rhode Island. They are listed as the “Complex Ophiocordyceps unilateralis” because the correct ID is trying to ascertained. It’s not likely to be the tropical species. Maybe it’s only a matter of time before it ends up here by climate change, perhaps? We’ll be on the lookout for any suspect ant behavior —zombies—in our area.
Are there analogous species or examples for Nantucket? On a recent nature walk that I was guiding, we found a few examples of fungi that are having a similar effect as the zombie strains: they are slowly changing the tissues of the host species that they inhabit. There are other species in our region that lack the same dramatic brain take-over but are still biotrophic. A biotrophic fungus lives on (or in) the host plant without killing it, at least initially.
On Nantucket we have plenty of other examples of fungi altering their hosts for their own benefit. One of the most prevalent on-island is the black cherry knot. Go to any black cherry stand around the island—they are everywhere— many are easily found along Madaket Road. Black cherry knot is the name of what looks like black crusty crags on the sides of some branches. This hard black canker is actually the damage left after fungal infection.
Black knot is caused by the fungus Apiosporina morbosa. As the name implies, the fungus is species-specific, growing on members of the Prunus genus including black cherry (Prunus serotina), beach plum (Prunus maritima) and many ornamental cherries. It grows within the branch of an eastern black cherry tree for several months with no outward symptoms of disease. As the fungus grows, it releases chemicals that make the tree grow extra plant cells that are unusually large. This unusual growth results in the swollen, woody galls. Galls are made up of both plant and fungal tissue. One year after initial infection, galls can be seen as a swollen area of the branch with a velvety olive-green covering of fungal growth. Older black knot galls on trunks are often cracked and may ooze sticky liquid. Spores are released during wet periods in the spring. The wind carries these spores to trees where they infect young green shoots or wounded branches.
Black knot may not look great on cherry trees, but ultimately, they don’t kill a tree, they just continue to feed off the host. At the Linda Loring Nature Foundation (LLNF), we don’t intervene on black knot infections. But now that I’m thinking about zombie and fungal manipulation of plant tissue, I may look at them a little differently.
Walking along the trails at the Linda Loring Nature Foundation, another common fungus this spring has us seeing pink. Azalea leaf fungus (Exobasidium vaccini) is more common in cool, wet spring weather and humid environments. This spring has been typical for the Gray Lady, providing perfect conditions for this biotrophic fungus. The resulting fungus grows first on young, spring leaves. This developing leaf tissue is absconded by the fungus, which grows and then creates fruiting bodies out of the plant leaf tissue. At LLNF we see this almost exclusively on black huckleberry (Gaylussacia baccata)— one of the most common shrubs on the island. However, this fungus affects members of the Ericaceae plant family generally. On huckleberry, the fungus creates large flower-like structures that are pink and dusty white. It’s easy to mistake them for a flower, except that they come from the leaf and look nothing like a huckleberry flower. The white, powdery look are actually spores produced by the fungus.
But is it harming the plant? This fungus is not considered a serious threat to the health of the infected plant. There is some research showing that flower production is ultimately affected, but there isn’t a population-level effect. For now, we admire the fungus for it’s beautiful manipulation. These foggy, humid days will surely have us seeing more.
There are many other biotrophic fungi, like rusts and mildews, which are parasitic, feeding off of their hosts. Some kill the host plant, others just do minimal damage. On the helpful side, mycorrhizal fungi are a suite of fungi that are hugely beneficial to plants but also gain nutrients in return for their assistance with nutrient transport and potentially communication— a truly symbiotic relationship.
Hopefully we will continue to keep the zombies at bay. We can always raise the drawbridge if necessary.
What fungal relationships have you noticed around the island?
Share on iNaturalist or reach out and let us know at Stbois@llnf.org
