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Freaky Fungus with an Affinity for Fruit

Closeup of a Prunus genus fungus growth on a tree branch
Akchamczuk/Getty Images/iStockphoto
Closeup of a fungus growth on a tree branch.

Have you ever made your way leisurely through the forest, admiring the balmy weather, the lush plants, the cute animals and the – black, invasion-of-the-body-snatchers-esque growths on the trees? No? Just me?

You’ve probably seen it before, even if you didn’t know exactly what you were looking at: some black, woody growth on cherry or plum trees. Black knot fungus, or Apiosporina morbosa, is a fungal agent that invades young trees of the Prunus genus, including most hard-pitted, fruit-bearing trees like cherries, plums, apricots, and peaches.

With a disease cycle lasting up to a few years, you might not even know this invader is there. Overwintering on branches of their latest victims, black knot will release spores in the spring which attach themselves onto their new host’s branches. Once they have made themselves at home, normally on twigs of the current or previous year’s growth, they begin stimulating cell genesis around the area of infection. It won’t be until the second year that small, lumpy growths can be seen. These quickly swell into dark, black, bulbous nodes later that season, often cutting off nutrients to the end of the twig, mixing fungal and host cells in a massive and frankly terrifying gall.

If you thought this was the end of your worries, don’t be fooled. While black knot, now done with its host, releases asexual spores to be carried by the wind and rain, the tree is left with a knotty husk of fungal tree cells that act as an open door for insects to make their way into the bark of the now-diseased tree. This cycle is one that hides just beneath the bark, invisible, and this creepy invader is not the only horror fruit-bearing trees and other woody plants face.

Our next invader crawls up from underground, creating finger-like growths that rise up as a symptom of black root rot. The dead man’s finger fungus deserves its name as it grows up from the dying roots of trees and produces pale, bluish protrusions from the ground, shooting spores from these “fingertips”! Its “fingers” will later turn black as a sheath-like film appears around them, giving them their dead, rotting look. This fungus loves cherry varieties and will move from root to root underground where it can live up to 10 years! Night of the Living Dead? More like Decade of the Living Dead!

Our next Prunus-lover might cause your tree’s bark to become sunken, yellow and…ooze? Although its name doesn’t do this creep justice, Pseudomonas syringae, or bacterial canker, is just as silent and deadly as our former fungi. Once infected with this bacterium, trees might form holes in leaves, flowers might blossom and then immediately die, or clear, snot-like ooze might start seeping from concave trunks. Whatever the symptoms, this fungus screams slime monster.

Fungus and disease give all of us the heebie-jeebies (and not a small bit of anger to fruit growers!), but, like the strange and wonderful world of horror, we also don’t want to look away! And the closer we get the more questions arise: how and why do these invaders pick their victims? Why the affinity for fruit? Are these trees more susceptible, or do hosts offer something that our invaders have evolved to exploit or enjoy? Unfortunately, I could not find the answers to these questions. But I can hypothesize!

Perhaps, like the alcon blue butterfly, who tricks her ant-hosts into raising her young until they are big enough to devour the colony, the fruit trees act as a helper to these diseases, forming galls that assist in their development without realizing (often too late) that they were the demise of their own twigs and branches, fruit and blossoms, trunk and roots.

It could simply be that the fungus thrives on a lack of biodiversity. In orchards, where hundreds of plum or cherry trees sit, ripe for the infestation, this fungus has evolved alongside human agriculture, taking advantage of our desire for fresh fruit and tidy rows of trees, learning which it had the best chance of infecting. We became an ally in its evolution, perhaps.

Or maybe it is something out of a horror movie! A sentient fungus who just so happens to love a good pitted fruit every now and again! Probably not, but we can fantasize. Unable to find the answer, I will have to settle on not knowing until further research can be done. I am simultaneously craving more and absolutely freaked out by what I already discovered. Exactly as I would hope a naturalist would be when encountering nature's horror show.

Today’s Field Note was written by Tyler Tennant in the Master Naturalist Course at the Montana Outdoor Science School. This is Allison De Jong for Field Notes, brought to you by the Montana Natural History Center, providing natural history education for schools and the public throughout Montana. To find out about upcoming events and programs at the Center, call 406.327.0405, or visit our website at

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