Black Knot in my Bag of Tricks

Emily Stone


Once black knot kills some twigs, the additional sunlight fosters lichen growth. Photo by Emily Stone.
Once black knot kills some twigs, the additional sunlight fosters lichen growth. Photo by Emily Stone.

A good naturalist always has a variety of useful things tucked away in their back pocket or in their bag of tricks. For me, this often this takes the shape of a piece of scrap paper, folded into eighths, with a list of facts or an outline of my hopeful lesson plan for the day (“hopeful” because things never go exactly as planned). Sometimes I literally carry a bag of tricks. It’s a backpack full of preserved grouse feet, a bundle of chimney sweep extension rods for measuring bog depth, magnifying lenses, lengths of yarn, bug jars, and a pocket knife, among other seasonally appropriate items.

Most of the time, though, my bag of tricks is metaphorical. It’s a filing cabinet in my brain that is stuffed to bursting with a messy array of games, activities, facts, stories, jokes, and demonstrations that I can access through a technique that’s part Google search, part random association, and part luck. 

I’ve been adding items to this bag of tricks for a long time. My 12-year stint as both a camper and counselor at Girl Scout Camp provided endless games. The Outdoor Education program at Northland College filled me up with facts and activities. Teaching with creative people in the redwoods expanded my assemblage. Little by little I’ve gathered useful tidbits from every person and place I’ve met. Often people will ask me how I know certain things, or how I know so much about nature. My answer is that it’s been both my job and my hobby to collect facts and fill my bag of tricks to the brim. 

Unfortunately, many things I once tucked safely away in a corner of my brain are now buried in dust, and will never be accessed again without the help of some very specific reminder. Other things are so close to the top that I grab them on pretty much every walk. 
Recently, while leading a snowshoe hike on the Friday of Birkie Week, I realized that I hadn’t yet written a column about one of my favorite nature nuggets. It’s right here in my metaphorical back pocket, so let me tell you about it.
Especially in winter, shrubs and trees adorned with knobby, swollen, black growths stick out like a sore thumb. They were first pointed out to me by Craig Prudhomme, my instructor for a field semester at the Audubon Center of the Northwoods. Years of teaching college students had honed his sense of humor toward a combination of nerdy and gross. “It’s arboreal fox scat!” he declared, referring to a recent discussion of gray foxes and their tree-climbing skills. Sometimes I repeat his line, but I rarely have the right audience for it. More often I bring up another common nickname for these thumb-sized black growths: dead man’s fingers (always said in the spookiest voice I can muster.) 

Black knot fungus attacks trees and shrubs in the Prunus genus, which includes cherries and plums. Photo by Emily Stone.
Black knot fungus attacks trees and shrubs in the Prunus genus, which includes cherries and plums. Photo by Emily Stone.

While fun, neither of those names is even close to being correct. Gray fox scat doesn’t mold itself around twigs, and dead man’s finger more commonly refers to another black, finger-like fungus that occurs at the base of tree stumps in contact with the soil. “Black knot” is the accepted common name of this odd growth, and its scientific name is Apiosporina morbosa.

This common fungal pathogen is native across the U.S. and Canada, and only attacks plums and cherries. This fungus can help you identify plants! Chokecherry is a very common component of our local forest understory and edge, and it also happens to be one of the most susceptible species. This combination of factors means that I can find some black knot on almost any nature walk. 

The crusty, coal-black galls we notice represent a late stage of the infection. The first symptoms include small, tan, swollen galls on the cherry’s newest twigs. At this time, the fungus is growing inside the tree and causing excessive growth in the tree’s own cells. The fungus overwinters in the galls. The next spring, those galls look like olive green velvet before turning brown and then black. The velvet expels asexually produced spores called conida that fly off on a humid breeze to find a new host. They aren’t very effective at infecting new plants, however. I’ve never noticed the olive green stage, but I think that’s because its two-week-long appearance coincides with the peak of mosquito season in June. 

The older, blackened galls are made of both fungal and plant tissues, and will girdle and kill the twig or branch they are on. Even if the fungus doesn’t completely encircle a large branch or trunk, the cracks it causes in the bark allow access to different fungi that will have a go at killing the tree. The black galls also release ascospores that were produced sexually. These can infect wounded tissue, but they can also penetrate directly through the intact surface of an elongating shoot when growth begins in spring. 

Black knot isn’t a threat to the health of our forests, but if it happens to infect one of your favorite trees, then aggressive pruning of infected twigs during the winter is your best defense. I’ve noticed, though, that when black knot kills some twigs, the increased sunlight fosters a rainbow of lichen growth. Death is a part of life in nature, just as sure as black knot is part of my bag of tricks. 


Special Note: Emily’s book, Natural Connections: Exploring Northwoods Nature through Science and Your Senses is here! Order your copy at 
For 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Come visit us in Cable, WI! Our new exhibit: “Bee Amazed!” opens on May 1, 2018.