Wednesday, November 25, 2009

Black Knot Fungus – Scourge of Adirondack Cherries

Nature is constantly at war with itself. Romantics tend to see nature as colorful sunsets, fox pups playing around their dens, and bluebirds feeding their young. People with a more utilitarian outlook see nature as either a source of food (deer, turkeys, blackberries), or something to be conquered at all costs (human needs come first). There is truth in all views, but not one of them is exclusively “correct.” Nature has its warm fuzzy moments, but in reality, it is, as the saying goes, “red in tooth and claw.” This holds true for plants as well as animals.

Sometimes I think it’d be kind of nice to come back as a tree. Trees can live a long time. They provide food and shelter for birds and other wildlife. They help pull pollutants from the air and make oxygen for us to breathe. But they are also food and shelter for insects and fungi and myriad other pathogens. And then there’s the weather: wind storms, ice storms, lightning – these can all take their toll.

Not far from my house there’s an area that was once cleared and is now rapidly returning to a cluttered, tree- and shrub-filled tangle of growth. Certain species dominate the growth, and in the shrub department it is blueberries, chokeberries, and choke cherries.

Choke cherries (Prunus virginiana) are a native shrub that can grow to 30 feet in height. Around here, however, every specimen I’ve seen has been shorter than I, courtesy of the local pruning service: Odocoileus virginiana, the white-tailed deer. The berries are full of antioxidants and are edible by people (best in jams and syrups, where you can counteract their astringency with a good dose of sugar). Keep in mind, though, that the plant is toxic to horses.

Choke cherries, like other cherries and plums, are susceptible to a native pathogen called Black Knot Fungus (Dibotryon morbosum). The patch of choke cherries that I visit weekly is riddled with this fungus. It looks like someone has stuck a bunch of burned corndogs on the branches. This time of year the blackened growths are as obvious as the nose on your face, but when the disease is in its earliest stages, it can be very difficult to detect. If you are growing cherries or plums commercially, or even for your own enjoyment at home, you will want to know how to detect this virulent pathogen as soon as possible.

Black knot begins its colonization when spores are released from the parent fungus. The spores come in two varieties. The first are asexual, called conidia, and they appear as an olive-green, velvety growth on the black knot cankers in their second spring of growth. From early spring to early summer, wind and rain break off the conidia and spread them to new infection sites. The second kind of spores are ascospores, and these are formed sexually through the fruiting structures of the fungus, which are found on knots that are beginning their third year of growth. Like the conidia, they are spread by wind and splashing rain from early spring to early summer.

Once the spores are airborne, either blown or splashed, some will land on young wood, such as twigs and branches. Here the spores settle in for the long haul, either entering the plant via wounds or directly inserting themselves through the bark. Often entry is at the crotch of the twigs and branches. If the weather is wet (wet being a relative term, for it only needs to be wet for a few hours), and the temperature is between 55 and 75 degrees Fahrenheit, conditions are ripe for infection.

So, the spores start to grow. Mycelium snake their way into and all along the wood of the tree/shrub. During the first year of growth, a small brownish blob may appear on the infected stem. It’s not terribly noticeable, which is why the disease is easily overlooked until it is well-established. Year two rolls around, and now the knot grows rapidly. At first it is soft and develops a greenish-brown color: this is the sign that the conidia are developing. As summer number two progresses, the knots, which are now rather large (they can grow up to a foot in length over time), start to harden and turn black.

Eventually the knots can encircle the twig/branch on which they are growing, effectively girdling it. The end result is a dead twig/branch above the knot. And even though the knot is now hard and crusty, its edges can continue to grow. Eventually, the oldest parts of the knot will break down, and this opens them up to invasion by boring insects (not insects that are dull conversationalists, but insects that will chew their way into the woody tissue of the plant, potentially bringing with them a whole new set of pathogens).

Some authorities consider black knot to be a minor disease, while others call it a serious problem. The latter are probably involved with commercial fruit growers, for whom black knot can indeed be a serious problem. But even if you only have a single cherry or plum, you want to know how to deal with this fungus, for if left untreated, it will work its way through the cherry and plum population, eventually killing off all the trees.

The first thing to do is routinely inspect your trees. You want to nail the fungus as early as possible, so know what the first summer’s growth looks like. If you miss it, and you don’t notice the knots until they are well-formed, it is still not too late. Grabbed your pruners and cut off the offending branch(es). You want to cut about eight inches below the canker to ensure that you are getting most of the mycelium inside the wood. Gather all your prunings and burn them. Or bury them deep in the ground.

If cankers have formed on the trunk of your tree (not as common, but still possible), dig them out with a knife and chisel, taking an additional inch of wood all around. If the resulting hole is greater than two inches across, paint it with shellac and cover with tree-wound dressing. You will also want to destroy all affected wild trees/shrubs in the immediate area. The recommended distance is 600 feet. If you have an orchard you need to protect, contact your local extension office to find out what dormant sprays and fungicides are recommended.

It’s a war zone out there. Fungi, insects and other pathogens are attacking trees and shrubs; trees fight back with sticky saps and toxic chemicals. Some plants call in the cavalry, in the form of insects that will attack the offenders (such as ladybugs vs aphids). The next time you go for a walk in the woods, think about this. Take a look around. See if you can find some evidence that all is not as calm as it seems.

Ellen Rathbone

Ellen Rathbone

Ellen Rathbone is by her own admission a "certified nature nut." She began contributing to the Adirondack Almanack while living in Newcomb, when she was an environmental educator for the Adirondack Park Agency's Visitor Interpretive Centers for nearly ten years.

Ellen graduated from SUNY ESF in 1988 with a BS in forestry and biology and has worked as a naturalist in New York, New Jersey, and Vermont.

In 2010 her work took her to Michigan, where she currently resides and serves as Education Director of the Dahlem Conservancy just outside Jackson, Michigan.

She also writes her own blog about her Michigan adventures.


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