Identifying and Managing Black Knot
This winter I’ve taken the time to walk through a nature preserve near my house. With the deciduous trees stripped of their leaves, it’s easy to see each branch and twig interwoven amongst the others. During these walks I’ve noticed a number of trees with growth like features. Large, lumpy, black growths usually occurring on the smaller twigs and branches. Well, what the heck are they?
Probably black knot. Here’s a closer look at what it is and what, if anything, you should do.
What is Black Knot?
Black knot is a fungus (Apiosporina morbosa). The black lumps are actually what the fungus uses to protect itself over the winter. But I’m getting ahead of myself; let’s start at the beginning. The black knot spores are spread through the air via wind and rain (much like the dreaded tomato blight). Young, green growth is the most likely to be affected but any wounded tissue is also a great candidate for the black knot fungus to settle in.
What to Look For
The spores are spread around during the warm, rainy spring months. When they find a good resting place, they begin to cause the tree to react in a similar way to how we would react to a bee sting: the tree’s surface begins to swell.
The first season with the fungus may not produce any noticeable problems but you may see a lightly swelled brown surface by the end of summer. The following spring the tree will begin to react more quickly, creating a light green lump as it produces more and more cells to fight the infection. By the end of this second season, the fungus will cause the light green lump to turn hard, black, and crusty looking. In the spring, spores will explode from this structure and start the cycle over again.
Are Your Trees Affected?
Remember that the black knot fungus only affects trees in the Prunus family – that is, the plums, cherries (including black cherry and chokecherry), peaches, apricots and a number of other fruit trees, plus buckthorn.
What To Do About It
If you have noticed black knot growths on the trees in your yard, it may be in your best interest to get out the pruners. Black knot, if your tree becomes heavily infested, can kill an otherwise healthy tree.
It’s most likely that you will notice the black lesions in the winter when the leaves have fallen. Prune out these lesions in the winter to keep the spores from spreading in the spring and destroy the trimmings – burn, bury or remove far from the site. Prune 10-20cm below the infection towards the trunk of the tree – the more the better. It’s a good idea to keep a bucket of bleach handy to disinfect your pruners between each cut (you should be doing this for any kind of tree pruning).
Give your trees a close inspection next time you’re out – you’ll be glad you did.