One of the more frustrating local diseases of ornamental cherry and plum trees is Black Knot. Customers frequently bring branches into the garden center with hard swollen blotches attached to it, hoping for a cure.
Unfortunately, there is no easy save for trees with black knot. If caught early, it can be pruned off and pruning remains the only viable option for removing black knot.
Here's what to do if you notice this fungal disease on one of your trees.
First, it must be noted that this disease spreads easily, including via your pruner. So your pruner should be disinfected between each cut. The goal is to remove as much possible disease that can be reached, while reducing the spread of the fungus.
Gradually the knots cut off the flow of water and nutrients to the rest of the plant, which is what cause the slow death of the plant if it isn't pruned off.
How To Prune Black Knot
There is a careful balance that must be maintained for the health of semi-dwarf fruit trees. Trimming off excess branches helps maintain the strength of the tree and keeps the size manageable for home gardeners.
When to Prune Black Knot
It's best to prune black knot in early spring. However, you can work on pruning off throughout the season once it is noticed on a plant.
How to Prune Black Knot
As previously mentioned, the pruners should be disinfected between each cut. Pruning cuts should be made at least four to eight inches below any swellings or knots because the fungus grows beyond the edge of the knot. We recommend immediately burning the branches and black knot that has been pruned off.
Want to read more about black knot? Read this article from The Morton Arboretum.