Black Knot Disease



Branches afflicted with Black Knot



My friend Janice called last week to ask about a problem she was having with her ornamental plum in front of her house. It had black growths on some of the branches. I was pretty sure it was some type of fungus, but I didn’t know what or how to treat it. I told her to take a branch into Countryside and ask for KC or Kelly. They are both certified nursery professionals and have training in disease identification.



Kelly O'Leary
Nursery Staff



Well, the diagnosis was Black Knot. Kelly advised her to prune out the affected branches, cutting at least 4" back from the infected sites and to carefully bag the branches and dispose of them. You don’t want diseased plant material going into a compost pile or back in the woods where the fungal spores can continue to spread. After cutting out the infected branches Kelly recommended treating the tree with Bonide Fruit Tree Spray or any spray containing captan.



Black Knot affects plum and cherry trees. The fungus disrupts the twig growth and causes a tumor like growth. At first the swelling is light brown in color but by the second year the swellings have become hard and black. It takes a year or two before the disease even becomes noticeable and by the time you do see indications of the disease, it is usually too far gone for any remedy to work. As with all fungal diseases, it is easier to prevent than to cure.



Close up of Black Knot fungus



The best time to prune out the diseased wood is in the late fall or early winter when the tree and the fungus are dormant. This way you won’t be inadvertently spreading more spores. When you do prune make sure you dip your pruners in rubbing alcohol between cuts to again insure that you don’t spread the disease to healthy tissue.

We got about half way through the job of trimming when we realized that we wouldn’t be left with much of a tree when we were through. Janice decided to just replace the tree with a more disease resistant one. This time she is thinking of a crab apple. Though crab apples are also susceptible to fungal infections such as apple scab and fire blight, new varieties have been introduced that are very disease resistant.