Black knot on ornamental prunus

Asked March 26, 2020, 11:47 AM EDT

I noticed thickened branches on my decorative prunus late summer and it spread to other branches and twigs throughout the fall and winter. It is definitely black knot. It is almost 18 ft X 18 ft. Should I cut down now before it blooms? Buds are swollen and ready to open. Most garden centers are closed and I do not have lawn service or arborist to consult. I can cut down with my sawzall but I cannot dig up stump. Should stump be removed? Can I plant another decorative tree or shrub in that area? If so, what do you suggest that will not be affected? Do I need to "cleanse" or kill anything in the soil? It provided shade to my porch and the more tender plants below. I have huge hosta and other bedding perennials underneath. Adjacent to this tree are several nine bark and hydrangeas.

Oakland County Michigan

3 Responses

Hello,
I am sorry to hear that your Prunus, Cherry?, has black knot. Depending on the extent of the infection, black knot can be controlled by cutting out infected twigs and branches 3-4 inches beyond the knot. It is advised that this be done in the winter. I'm attaching two articles that give more guidance about controlling black knot. One from MSU Extension and the other from University of Minnesota Extension.
https://extension.umn.edu/plant-diseases/black-knot https;//www.canr.msu.edu/news/controlling_black_knot_in_michigan

As you can see in the articles, there are fungicides that can be used after you have cut out the affected twigs. If you do choose to use a fungicide (and let's hope that soon, when life returns to normal, we will be able to visit a nursery), make sure that you read the instructions on the label to ensure that this fungicide can be used on cherries. Do not use a fungicide unless the species is listed.

If you decide to remove and replace the tree, it would be wise not to plant a tree in the prunus family--no cherries, plums, apricots, peaches. You could try a hardy crabapple or magnolia, considering the size of the area. As to the root, if you wish to plant in the area, you would have to remove the root--there are companies that grind stumps which will allow you to plant in the area. If you do hire a stump removal company, you will have to move the perennials out of the way while they are grinding it.

Finally, if you which to consult a certified arborist, and probably even in these times, they will still answer the phone, go to www.treesaregood.org, the website of the International Society of Arboriculture, to find one in your area.

Thank you for your question and good luck with your tree.

The black knot is in almost every area. Branches are contorted and thick without the blisters bursting through the bark. I am going to cut off as much as I can but I don't have anything to seal the open cuts other than paint. Is there anything to use while the nurseries are closed? I have some fungicides in my shed. Right now it is too full with mowers etc to reach. I am burning all the infected material. Thanks for your help. Stay safe.

Hello, using latex paint is just as effective as any limb sealant and you probably have some on hand.

Good luck with your tree.