Tree Fungus - Ask Extension
We have a 23 year old plum tree and we think it has Black Knot fungus. It is a memorial tree and we’d like to save it, if possible. What do you reco...
Knowledgebase
Tree Fungus #787523
Asked April 21, 2022, 11:38 AM EDT
We have a 23 year old plum tree and we think it has Black Knot fungus. It is a memorial tree and we’d like to save it, if possible. What do you recommend?
Isabella County Michigan
Expert Response
Hello Ann
This tree has a disease common to plums called black knot. All the knots need to be cut out of the tree, and treat the tree with a chemical spray to prevent new knots forming.
The procedure to do this is described below. If you would prefer, a certified arborist can be hired to do the pruning for you. Find them by zip code search here— http://www.treesaregood.org
Pruning all knots out may not leave enough canopy for the tree to survive, but pruning and spraying, as described below, is worth a try for a couple seasons to see if the tree can recover.
Here is the control information for black knot:
“Because the knots are easily visible and localized, removing infected twigs and knots from branches is an important part of disease control. This should be done during the winter dormant period when the knots are easily visible. Cuts should be made several inches (4 to 6 inches) below any visible signs of the knot. Cuts can also be made during the growing season if you see knots expanding, but make doubly sure you cut well past any swelling. Burn all infected knotted prunings before the trees break dormancy in the spring. Spores can develop and spread from knots left on the ground or in brush piles.
On large, main branches and trunks, knots should be cut out with a knife or chisel. At least 1 inch of healthy bark around the knot should be removed with these cuts. Taper the cuts to a point at upper and lower ends to promote regrowth of the bark over the wound. If possible, nearby wild plum and cherry trees showing black knots should be destroyed.
Commercial plum growers routinely spray and focus on removing black knots on infected twigs and branches in the winter. Fungicides are applied starting at “green cluster” and repeated at seven- to 10-day intervals until shoot growth ends around mid-June. Fungicides containing chlorothalonil (FRAC Group M5) are the standard treatment to control this disease. Chlorothalonil sprays are applied during the prebloom and bloom period and also control blossom brown rot. Chlorothalonil cannot be used after shuck split on exposed fruit if that fruit is going to be harvested for human consumption. Chlorothalonil can be used after bloom on ornamental trees where there is no fruit harvest. Fungicide choices for commercial fruit growers after shuck split include Indar (fenbuconazole, FRAC Group 3) and Topsin-M (thiophanate methyl, FRAC Group 1). Sulfur is not an effective control material for this disease.”
The complete article, with pictures is here— https://www.canr.msu.edu/news/controlling_black_knot_in_michigan
These backyard fruit references show the bud growth stages including “green cluster” and explains more on care of your plum tree. You can follow the guide even if this is an ornamental plum to protect the leaves and branches from black knot. You can skip the sprays that are later in the season, which are to protect fruits as they develop on edible plum trees, if your is not an edible plum.
Follow these guides for care, watering, pruning, fertilization to have the best chance of returning the tree to health.
https://www.extension.purdue.edu/extmedia/ID/ID-146-W.pdf
https://plantpathology.ca.uky.edu/files/mw_home_fruit_productn_b591.pdf
It would also benefit the tree to carefully remove a circle of sod around the base, and cover the area with 2-3 inch es deep of wood chip or shredded bark mulch. Place the mulch so it does not touch the bark of the tree. Keep the tree watered during dry periods this summer and fall.
I wish you the best with your tree.
This tree has a disease common to plums called black knot. All the knots need to be cut out of the tree, and treat the tree with a chemical spray to prevent new knots forming.
The procedure to do this is described below. If you would prefer, a certified arborist can be hired to do the pruning for you. Find them by zip code search here— http://www.treesaregood.org
Pruning all knots out may not leave enough canopy for the tree to survive, but pruning and spraying, as described below, is worth a try for a couple seasons to see if the tree can recover.
Here is the control information for black knot:
“Because the knots are easily visible and localized, removing infected twigs and knots from branches is an important part of disease control. This should be done during the winter dormant period when the knots are easily visible. Cuts should be made several inches (4 to 6 inches) below any visible signs of the knot. Cuts can also be made during the growing season if you see knots expanding, but make doubly sure you cut well past any swelling. Burn all infected knotted prunings before the trees break dormancy in the spring. Spores can develop and spread from knots left on the ground or in brush piles.
On large, main branches and trunks, knots should be cut out with a knife or chisel. At least 1 inch of healthy bark around the knot should be removed with these cuts. Taper the cuts to a point at upper and lower ends to promote regrowth of the bark over the wound. If possible, nearby wild plum and cherry trees showing black knots should be destroyed.
Commercial plum growers routinely spray and focus on removing black knots on infected twigs and branches in the winter. Fungicides are applied starting at “green cluster” and repeated at seven- to 10-day intervals until shoot growth ends around mid-June. Fungicides containing chlorothalonil (FRAC Group M5) are the standard treatment to control this disease. Chlorothalonil sprays are applied during the prebloom and bloom period and also control blossom brown rot. Chlorothalonil cannot be used after shuck split on exposed fruit if that fruit is going to be harvested for human consumption. Chlorothalonil can be used after bloom on ornamental trees where there is no fruit harvest. Fungicide choices for commercial fruit growers after shuck split include Indar (fenbuconazole, FRAC Group 3) and Topsin-M (thiophanate methyl, FRAC Group 1). Sulfur is not an effective control material for this disease.”
The complete article, with pictures is here— https://www.canr.msu.edu/news/controlling_black_knot_in_michigan
These backyard fruit references show the bud growth stages including “green cluster” and explains more on care of your plum tree. You can follow the guide even if this is an ornamental plum to protect the leaves and branches from black knot. You can skip the sprays that are later in the season, which are to protect fruits as they develop on edible plum trees, if your is not an edible plum.
Follow these guides for care, watering, pruning, fertilization to have the best chance of returning the tree to health.
https://www.extension.purdue.edu/extmedia/ID/ID-146-W.pdf
https://plantpathology.ca.uky.edu/files/mw_home_fruit_productn_b591.pdf
It would also benefit the tree to carefully remove a circle of sod around the base, and cover the area with 2-3 inch es deep of wood chip or shredded bark mulch. Place the mulch so it does not touch the bark of the tree. Keep the tree watered during dry periods this summer and fall.
I wish you the best with your tree.
