Japanese Lilac Tree with black knot fungal disease

Asked April 27, 2016, 9:33 PM EDT

Hello, We have a Japanese Lilac tree in our front yard which we recently noticed had black knotty growths on multiple branches. We did not notice this last year or over the winter. In a google search, it appears that this may be 'black knot disease', technically known as fungus Apiosporina morbosa. from the online recommendations, we have cut off all branches we could reach which had the knots. Specific questions: Have you seen this in our area (we are in Chaksa)? Do you recommend removal of the tree to ensure that the fungus does not spread to several other new trees we have planted (autumn blaze maple and a whitespire clump birch)? If removal is not necessary, what is the treatment plan? Thanks in advance for your attention to this request. Kindest regards, Katherine McCabe (612) 801-0481

Carver County Minnesota

8 Responses

While this certainly looks like Black Knot, Japanese Lilacs are not susceptible to this disease. Do you mean that your tree is a Japanese Plum tree?

Besides pruning out the infected branches, you may chose to spray the tree, as well. The tree does not have to be removed as long as the infection is controlled.

Burn all of the wood that you prune off in order to limit the possibility of spread.

Here is a fact sheet from the University, along with treatment recommendations:

Thank you for your quick response! We are not exactly certain what our tree is, so we've included several additional photos. Please note that the photo of the gall uploaded with the original question, is a photo I found online which very closely resembled the growth in our tree.

Are you able to determine the variety of our specific tree?

Thank you,

One final image of the full tree.

Hi Katherine,

This is definitely not a Japanese Lilac! I suspect that it's a cherry, of some sort - probably a Black Cherry, or a hybrid. And that would explain the Black Knot disease.
(I have what was supposed to be a cherry-plum hybrid that was damaged early in it's life. The resulting tree is much larger than the advertised 6 feet, and produces simple - black cherries. It too, suffers from Black Knot).
You can tell by how the flowers cluster in what is called "racemes".

In any case, the fact sheet that I'd send will contain the information you need to take care of this tree. If you decide to get rid of it anyway, make sure to dispose of the wood and branches properly. A real Japanese Lilac tree could be a fine replacement tree!

Well, you've helped us solve the mystery of what our tree is - thank you! I have the fact sheet you'd originally sent with how to treat it. The the U/ MN Arboretum have a list of recommended tree services who we could contact?


Excellent - thanks for forwarding this.

I think you mentioned above that you currently have a cherry variety tree with black knot. Do you have to have it treated each season?

This is my first experience using the Master Gardener Ask the Expert tool. You have made it a great experience and I'll be sharing this with anyone I come across who needs advice.

Many thanks!

Thank you for your kind words!

I have never sprayed this tree, nor used any systemics. I simply cut out the branches that become infected. It can take 2-3 years before I see any new fungal growth. this tree is situated inbetween two houses and gets full sun for just a few hours a day. The growing conditions tend to be more humid than if the tree was situated out in a lawn - perfect for fungal infections. So far the tree is thriving!

Like the publications says, this is rarely life-threatening for a tree, unless the disease has gone undetected and un-treated for many years. Your tree looks very healthy, and should do just fine with some judicial pruning.