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Black knot fungus on my plum tree #760033

Asked July 05, 2021, 11:30 AM EDT

I have a single plum tree in my front yard, now infected with black knot fungus. I read that I must prune off the infected branches (there are a lot of them!) and treat the tree with fungicide. Which fungicide is best? Other advice? There is a peach tree nearby, but it seems ok. Thanks.

Washtenaw County Michigan

Expert Response

Yes, one of the best ways to manage black knot is to prune out all the "knots" (cut 6 to 8 inches below them). Fungicides can be used in combination with this pruning/sanitation and applied at the white bud and petal fall growth stages. More information here: Black knot, MSU Integrated Pest Management

This is a great publication from Purdue Extension that has images of what the "white bud" and "peal fall" stages look like and recommends using the fungicide Chlorothalonil: Managing Pests in Home Fruit Plantings

Thank you so much!  So the pruning is done now and the fungicide is applied next year?

From: Ask Extension [mailto:<personal data hidden>]
Subject: Black knot fungus on my plum tree (#0029964)
Date: Thursday, July 8, 2021, 9:45 AM

The Question Asker Replied July 08, 2021, 10:45 AM EDT

Sure thing! I would wait to prune until the dormant season (late winter). Pruning right now can create open wounds that would be a site for further infection from other pathogens or pests - best to avoid that! The fungicide will be applied in spring based on when the tree is at the "white bud" and "petal fall" stages (reference images in this publication from Purdue). Fungicides are used for preventing further infection. You can imagine that they sort of create a barrier on the foliage and branches that prevent fungal spores from taking hold. They don't really "cure" the tree and that's why it is important to use pruning/sanitation in combination with them to limit how much pathogen is present.

Thanks!  I was ready to get out my shears.
I read that it is good to burn the pruned branches.  We have a wood stove and could burn them in late winter.

From: Ask Extension [mailto:<personal data hidden>]
Subject: Black knot fungus on my plum tree (#0029964)
Date: Thursday, July 8, 2021, 10:53 AM

The Question Asker Replied July 08, 2021, 11:41 AM EDT

Yeah, I have also heard you can burn them. Otherwise you can bag them up and put them in the trash. Avoid composting because those "knots" can still spread the pathogen. Also, when you prune be sure to sanitize your shears with 10% bleach after each cut, and definitely before pruning on another tree. 

Now that I am not pruning until late winter, I have another question.  If the plums on the tree ripen, are they safe to eat.?

From: Ask Extension [mailto:<personal data hidden>]
Subject: Black knot fungus on my plum tree (#0029964)
Date: Thursday, July 8, 2021, 1:39 PM

The Question Asker Replied July 08, 2021, 2:25 PM EDT

The fruit should be okay to eat. Black knot usually affects the branches and not the fruit (maybe fruit spurs).

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