I have several plum trees in my front yard in Indian Hill, Hamilton County,...

Asked January 22, 2016, 3:24 PM EST

I have several plum trees in my front yard in Indian Hill, Hamilton County, that have recently become infected with black knot fungus. My 10 year old trees now look like they have ugly black cigars growing on them! At various places in the tree limbs. I know I have to prune them off, but what else should I do? What fungicide do I use and when? Please advise. I don't want to hack my beautiful trees up!

Hamilton County Ohio

1 Response

This is one of the most difficult fungal diseases to effectively manage on susceptible Prunus species which of course includes both ornamental and fruit plums. Once your trees have become infected, you will probably never be totally free of the disease which means that you will probably have to do some pruning each year even if you make fungicidal applications. Unfortunately, I'm speaking from experience: I've been engaged for a couple of years in trying to reduce the impact on my own, large Canada Red cherry.

Here are the major challenges that you are facing:

  1. Host susceptibility ranges within the genus Prunus; however, since your trees are infected, they are obviously susceptible.
  2. The fungus is common on wild trees including wild black cherry which is common in the woods in your area. This means your dealing with a continual reservoir of fungal spores (inoculum) from surrounding trees. Pruning to remove the inoculum from your trees will not prevent spores from drifting in from your surroundings.
  3. Pruning is recommended and you must make cuts a least 6-8" below the infections (= "cigars") on your tree. This is the perfect time of the year because you can see the infections and they are not active. However, since your trees are susceptible and you have inoculum coming into your landscape, you will probably need to prune away new infections each winter ... you may find that you're slowly whittling away your trees!
  4. Fungicides are not long-lasting and they are only preventative, not curative. This means the fungicide can only act to protect against infection, not cure an infection. Since they are not long-lasting, you would need to make multiple applications following the re-application directions on the product label. And, since infections can occur over a long period of time, your applications will need to start at bud break and continue throughout much of the season, depending on environmental conditions.

This all adds up to something I'm sure you don't want to hear: you may be better off slowly replacing your plums with less susceptible trees. Frankly, that's probably what I'm going to do with my own cherry.

To learn more about this disease, here is a pretty good Fact Sheet: http://www.apsnet.org/edcenter/intropp/lessons/fungi/ascomycetes/pages/blackknot.aspx

I'm sorry that I can't provide a more optimistic prognosis.