Black Knot Disease in plum trees

Asked September 10, 2019, 8:11 PM EDT

I understand that I must prune out the branches with the galls. I notice that infected branches will also have twigs that are abnormally shortened and look "scrunched up" I'm assuming that these are infected and will also have to be pruned. After that (winter) what sort of fungicide should I use, and do I use this on the plum trees that are not infected? And can this disease infect red maples? The tall ones in my adjacent woods also have a gall, not as black.

Newaygo County Michigan

1 Response

No, You only need to prune out the black knots that you see. There are infections on some of the new shoots that will develop next year and those need to be pruned out next year. It takes about a year for the infections to swell and become noticeable.
No, the Maple does not get this disease, only some members of the Prunus genus mostly plums and some cherries.
You do need to spray the new growth next year. You pretty much need to protect the green growing tissue of the shoots from the beginning of growth until they stop growing (when there are no more new leaves appearing at the ends of the shoots). Here is a link to an article
This article was written for commercial growers. Chlorothalonil is the best material. Chlorothalonil is often sold to home owners under many names. You can find it listed in the ingredients on the container. If you plan on eating the plums you will not be able to use this material after shuck split when the fruit emerges. If they are purely ornamental plums you can use as long as you want. Read the label and follow the instructions. Try to apply the material before rains and reapply about every 10 days to cover up new growth.
For fruit you are going to eat you need to change to a different fungicide.
Captan is a common fungicide (again listed in the ingredients) but it can cause some shoot holes in the leaves and mark the fruit if the conditions are not right during the application. This occurs soon after bloom when the fruit and new leaves are tender and don't have a well developed waxy cuticle. We are not supposed to endorse or use specific product names on this forum so you need to contact me directly for another recommendation of a fungicide to use when there is fruit on the tree.