Fungus on Plum Tree

Asked March 16, 2020, 3:16 PM EDT

We have a black fungus on the branches of out Japanese plum tree. It doesn't seem to be on the main body or on the larger limbs. We want to know if the tree will be dead or if we can save it.It has some new buds on it this spring. Any help you can give would be appreciated. Ii is blooming but the blooms are small and sparse, not as big as usual. Also there is sap running out of the bark in a couple of spots on the trunk of the tree.

Baltimore County Maryland

1 Response

This looks like a common fungal disease of Plums called, appropriately, Black Knot.

As the information indicates, there is no fungicide to use on the disease; the symptomatic branches (twigs) need to be removed. Given the size of your tree, we realize this is might be challenging. If you have a pole pruner, this will help.

The trunk ooze could be related to the Black Knot or it could be a separate issue, such as a trunk canker (typically a different fungus) or wood-boring insect. It is not uncommon for pests and diseases to overlap, as stressed plants are more susceptible to both. For this, similarly, nothing can be done except to try to keep the tree in relative good health. The past two springs of excessive rain, followed by last summer/fall's drought likely caused much root damage on many garden plants. Suffering plants were then set up to be attacked by insects or pathogens; this could be what promoted this outbreak. If surrounded by turfgrass, it is very important that trees be protected from accidental injury from lawn equipment though edging guards or a ring of mulch or non-turf around the trunk. Cuts and scrapes provide points of entry for both wood-boring insects and spores.

The poor flowering is tied to either the fungal infection cutting off nutrient and water supply to the branch tips, where the flowers were forming, or tied to root loss. Plants with root stress will typically abort flowers (or buds), fruit, or foliage before branches themselves die back. Unless the galls can be removed, it's unlikely flowering will improve measurably in future years.

Given the extent of the damage, iIf treatment is too daunting and you feel the tree should be replaced, there are many attractive flowering trees of similar height that are unrelated to plum and will not contract this disease. Site conditions will help dictate which are best, but in general, Redbuds, Dogwoods, Fringetrees, Serviceberry, and Magnolias are among the most popular.