Garden Q&A: What is this odd growth on my apple tree?

Q: I noticed this odd growth on my apple tree. I don’t think it’s causing damage because the canopy looks normal, but I think it’s becoming more extensive so I’m concerned.

A: This funky growth is not a disease, nor is it infectious. It’s called burr knot, and it’s akin to root bumps on a tomato stem. They can develop on both trunks and branches, and may occur on a variety of tree species but are most commonly encountered on apples.

Some apple rootstocks (named M. 7, M. 9, and M. 26) are more prone to burr knot, often at the graft union where trees were grafted to their rootstock. Conditions of shaded bark, high humidity, and warm weather are conducive to the formation of burr knots, which begin at a node — the place of attachment of a former leaf or branch when the tree was much younger.

This is one good reason to not wet the trunks of trees when irrigating, and to make sure trees are planted at the proper depth and not over-mulched. Similarly, use caution if wrapping the trunk with a plastic or fabric protector because these shade the bark and trap moisture.

There are several vulnerabilities that burr knots can create in the tree. The gnarled and fissured bark can be easier points of entry for wood-boring insects and infectious bacteria like fire blight, plus it might be more susceptible to winter cold damage than the rest of the wood. If a knot enlarges or coalesces enough with others to interfere with sapwood transport, then the canopy or root system may begin to suffer decline because the carbohydrates produced by photosynthesis won’t move as freely to where they are needed for growth. If you have a tree with burr knot, there isn’t much you can do to halt knot growth and there is no chemical to cure the condition. You should monitor the tree for decline.

Q: What’s causing a weird knobby swelling on some of my Euonymus stems?

A: Probably an infection called crown gall, which creates deformities on shrub stems, often near the soil line. Euonymus, forsythia, and roses are common victims. Galls can begin semisoft but harden with a cracked surface over time, though plants won’t necessarily display symptoms the same season or year they were infected.

The bacteria responsible live in soils and can infect a wide variety of plant species, though what kills the plant or causes dieback is often a secondary infection where another pathogen took advantage of the weakened tissue as a point of entry. The crown gall bacteria themselves need a wound on the plant in order to begin an infection, which can arise from bark cuts during planting to soil-dwelling insect feeding or even contaminated sap spread from branch to branch. (from use of unsterilized pruning tools).

No curative treatments exist. Pruning out any visible galls is only a temporary fix because the bacteria are distributed throughout the plant. Prevention takes the form of simply avoiding plant injury (like root wounds from tilling or transplanting), trimming broken branches with clean and sharp tools, and growing species naturally more resistant to this disease.

Healthy soils with lots of diverse microbial life may also help to suppress crown gall. For example, beneficial microbes may outcompete the crown gall bacteria on root surfaces. Soil health is improved by adding organic matter and avoiding compaction and over fertilization.

University of Maryland Extension’s Home and Garden Information Center offers free gardening and pest information at extension.umd.edu/hgic. Click “Ask Extension” to send questions and photos.