By Tim Clancy.
One of the best things about working with and studying trees is the availability of subjects. If you’re a doctor you wait for patients to come to your office, if you’re a mechanic you wait for a car to roll into your shop and so on. If you admire trees all you need to do is step outside almost anywhere in the world and you can find a tree to look at. A friend asked me to come and have a look at an American sweet gum (Liquidambar styraciflua) near his house. It was losing leaves at the top and he was curious as to my thoughts on why this was occurring.
While I was waiting for my friend at the agreed upon location, I noticed a California pepper (Schinus molle) that was struggling. Upon closer examination of the trunk I saw what can only be described as a family of crown galls. Galls in general are described as being abnormal growths that look like warts or knobs or lumps growing on a plant. Sometime galls are caused by insects that injure the plant and the plant’s response is the abnormal growth. Other times the gall is caused by an organism other than an insect. In the case of crown galls that organism is Agrobacterium tumefaciens a bacterium that lives in the soil.
In the case of woody plants, they typically grow on roots and trunks somewhere near the area where the trunk and roots transition. This area is sometimes referred to as the crown or root crown hence the name crown gall. Now as an arborist I must admit I find them fascinating, these abnormal growths. The particular tree I was looking at was full of them as you can see in the photograph.
This affliction occurs on plants around the world and “crown gall” disease has been studied extensively. It can be problematic for many species of plants including apples, walnuts, and grapes. As a matter of fact, Agrobacterium spp. is considered to have the widest host range of all plant pathogens.
In some cases, crown galls can kill trees. I have not personally seen this and in most cases trees I have seen with crown galls have functioned quite well. It turns out this is a good thing since there is not much you can do about crown galls once they arrive. If you catch it early and it’s on a stem you can remove, then that’s the best course of action. However, if it’s on the trunk as in this case, pruning it out will leave a big wound on the trunk perfect for re-infestation by the gall bacterium or other diseases. So there really is not much to do on ornamental trees when it comes to galls. Fortunately, they rarely kill trees here in San Diego. What about the tree in the picture you say? Well its not completely dead yet but will be soon and I am pretty sure it was attacked by another more ruthless foe, Armillaria root rot.
The American sweet gum, well that’s another story.