Issue: August 17, 1998

Knots on apple roots - wooly apple aphid

I have had numerous sprouts come up from my apple tree. I decided to dig some of them to transplant so that I could have more apple trees. This is a very good apple and I wanted more. When I dug the sprouts, I discovered that they were attached to large roots from the tree and that these large roots had knots, or tumors, on them. I also noticed some white, fuzzy, things on these knots. Did I hurt my tree by digging these roots up? The tree has been losing branches for the last few years, and I wonder if it was too weak to have the roots damaged like that. By the way, what causes the knots on the roots?

While I would not recommend digging the large roots from under an apple, or any other tree, in this case you probably did less damage than the "wooly apple aphids" that you described. These are aphids that attack apples and cause serious problems. They can attack the bark, twigs, and the roots. In the roots they cause galling, or the formation of the knots that you described. These galls interfere with the flow of water and minerals from the roots to the top of the tree, probably explaining why you have seen branch dieback.

The wooly apple aphid is a small insect which covers itself with white, waxy filaments which give it the "wooly" appearance. Their color varies with the seasons and whether the aphids are winged or without wings. These colors range from dusky green and dusky brown, to pinkish, reddish, to rusty yellow or brown. However, the waxy coating usually conceals these colors, and their small size makes the color less important unless you observe them through a microscope.

On the apple trees, they are commonly found on the twigs, roots, and especially in crevices in the bark where they may be numerous. Even if not observed above ground, they may be present on the roots and causing problems like you have described. One of the symptoms of their presence underground, though also a symptom of other things, is the development of numerous sprouts from the roots where the galls have formed.

According to Dr. Mike English, NMSU Extension Entomologist, once the roots are infested, the tree is doomed to a slow death. It is not possible with reasonable means to eradicate the aphids from the roots. In time the tree will die. Initially individual branches will die and leaves will drop from other branches. In spite of infestation, the tree can continue to grow and produce apples for years.

Crown gall is a disease of apples and other members of the rose family. This disease causes the formation of galls on the roots, base of the trunk, and sometimes on the branches of the trees. It may be confused with the wooly apple aphid galls, but the white, waxy covered, insects are not present. Since you described the presence of the insects, they are the most likely cause of your problem.

Regarding your plan to dig the sprouts to start new trees, these sprouts are from the rootstock and will not produce the tasty apples you described unless the tree was started from seed and never grafted. If these are from the rootstock, the new plants would need to be grafted with the variety that you like. However, you now know that this rootstock is susceptible to the wooly apple aphid, and these sprouts are probably infested, so I would not recommend using them. The solution to the problem is to plant apple trees grafted onto wooly apple aphid resistant rootstocks. When you plant new trees and when it is time to replace the tree you wrote about, if you choose to plant apples, be sure to use the resistant rootstocks.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

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