What is crown gall?

It is a “parasitic infection”, sometimes called “tree cancer”, that causes tumor-like growth at the crown and soil level of the tree, along the roots, on the trunk above the crown, and sometimes at the grafting site.

What causes it and where does it come from?

Walnut crown gall is caused by a bacteria called Agrobacterium Tumefaciens. The bacteria is indigenous to the soils in which walnuts are grown. The bacteria has the ability to infect the tree naturally as it comes into contact with the thin, delicate tissue that is part of the crown and roots of the young seedling and tree. In addition, infection may result from injury (abrasions, cuts) to the young tree when the nursery “harvests” the young trees for sale, and when the trees are transplanted into an orchard. Other injuries and resulting infections may occur later in time as a result of gophers, ground squirrels, discing, sucker removal - any injury exposing the cambium, phloem and xylem on the trunk and the roots.

Do all walnut trees get crown gall?

Yes and no. Not every tree in an orchard will get crown gall. Infection rates may get as high as 70% or more, but each tree has different degrees/levels of infection most of which can be successfully controlled if done early in the life of the tree and then treated thereafter as needed. In addition, there are two primary walnut root stocks: Paradox root stock and Northern California Black walnut root stock. The various varieties of walnuts grafted to Paradox root stock are far more susceptible to crown gall infection than the same varieties grafted to Northern California Black root stock. Both root stocks get crown gall, but Paradox root stock has a much higher incidence of infection.

What happens if I do not remove the galls from my trees?

Not all trees are infected at the time they are transplanted to the orchard site and not all trees are infected at the same time. This means galls that begin to emerge from the soil at the crown will be at different ages of development, sizes and locations. Those trees that had crown infections at an early age will see the galls quickly become immense in size – two and three times the diameter of the trunk and may encompass 180 degrees or more of the trunk within a year or two. Left untreated, the gall will continue to surround the trunk. The tree will show the effects of this with stunted growth, and sooner or later will not be able to withstand the summer heat. The leaves will begin to yellow indicating reduced transpiration and nutrition causing the tree to likely die in a season or two. Young trees need to be treated as soon as possible. Trees that become infected much later (7, 8, 9, 10 years old) have established a formidable root pattern and can withstand for a number of years a few large galls. However, as the bacteria/the gall continues to infect fresh tissue at its outer edges, the degree of encompassing the trunk increases. Gall removal even at 300 degrees may save the tree. At 360 degrees the tree is probably doomed.

If I remove the gall tissue from the tree, will it grow back?

Unless all, or a high percentage of the gall tissue is removed, especially one or two inches above and around the perimeter of the gall site, it will grow back. The gall tissue spreads because the bacteria inside the gall is constantly re-infecting the healthy tissue surrounding the gall site. By removing the “good” tissue around the gall and treating/disinfecting the exposed area, it is possible to end the expansion of the gall and prevent it from growing back. However, the removal of a large gall on one side of the tree may trigger the growth of a latent gall or galls at other sites on the crown. Therefore, yearly monitoring of the orchard for new galls and removal of the bigger ones is essential in sustaining the vigor, health and production of the trees and the orchard.

Once I remove the gall, may I refill the hole around the crown with dirt?

No. Removing the gall has created an open wound and immediately replacing the dirt increases the chances of re-infection. If you have followed the directions in successfully removing and disinfecting the wound site, leave the hole open for about a year. Then blow out the hole again and check for any re-infections. Remove and disinfect/treat any new gall sites. Wait a few months so the new wounds have a chance to dry and harden and become less susceptible to re-infection, then you may fill in the hole.

How do I know if the chemical is working?

If you have followed the directions in correctly removing and applying Gall Buster, field testing has shown that after a year’s time there may be a reinfection rate of just 2% of the surface area/square inches at the original gall removal site. The removed gall may have represented 25, 50, 100+ cubic inches of gall tissue which “captured” the water and nutrients for its own use at the expense of the tree. Without the parasitic effects of the gall, the tree will be able to hold its own against the elements, begin the process of surrounding the wound site with healing tissue and within a season or two resume a normal growth pattern indicated by putting out new leaders.

Is there an easy way to remove crown gall?

Small galls can easily be removed with an axe, cored out with a knife, air drill or an air chisel and then sprayed with Gall Buster. Much larger galls at and below the ground surface can be quite large, deep and sometimes intertwined with multiple roots. The most effective way of controlling crown gall in this case is to remove from the crown and roots as much of the gall as possible. This will assure the greater chance of saving the tree. There is no instrument at this point in time other than an axe, an air chisel, a chain saw that can fairly quickly remove the gall tissue. The chain saw is by far the most radical and most damaging to surrounding healthy tissue.


Summary and the Drill-and-Fill Method

Remember the gall is a cancer in that the cell tissue of the tree at the point of infection has been genetically modified by the DNA of the bacteria to form a fast growing irregular cell pattern that becomes the tumor – the gall. The gall becomes one with the tree; therefore, most gall tissue/cells need to be removed, or at least rendered less harmful in size. Gall Buster will soak into the cambium and will kill most tumor cells missed by the initial removal process.

Finally, for those young trees with gall encompassing about 300 degrees of the tree, or those growers who want to reduce the size of the tumors/gall as expeditiously as possible because of a time factor, using the Drill-and-Fill method may be a temporary solution. If done correctly, this method will immediately stop the growth/spreading of the gall. In about six weeks, most of the gall will crumble away; however, there will still remain a viable layer of gall tissue where the drill holes did not go deep enough and the chemical did not reach the lowest levels of the gall. Eventually the gall site will have to be cleaned for the best results. In the meantime, the Drill-and-Fill method will have dramatically reduced the cubic inches of the gall to the benefit of the tree.