Crown gall—the tumors or galls that can appear on the roots, trunk, and crown of a tree—is nothing if not unsightly, but these abnormal masses that appear on walnut trees stem from a more involved process.
Agrobacterium tumefaciens
While a nuisance to growers and a hindrance to trees, Agrobacterium tumefaciens— the bacterium responsible for crown gall—has actually made an impact on humankind and medical advancement due to its ability to transform a cell into what the bacterium needs it to be, making it a key player in genetic engineering.
But to put it succinctly in regards to orchards, A. tumefaciens is everywhere. However, being everywhere doesn’t necessarily mean that it will cause disease (pathogenic) to your trees. To become a pathogen, the bacterium requires a wound or natural opening in order to enter the tree. Once it enters, the bacterium takes a piece of its own DNA—called a plasmid—and inserts it into the plant cells of the tree, which then causes the crown gall bacteria to self-proliferate. (It is this plasmid that makes the bacteria pathogenic. Not all carry this plasmid.) The bacterium forces the plant to generate a very special, selective food source that only that bacterium can eat, and other competing bacteria cannot eat it. It’s self-sustenance at its finest.
Crown gall is unique in that it is motile, is not harbored in grasses, does not cause necrosis, and, therefore, isn’t credited with directly killing trees. It can send the host tree into a decline, but the tree will remain alive, and can remain so for a very long time. (It’s possible to find trees that are 20-30 years old that have crown gall.) It does, however, affect tree size, which can have a domino-like effect in terms of yield. Irrigation issues come into play when trees are smaller because they are receiving more water when compared to larger, healthy trees in the same row. This also causes the infected trees to age more quickly.
Research
A study conducted by Lynn Epstein and colleagues and published in 2008 found that for every quarter of the trunk circumference that had galls, there was a 12 percent decrease in cumulative nut yield over the first four years. Basically, larger trees have a higher yield, unless they’re infected by crown gall.
Crown gall also predisposes the trees to other pests and diseases. A closely related pathogen to crown gall is hairy root, or Agrobacterium rhizogenes. Sometimes both crown gall and hairy root are found on the same tree. Crown gall also predisposes trees to thousand cankers disease, which is a fungal pathogen and introduced to the tree via the walnut twig beetle. In past studies, it was found that trees with thousand cankers were more likely to have crown gall, but trees without crown gall are less likely to become infected with thousand cankers.
In related studies conducted by Elizabeth Fichtner, Ph.D, University of California Cooperative Extension (UCCE) Tulare County farm advisor, insect pest larvae were excavated from trees both with and without crown gall, with a higher number of larvae present on the roots and crowns of trees with crown gall. Not only that, but pests feed on the galls, and when the insects were taken to the lab and swabbed, A. tumefaciens was inside their digestive tract. Though this has yet to be proven scientifically and is currently just a hypothesis, this suggests that there is the potential for insects to spread the bacteria.
What does this mean for growers and the management of the crown gall pathogen?
Rootstock Selection
Sometimes curbing infection can start at the beginning with rootstock selection. For one, crown gall cares less for black walnut than it does for other types. Though black walnut is still susceptible, it seems to have a lower frequency of crown gall. Paradox seedlings are highly susceptible. RX-1 seems to show moderate resistance to the pathogen, but is still susceptible.
Any clones grown in sterile conditions seem to have a lower instance of crown gall, but this is particularly in part due to their initial growing environments. They’re not genetically more resistant, but rather grown in lab conditions that exclude exposure to the pathogen. If the pathogen isn’t present, then the trees cannot be infected.
There is a trade-off with the RX-1 rootstock, however. While it may be more resistant to crown gall, it also has moderate vigor. Since this is a less vigorous tree, it doesn’t yield as well as other trees.
Exposure to Crown Gall
But, how does crown gall make its way to your walnut trees in the first place? Since A. tumefaciens is everywhere, it may colonize your soil, but it cannot affect your trees unless it makes its way through an opening. There are a few ways that crown gall bacteria can make it into plants.
Dirty grafting knives or pruning tools can spread bacteria, and while this seems like an obvious culprit, how it happens isn’t so obvious. A crew can start with clean tools on uninfected trees, but if those tools happen to touch the soil, which is infested, and then create pruning wounds before re-sterilizing the tools, crown gall has now been given an ideal opportunity to infect the tree. The transfer of that bacteria doesn’t stop with that first cut. Dirty tools can continue to infect plants for several cuts as you make your way down the row. The rate will decrease, but bacteria transfer can happen for up to several cuts.
Infested budwood can also spread bacteria to previously uninfected trees. If nursery crews collect budwood from a tree that does not contain crown gall, and the budwood falls on infested soil, crown gall bacteria can now infest that budwood. If that infested budwood is then used to bud a tree, symptoms of crown gall may appear at the graft union or other areas of the tree.
Also, clean seeds that fall into soil infested with A. tumefaciens have a higher possibility of becoming infested the longer they sit in the soil. This is another way that crown gall can make its way into later generations of walnuts.
It’s completely possible to purchase healthy trees from a nursery that have the bacteria on them, and to never have known the plants carried the bacteria until the tree was wounded. Plants can be asymptomatic, meaning they can carry the bacterium without showing symptoms, the way humans can carry a common cold or flu virus and not be aware. Crown gall bacteria also translocates through the plant, so even if A. tumefaciens is found in the soil, galls can appear anywhere throughout the tree.
Prevention
What can growers do to prevent or manage crown gall in their orchards? Sanitizing tools is an immediate practice that can be put into place. Bleach is an easy and cost-effective way to disinfect pruning shears, grafting knives, and other tools, as it takes low concentrations to kill the pathogen. It’s best to spray tools with the bleach solution, which can provide up to a 93 percent reduction in crown gall bacteria; dipping tools increases the number of solids that appear in the solution, which dramatically decreases the solution’s effectiveness, and can even render it useless. The downsides to bleach are that it can corrode tools and that it’s phytotoxic.
There are some commercial solutions available on the market that are non-corrosive, not phytotoxic, and maintain their efficacy even as the particulate matter accumulates in the solution, meaning tools can be dipped. These quaternary ammonium compounds are cationic disinfectants and quite effective. However, these solutions cost much more than bleach, and require a much higher amount of the active ingredient in order to be effective.
Infected Trees
For trees already infected with crown gall, the decision to remove the tree or just remove the gall becomes a decision based on a few factors: your economics, the severity of the gall, and the age of the tree. Crown gall researchers suggest that if first leaf trees are found to be infected, just remove them and replant. In second leaf trees, treat smaller galls, but remove and replant only the trees that have large galls—galls that are bigger than a quarter of the diameter of the trunk. For those in third through seventh leaf, treat the galls unless the tree is completely girdled. Trees older than this are just let go and left untreated.
While there aren’t chemical sprays that are effective for use against crown gall, there are biocontrol agents that can be used to dip cuttings and prevent infection. These are made from Agrobacterium radiobacter and are found under the names Galltrol A, Norbac 84C, Nogall, and Diegall.