Genetic alteration of organisms by inserting genes from other species is an astounding achievment of modern biology - until you consider that Mother Nature has been doing it for millions of years. A humble bacterium with a 25-cent name, Agrobacterium tumefaciens, was changing the genetic makeup of plants long before humans developed opposable thumbs, let alone picked up a test tube.
The crown gall bacterium uses its genes as a weapon to attack plants. It enters the plant from the soil through wounds on the roots or lower stem (hence the name crown gall, because the base of a plant is called the crown). Once inside, it slips a chunk of its DNA, called a plasmid, into a host cell. The fun begins when the plasmid links up with the plant's own DNA. The altered plant cells start dividing rapidly and uncontrollably, and the root or stem develops a tumor-like swelling. At first these swellings, called galls, are whitish, rounded, and fairly soft. As they enlarge, galls become dark and woody with a rough, irregular surface. Galls can range from pea-size to softball- size.
All of these changes make the crown gall bacterium happy, because it can feed easily in developing galls. The plant is less fortunate, though. The expanding gall squeezes and finally crushes the water-conducting tissues, constricting the flow of water to the upper parts of the plant by as much as 80%. Galled plants become stunted and leaves may become undersized. Reduced vigor means that infected plants are more vulnerable to environmental stresses, especially winter injury.
Over 40 plant families can be attacked by crown gall. In Iowa, the most commonly attacked plants are probably certain species of euonymous (a woody shrub) and roses. Other hosts include willow, poplar and other shade trees, nut trees, pome fruits (apple and pear) and stone fruits (cherry, peach, plum), tomatoes, honeysuckle, and raspberries. Because the crown gall bacterium can be spread on tools used to make grafts, large losses can occur in nurseries, where entire infected plantings may have to be destroyed.
At the nursery, crown gall prevention starts with clean, careful propagation methods. Grafting knives are sterilized frequently with alcohol to kill bacteria, and new grafts are wrapped to reduce the risk of infection. On the home front, it's prudent to look over nursery stock carefully and reject any plants with crown gall symptoms. Just as important is taking care to avoid making deep wounds in trees and bushes. This precaution is especially important to keep in mind during transplanting.
Chemical control is not a viable option for crown gall. However, a biological control material called Galltrol is commercially available. Galltrol is a live culture of a close relative of the crown gall organism by the name of Agrobacterium radiobacter. The difference is that the Galltrol bacterium is defanged; unlike crown gall, it lacks the ability to attack plants. To use Galltrol, you mix the bacteria in a bucket of water, then dip the host plants into the mixture immediately before transplanting. The good bacteria stick to the plant surfaces and block the entrance of crown gall bacteria. Galltrol can also be used as a spray or soil drench treatment for crown gall prevention. Galltrol is available from AgBioChem Inc., 3 Fleetwood Ct., Orinda, California 94563 (phone: 510-254-0789).
Because eradicating crown gall from infected plants is difficult or impossible, the best recourse for infected plants is to remove and destroy them. The crown gall bacterium can survive in contaminated soil for at least two years, so avoid replanting susceptible plant species in infested sites during this time period.
The most important take-home messages for crown gall control are to inspect plants that are potential crown gall hosts before you buy them and to avoid wounding potential host plants. These simple precautions vastly reduce the risk of a invasion by crown gall bacteria. No matter how clever these little genetic engineers are, you don't want to witness their wondrous handiwork in your own landscape.
This article originally appeared in the May 19, 1995 issue, p. 73.