Crown gall (Agrobacterium tumefaciens) occurs in more than 140 plant genera, according to the University of Illinois. It afflicts woody and herbaceous plants by entering their root systems and altering their genetic makeup. The plant develops a large tumor growth on its root system or crown base. The gall tumor appears round, soft and slightly white in color. As the gall grows, it develops a rough, woody surface. Its color deepens to dark brown.
The crown gall bacteria is located in soil. It infects the root system of the plant through small wounds that are sustained from over-cultivation, nematode damage and insect feeding. The bacteria finds the wounds on the root system by the phenolic compounds and sugars that the injured plant releases from the wound site, according to the University of Massachusetts.
Crown gall bacteria enters the plant's root system and quickly begins to slip its own DNA, known as a plasmid, into the plant's cell. The crown gall bacteria's DNA links with the plant's DNA, which causes a rapid cell division to begin due to abnormal hormone production, according to Virginia State University. The hormones, auxins and cytokinins, cause the development of a large tumor or gall on the plant.
As the gall grows, it quickly begins to choke off the plant's water supply by up to 80 percent, according to Iowa State University. The plant quickly loses its vigor. The leaves often appear smaller and the overall growth of the plant is slowed. Leaves may turn yellow and drop from the plant.
As the gall grows, it begins to decompose and break down. When the gall cracks open the bacteria returns to the soil to re-infest new plants. Once back in the soil, the bacteria remains active and viable for up to two years, according to the University of Maine.
No cure exists for crown gall infection; rather, the plant must be lifted and destroyed. Removal of the gall has no effect because the crown gall bacteria invades the entire plant system and will only form a new gall in a new location on the plant. In areas where the soil is known to harbor the crown gall bacteria, the gardener should only plant crown gall-resistant plants, shrubs and trees. Conifers, golden rain trees, tulip trees and beech trees offer resistance to crown gall.