Crown gall is a common plant disease characterized by the formation of tumor-like growths, or galls, on the plant’s surface. This condition affects a wide variety of plants, including woody ornamentals, shrubs, and economically important fruit trees like stone fruits and grapes. Crown gall disrupts the plant’s normal functions, leading to reduced vigor and yield, especially in younger plants. Understanding the biological mechanism and visible symptoms is key to effective management.
The Pathogen and Its Mechanism
Crown gall is caused by the soil-dwelling bacterium Agrobacterium tumefaciens, which manipulates its host using a unique mechanism. The bacterium carries the tumor-inducing (Ti) plasmid, which contains a segment of genetic material known as the T-DNA.
When the bacterium encounters a fresh wound, it transfers the T-DNA into the plant cell. Virulence genes activate a process that cuts the T-DNA segment and transports it into the host cell, where it integrates into the plant’s genome.
The integrated T-DNA codes for the uncontrolled production of plant hormones, specifically auxin and cytokinin. This hormonal imbalance forces the plant cells to divide rapidly, manifesting as the visible gall. The T-DNA also instructs the plant to produce opines, which the bacterium uses as a specialized food source.
Identifying Symptoms and Host Range
The presence of galls is the definitive sign of this disease, though their appearance changes as they develop. Galls begin as small, pale, soft outgrowths, often resembling callus tissue. Over time, these masses enlarge, darken to brown or black, and become rough, hard, and woody.
Galls most often form at the crown (the junction of the stem and roots), but they can also appear on roots, lower stems, or higher branches. Gall size is highly variable, ranging from less than a half-inch to several inches in diameter.
Young or severely infected plants suffer from the disrupted flow of water and nutrients caused by the disorganized tissue within the gall. This disruption leads to stunted growth, yellowing foliage, and reduced fruit yield, sometimes resulting in the plant’s death.
Crown gall has an extensive host range, affecting woody and herbaceous varieties. Susceptible plants include ornamentals like roses and willows, and commercially grown fruits. Stone fruits (cherries, plums, and apricots) and grapes are particularly vulnerable. The soil-borne bacteria can survive for years, making replanting susceptible species in previously infected areas a risk.
Control and Prevention Strategies
Minimizing injury to roots and stems is the most effective preventative measure, as the bacterium enters the plant only through fresh wounds. Gardeners should be careful when cultivating around susceptible plants to avoid damage from tools or insects. Primary prevention also involves using certified disease-free nursery stock and thoroughly inspecting the roots and crowns of new plants for suspicious lumps before planting.
For young or infected plants, the recommended action is often to remove and destroy the entire plant, including the surrounding soil, to reduce the pathogen’s presence. If galls are isolated on a stem or branch of an established, valuable plant, they can be carefully pruned out. All pruning tools must be immediately disinfected using a solution of bleach or alcohol after use to prevent spreading the bacteria.
In commercial settings or for high-value crops, a biological control agent can be used during planting. This involves treating the roots of bare-root plants with a non-pathogenic bacterium, typically Agrobacterium radiobacter strain K84. This beneficial strain colonizes wound sites and produces agrocin 84, a natural antibiotic that inhibits the growth of the harmful crown gall bacteria. This proactive step significantly protects young plants from initial infection.