Crown gall is a plant disease characterized by the formation of abnormal, tumor-like growths (galls) on the plant body. These galls result from uncontrolled cell division and enlargement in the infected tissue. The presence of galls significantly reduces the plant’s overall vigor, leading to stunted growth, reduced fruit or flower production, and increased susceptibility to environmental stresses like drought and winter injury. While the disease rarely causes the immediate death of a mature plant, young or heavily infected plants can decline rapidly and may eventually die.
The Primary Sites of Crown Gall Formation
The name “crown gall” points directly to the location most frequently affected: the crown, where the main stem or trunk meets the root system at the soil line. This region is highly susceptible because minor injuries from cultivation, freezing, or insects provide an entry point for the causative bacteria.
Galls commonly form on the main root system below the soil surface, especially on lateral roots or at grafting unions. Initially, these growths are small, whitish, and soft, resembling unorganized callus tissue. Over time, they expand, becoming irregularly shaped, rough, and developing a hard, woody, or corky exterior.
Although the crown and roots are the most common sites, galls can also appear higher up on the plant, including the stems, branches, and leaves of certain susceptible species. On grapevines and caneberries, for example, galls frequently develop along the canes and aerial parts. When the infection becomes systemic, galls can manifest at various points far from the initial infection site.
The Bacterial Cause and Infection Mechanism
The plant tumors are caused primarily by the soil-dwelling bacterium Agrobacterium tumefaciens. This pathogen is unique because it performs a natural form of genetic engineering, transferring its own DNA into the host plant’s genome. Infection can only occur through a fresh wound, as the bacteria need an opening to access the underlying plant cells.
Wounded plant cells release phenolic compounds, such as acetosyringone, which act as a chemical signal to attract the bacteria and activate their virulence genes. The bacteria carry a large tumor-inducing (Ti) plasmid, which contains a specific segment of DNA known as the T-DNA (transferred DNA). Once activated, the bacteria use the Type IV secretion system to export the T-DNA into the plant cell.
The T-DNA then travels to the plant cell’s nucleus, where it integrates into the host cell’s chromosomes. This integrated T-DNA carries genes that code for the production of plant growth hormones, specifically auxins and cytokinins. The plant cell is genetically reprogrammed to overproduce these hormones, resulting in the rapid, uncontrolled proliferation of cells that forms the gall. The T-DNA also forces the plant cell to produce unique compounds called opines, which serve as a specialized food source that only the Agrobacterium can utilize.
How Crown Gall Spreads and Host Susceptibility
The Agrobacterium pathogen is highly persistent and survives for extended periods in the soil, remaining viable for years. Movement is primarily facilitated by water (splashing rain or irrigation runoff) and the movement of contaminated soil. Infected nursery stock is a common method of long-distance transmission, introducing the disease to new gardens and fields.
The bacteria are also readily spread on gardening and farming equipment, including shovels, cultivators, and pruning tools, especially when sanitation is neglected. Any practice that causes a wound to the plant’s roots or lower stem, such as grafting, pruning, or damage from insects, creates a potential entry point for the pathogen.
Crown gall has a remarkably broad host range, affecting over 600 species across more than 90 plant families. The disease is particularly problematic in woody plants and commercially cultivated species. Common susceptible hosts include:
- Stone fruits (peaches, plums, and cherries)
- Pome fruits (apples)
- Roses
- Grapes
- Willows
- Poplars
- Various ornamental shrubs
Managing and Preventing Crown Gall Disease
Management strategies center on prevention and the removal of infected material, as there is no cure for severely infected plants. The most effective preventive measure is to purchase certified disease-free nursery stock and thoroughly inspect the roots and crown of young plants before planting. Gardeners should avoid planting in areas with a known history of crown gall or consider replacing the top layer of soil with clean material if replanting is necessary.
Minimizing mechanical injury to the crown and roots is essential, meaning deep cultivation near susceptible plants should be avoided. When pruning or working with plants, tools must be sanitized frequently, such as by soaking them in a 10% bleach solution or using a commercial disinfectant. This sanitation prevents the physical transfer of bacteria.
For plants with small, accessible galls, the growths can sometimes be surgically removed by cutting well below the galled tissue, though new galls may still form elsewhere. Severely infected plants should be dug up and destroyed completely to reduce the concentration of bacteria in the soil. Biological control agents, such as non-pathogenic Agrobacterium radiobacter strain K84, can be applied to young roots or cuttings before planting. This strain produces an antibiotic that inhibits the growth of the harmful, gall-forming bacteria, offering protection against initial infection.