Crown gall

The causative agent of this disease is the soil-borne bacterium Allorhizobium rhizogenes, formerly known as Agrobacterium tumefaciens. This pathogen is unique for its ability to perform natural genetic transformation of plant cells.

The bacteria contain a tumor-inducing (Ti) plasmid. When the plant is wounded, the bacteria enter the tissue and transfer a specific segment of their DNA into the plant's genome, forcing the plant to produce hormones and nutrients.

This genetic modification leads to uncontrolled cell division, which results in the formation of abnormal, cancerous growths known as galls. These structures provide a protected habitat and nutrients for the bacteria.

The pathogen is highly persistent in the soil, capable of surviving for several years without a host, which makes it a persistent threat in nurseries and orchards.

Crown gall has a broad host range, infecting more than 600 species of dicotyledonous plants, including top fruits, ornamentals, and various vegetables.

The primary symptom is the development of rough, tumor-like galls on roots, the root collar, or stems. Initially, these are small, pale, and soft, but they gradually enlarge and harden.

As the galls mature, they become dark, woody, and frequently crack open, providing entry points for secondary pathogens and decay organisms.

Above-ground symptoms include stunted growth, chlorosis, reduced vigor, and increased susceptibility to environmental stress such as drought and frost.

In severely infected young plants, the structural integrity of the root system can be compromised, leading to premature plant death and yield failure.

Regular visual inspections, especially of the root collar during transplanting, are essential to identify the disease before it establishes in the garden or orchard.

Allorhizobium thrives in moist, alkaline to neutral soils (pH 7.0–8.0). Acidic soils are generally less conducive to the survival and infection process of the bacteria.

Warm temperatures (20–30°C) favor bacterial activity. The presence of open wounds is necessary for the initial infection, which makes post-planting and cultivation care critical.

Insects and nematodes feeding on root systems often create the mechanical damage required for bacterial entry, facilitating the spread of the disease.

Poor drainage and excessive watering can create a favorable environment for the pathogen, as the bacteria move through soil water to find new infection sites.

Continuous cropping of susceptible species without proper rotation encourages the build-up of high bacterial populations, making it nearly impossible to grow healthy plants on that plot.

Crown gall causes significant physiological damage by disrupting the plant's vascular system. The galls act as a sink for nutrients, starving the rest of the plant.

The disease severely impacts fruit production and marketability. Infected trees often exhibit decreased fruit size, quality, and overall longevity.

In commercial nurseries, crown gall is considered a quarantine disease. Even a single infected plant can lead to the condemnation of the entire shipment, resulting in massive economic losses.

The physical damage caused by the tumors can weaken the tree's anchorage, making it susceptible to toppling during strong winds.

Furthermore, because the bacteria persist in the soil for years, the economic impact is long-term, requiring soil treatment or switching to non-host crops.

Prevention is the only truly effective strategy. Always source plants from reputable, disease-free nurseries and inspect roots thoroughly before planting.

Biological control using specific non-pathogenic strains (like Agrobacterium radiobacter strain K84) has proven effective when applied to roots before planting as a preventative measure.

Proper sanitation is vital; disinfect all tools used for pruning and grafting between plants using alcohol or appropriate bleach solutions to prevent cross-contamination.

Avoid injuring the roots during cultivation and pest management. Control populations of soil-dwelling pests like nematodes and grubs to minimize potential infection points.

• Remove and destroy severely infected plants including the root system.
• Avoid planting in infested soil for several years.
• Select resistant cultivars where possible.
• Maintain slightly acidic soil conditions to discourage bacterial growth.