Crown Gall
Introduction
Crown gall is a type of plant disease caused by the bacterium Agrobacterium tumefaciens. This disease is characterized by the formation of tumor-like growths, typically at the crown of the plant, where the stem and root meet. The disease can affect a wide range of dicotyledonous (broad-leaved) plants and some monocotyledonous (narrow-leaved) plants.
Etiology
The bacterium Agrobacterium tumefaciens is the causative agent of crown gall disease. This bacterium is a Gram-negative, rod-shaped bacterium that is commonly found in the soil. A. tumefaciens is unique among pathogenic bacteria because it is capable of transferring a portion of its own DNA into the plant cells it infects, a process known as horizontal gene transfer. This DNA transfer is facilitated by a plasmid (a small, circular piece of DNA) known as the Ti (tumor-inducing) plasmid.
Pathogenesis
The pathogenesis of crown gall disease begins when A. tumefaciens enters a plant through a wound. Once inside the plant, the bacterium uses its Ti plasmid to transfer a segment of DNA known as T-DNA (transfer DNA) into the plant cell. This T-DNA integrates into the plant cell's genome and causes the cell to produce two types of compounds: auxins and cytokinins, which are types of plant hormones. The overproduction of these hormones leads to uncontrolled cell division and the formation of a gall or tumor.
Symptoms
The primary symptom of crown gall disease is the formation of galls or tumors at the crown of the plant. These galls can vary in size from small, pea-sized growths to large, woody tumors. The galls can disrupt the normal growth and development of the plant, leading to stunted growth, reduced yield, and in severe cases, death of the plant. Other symptoms can include yellowing of the leaves (chlorosis), wilting, and reduced vigor.
Diagnosis
Diagnosis of crown gall disease is typically based on the characteristic symptoms of the disease. Laboratory tests, such as PCR (polymerase chain reaction) or ELISA (enzyme-linked immunosorbent assay), can be used to confirm the presence of A. tumefaciens in the plant tissue.
Management and Control
Management of crown gall disease involves a combination of cultural practices, biological control, and chemical control. Cultural practices include avoiding injury to plants, as wounds can provide an entry point for A. tumefaciens. Biological control involves the use of non-pathogenic strains of Agrobacterium to compete with the pathogenic strains. Chemical control can involve the use of bactericides or soil fumigants, although these methods are not always effective and can have negative environmental impacts.
Impact
Crown gall disease can have a significant impact on agricultural production, particularly in the nursery and orchard industries. The disease can reduce yield and quality, and in severe cases, can result in the loss of entire plants. In addition to the economic impact, crown gall disease can also have ecological impacts, as the disease can alter plant communities and disrupt ecosystems.