Biology (Year 12) - Specific Disease Studies
What is crown gall? Crown gall infection is caused by various bacteria of the genus Agrobacterium, usually Agrobacterium tumefaciens. The bacterium can live for many years in soil and can multiply in the root zone of susceptible plants. It infects the plant through a wound, usually at ground level or on the roots, and transfers through a tumorigenic (cancerous) factor to some plant cells. How is crown gall transmitted? The Agrobacterium tumefaciens can be transmitted from one host to another directly (via infected plant root to susceptible plant root contact) or indirectly (via grafting tools, car tyres, infested soil reservoirs, or movement of infested soil by boots or rain). The bacteria require a wound as a portal of entry. What is the typical pathology of crown gall? Symptoms include roundish rough-surfaced galls (woody tumour like growths), several centimetres or more in diameter, usually at or near the soil line, on a graft site or bud union, or on roots and lower stems. The galls are at first cream-coloured or greenish and later turn brown or black. As the disease progresses, plants lose vigour and may eventually die.
What is the life cycle of crown gall? Bacterial spores released from infected plant galls. Direct or indirect modes of transmission occurs, where spores are transmitted by contact or by vehicle into wounded roots in a susceptible plant (the portal entry is through a wound). Bacterium attaches to a wounded plant cell and multiplies, and then transfers a plasmid into plant cells. Plasmid contains genes for uncontrolled cell growth. Plant cell genome transforms and starts to divide, rapidly forming galls. Galls grow around the crown of the plant and stunted plant growth results. Spores can be released to continue the cycle.
What is crown gall's pathogenicity? The invasion of the bacterial cells is the disease causing factor in crown gall disease. The plant’s response to the invasion by forming tumour-like growths over wounds to stop the spread of the infection which eventually leads to a limitation in the flow of nutrients and water. Transfer of plasmids, meaning it can make the compounds needed to make the proteins it needs. Hijacking the machinery in the plant.
What are the treatments/prevention/control measures? Unfortunately, the best course of action for plants affected by crown gall is to remove and destroy the infected plant. The bacteria can persist in the soil for two years after the plant is gone, so avoid planting any other susceptible plants in the area until the bacteria dies out for lack of a host plant.
Prevention is an essential aspect of dealing with crown gall. Inspect plants carefully before you buy them and reject any plants with swollen knots. The disease can enter the plant in the nursery through the graft union, so pay particular attention to this area. To prevent the bacteria from entering the plant once you get it home, avoid wounds near the ground as much as possible. Use string trimmers with care and mow the lawn so that debris flies away from susceptible plants.