Crown gall disease (tumor) is a type of cancer disease that occurs by forming tumors in the root and root collar of many woody and herbaceous plant species. The occurrence of the disease is caused by 4 types of soil and plant pathogens:
- Agrobacterium tumefaciens
- Agrobacterium rhizogenes
- Agrobacterium rubi
- Agrobacterium vitis
Among these pathogens, the most common pathogen causing this disease is the bacterium Agrobacterium tumefaciens.
The bacterium that causes this root cancer disease is rod-shaped and has a peritric whip with dimensions of 0.4-0.8x1.0-3.0 µm. Virulent bacteria carry several large plasmids. One of these plasmids has genes that cause tumor formation and is called a Ti-plasmid. Bacteria that do not have Ti-plasmid or lose Ti-plasmid by heating are not virulent. The agent is Gram-negative and not acid-resistant. The optimum development temperature is 25-30°C, the minimum is 0°C, and the maximum development temperature is 37°C. The thermal death point of the bacterium is 50-52°C.t
The agent is able to live in the soil for a long time and maintain its virulence in the treated soil. It has been found that it spreads more on alkaline soils than on those with a slightly acidic character. Bacteria can maintain their vitality for many years as saprophytes in the soil. When a host plant is planted in infected soils, it enters the plant from fresh wounds opened for reasons such as cultural processes, vaccination, insect and nematode feeding in the parts of the root or trunk close to the soil. Bacteria that multiply in intercellular spaces genetically transform plant cells, stimulating excessive and uncontrolled cell division, causing tumor formations. Bacteria re-enter the soil from the surface of these galls. The disease is spread by infected seedlings and soils.
Symptoms and Spread
The disease occurs with fruit trees, at the roots of some forest and park trees and in parts of the trunk near the soil surface. Although the location of the disease is the crown (root collar) of the trees, rarely it may occur in the root and the parts above ground. On thin and deep roots, the disease almost never occurs. The rate of occurrence of the disease increases as the roots thicken and approach the crown.
The disease begins as a result of excessive proliferation of parenchyma cells where the bacterium infects. The first symptom of the disease is a small tumor. During the growing season, a visible gall can be seen in 2-4 weeks. At the beginning of the formation of tumor, the tumors are white or skin-colored and rather soft, not easily distinguishable from the scar tissue (callus). Tumor tissue usually develops faster than scar tissue. With the decay of cells on the outer surface of the tumors, the color of the tumors turn into dark coffee-black. Some galls can be easily broken down and easily separated from plant tissue. Some galls, on the other hand, are more woody and hard in structure. The dimensions of the galls can vary from the size of a nut to a diameter of up to 30 cm. Some of the tumors completely rot and lose their activity, while others may show fresh developments again from any surface in the following season.
The disease, in addition to the formation of tumors in plants, also causes symptoms such as stunting together with small and chlorotic leaf growth. Infected trees with these small-sized leaves become weak and inefficient. Pests of various types and sizes can settle on the lower parts of the leaves of almond trees with water stress. The most common symptom is yellowing or tanning of the leaves, followed by defoliation.
Sick plants are much more susceptible to adverse environmental conditions, especially winter damage. Plants with tumors in the root collar or in the main roots develop poorly, and a decrease in their yield occurs. Heavily infected plants can die.
In nurseries, tumors of the size of hazelnuts or walnuts are seen in the root collar and the roots of the seedlings. Tumors are initially cream-colored, then turn darker and their surface cracks. Seedlings that are severely affected by the disease can not develop well. Young trees are very affected by infection and dry out in a short time. Older trees, on the other hand, produce little and poor-quality fruit.
Measures of prevention
- In soils with heavy and high water retention capacity, nursery and fruit growing facilities should be avoided when possible. Drainage channels should be opened if a facility is already established.
- When setting up a nursery, it should first be checked whether the nursery soil is contaminated with this bacterium.