Causal organism: Agrobacterium tumefaciens
Symptoms
- The disease first appears as small overgrowths or galls on the roots, crown, trunk or canes. Galls usually develop on the crown or trunk of the plant near the soil line or on underground roots.
- Above ground or aerial galls may form on canes of brambles of highly susceptible cultivars of pear. Aerial galls are not common on fruit trees.
Below ground symptoms
- In early stages of development galls appear as tu¬mor-like swellings that are more or less spherical, white or flesh-colored, rough, spongy (soft) and wart-like.
- They usually form in late spring or early summer in each season. As galls age, they become dark brown to black, hard, rough, and woody. Some disintegrate with time while others may remain till the life of the plant.
Above ground symptoms
- The tops of infected plants may appear normal. In severe infection, plants may be stunted; produce dry, poorly-developed fruits, or show various deficiency symptoms due to impaired uptake and transportation of nutrients and water.
Causal Organism
- The crown gall bacterium is soil-borne and persists for a long time in the soil in plant debris. It requires a fresh wound to enter and infect and initiate gall formation.
Infection process
- Wounds that commonly serve as infection sites are those formed during pruning, machinery operations, freezing injury, growth cracks, soil insects and many other factors that causes injury to the plant tissue.
- Bacteria are abundant in the outer portion of primary galls which is often sloughed off into the soil. In addition to primary galls, secondary galls may also form around other wounds and other portions of the plant in the absence of the bacterium.
- The bacteria overwinter inside the plant (systemically) in galls or in the soil. When they come in contact with wounded tissue of a susceptible host, they enter the plant and induce gall formation, thus completing the disease cycle.
- The bacteria are most commonly introduced into the planting site or orchard planting materia
Management
- Obtain clean (disease free) nursery stock from a reputed nursery centre and inspect the roots and crowns critically to make sure that they are free from galls. Avoid planting healthy materials in the sites infested with the bacteria in the previous year.
- Avoid all types of root, crown, and trunk injuries due to careless cultivation and other machinery operations, and also control soil insects. Any practice that reduces injury is highly beneficial. Preventing winter injury (especially on pears) is also beneficial.
- In pears, the double trunk system of training is a useful system for minimizing losses due to crown gall. In case one trunk is infected, it can be removed and other trunk can be pruned leaving a large number of buds until the second trunk is renewed. Galls on the upper parts of the trunk or canes can be removed by pruning.
- A relatively new biological control agent for crown gall is available for pear, pear, stone fruit, blueberry, brambles, and many ornamentals.
- The agent is a nonpathogenic strain of bacterium (Agrobacterium radiobactor strain K - 84) that protects the plants against infection by the natural presence in the soil.
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