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Disease Cycle and Epidemiology

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Disease cycle of crown gall caused by Agrobacterium tumefaciens
(Courtesy C. Kado).

Disease Cycle and Epidemiology

Agrobacterium tumefaciens naturally resides on the rhizoplane of woody and herbaceous weeds. Its presence in soils originates from galls that were broken or sloughed off from infected plants during cultivation practices or disseminated as infected plant material. Irrigation aids in further dissemination of the A. tumefaciens bacterial cells. Agrobacterium tumefaciens is also spread by infected and infested planting stocks originating as nursery stock from uncertified sources. Secondary spread then occurs through pruning and cultivation equipment, particularly when galls are removed manually with the same cutting tools used in pruning. Tilling equipment can be contaminated by cutting through galls at or near the base of trunks of infected trees. Rogueing (removal) of infected trees and replanting in the same spot where the infected tree had grown is poor practice because sloughed off galls serve as sources of abundant populations of A. tumefaciens.

Although a single cell can be transformed and grow into a gall, the size of the initial tumor depends on the amount of inoculum. Thus, if a large number of cells of A. tumefaciens enter a plant wound, disease will be more severe and a large gall will be seen rapidly growing at the infection site (Figure 6). Plants that are systemically infected (such as grapevines) will harbor A. tumefaciens for extended periods of time in the absence of overt symptoms. Tissue injuries induced by accidental wounds made during cultivation practices or by frost will elicit new infections with the appearance of numerous galls along the vascular system of the host (Figure 4).


Figure 4.	Figure 6.