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Description of Agrobacterium tumefaciensThis is the most widely studied species in the genus. Strains of Agrobacterium are classified in three biovars based on their utilisation of different carbohydrates and other biochemical tests. The differences between biovars are determined by genes on the single circle of chromosomal DNA. Biovar differences are not particularly relevant to the pathogenicity of A. tumefaciens, except in one respect: biovar 3 is found worldwide as the pathogen of gravevines. This species causes crown gall disease of a wide range of dicotyledonous (broad-leaved) plants, especially members of the rose family such as apple, pear, peach, cherry, almond, raspberry and roses. Becazusde of the way that it infects other organisms, this bacterium has been used as a tool in plant breeding. Any desired genes, such as insecticidal toxin genes or herbicide-resistance genes, can be engineered into the bacterial DNA, and then inserted into the plant genome. This process shortens the conventional plant breeding process, and allows entirely new (non-plant) genes to be engineered into crops.