Bacteria are single-celled prokaryotic (without nuclei) organisms, which are very different from eukaryotic and mostly multicellular fungi. Bacteria can cause plant diseases in addition to their roles in important human and animal diseases. For example, fire blight of apple and pear and citrus canker have ravaged fruit tree groves in the United States and elsewhere. Another important bacterial plant pathogen is Agrobacterium tumefaciens, which causes a disease called crown gall on a wide variety of plants. The bacterium is able to alter the genetic information in plant cells and is thus called a natural genetic engineer.
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