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A virus has either a DNA or an RNA genome and is called a DNA virus or an RNA virus, respectively. The vast majority of viruses have RNA genomes. Plant viruses tend to have single-stranded RNA genomes and bacteriophages tend to have double-stranded DNA genomes.
1 The presence of viruses in host cells often results in disease. 2  400 or more viruses are known to attack plants (2000 viruses are described for plants, animals, bacteria, etc.). 3  viruses are generally specific, what infects a plant does not cause disease in an animal, and vice versa. 1971 - T. O. Diener discovered viroids, which only consist of nucleic acids. 2  Smaller than viruses, caused potato spindle tuber disease (250-400 bases long of single-stranded circular molecule of infectious RNA). 3  About a dozen other viroids that cause disease in a variety of plants have been isolated.