Viruses are very small pathogens, composed of nucleic acid (DNA or RNA) with a protective protein coat. Viroids are even smaller pathogens, consisting only of pieces of RNA. Viruses and viroids cause many important plant diseases including tobacco mosaic, the “guinea pig” of early virus studies. Viruses are visible only with an electron microscope, can replicate (copy themselves) only when inside a living plant cell, and usually require an insect to move from plant to plant. Important molecular techniques now commonly used to detect many plant pathogens were first developed to detect plant viruses..
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