Plant virus and viroid diseases can be traditionally detected by bioassay on suitable plant cultivars. This assay is very sensitive, but unfortunately it is laborious, expensive, and time consuming. The nucleic acid hybridization assay is based on the formation of a duplex "target-probe" between the nucleic acid of a pathogen and a pathogen-specific complementary nucleic acid. The duplex formation process is termed the hybridization reaction. An important step in the dot-blot nucleic acid hybridization assay is denaturation of nucleic acids, because for successful binding to nitrocellulose, the nucleic acid should not have a secondary structure. The quantitative aspects of thermodynamics and kinetics of nucleic acid hybridization are discussed in detail elsewhere. After the washing procedure, products of the nucleic acid hybridization assay can be detected using an appropriate method. Duplex developing procedures may vary, depending on the kind of label attached to the probe molecule.