SUMMARY Apple scar skin viroid (ASSVd), Pear blister canker

are widely distributed and economically important pome fruit viroids, all belonging to genus Apscaviroid. For a rapid and efficient detection method of these vi-roids, we have developed a one-tube RT-PCR protocol that amplifies simultaneously ASSVd, PBCVd and ADFVd from their respective hosts. Multiple alignment and comparison of the nucleotide sequences of ASSVd, PBCVd and ADFVd allowed us to select a conserved region and design a primer pair for their polyvalent amplification in a single RT-PCR reaction. These newly designed primers amplify a specific DNA fragment which contains a major portion of each viroid: ASSVd 267 bp, PBCVd 261 bp and ADFVd 252 bp. The development of this RT-PCR protocol enabled us to identify different isolates of the three viroids and to evaluate their distribution in cultivated and wild pome fruit germplasm. Key words: genus Apscaviroid, pome fruit viroids, polyvalent detection, single primer pair, RT-PCR. Viroids are plant pathogens wich may cause severe diseases (Garnsey and Randles, 1987). Viroids are infec-tious, low molecular weight single-stranded RNA mole-cules 246 to 399 nucleotides in size, have autonomous replication and are currenly classifies in two families and seven genera (Flores et al., 2000). Among important viroids in the genus Apscaviroid. Apple scar skin viroid (ASSVd), the type member of this genus, was the first viroid reported to infect pome fruit trees (Malus, Pyrus and Cydonia spp.) (Hashimoto and Koganezawa, 1982). The symptoms induced by ASSVd infections are restricted to fruits and include color dap-pling, cracking, scarring and distortion. Three other diseases, apple dapple (Hadidi et al., 1990), pear rust