This paper describes an mRT-PCR assay for the simultaneousdetection of three viruses infecting pear trees. Pear is a perennialfruit crop, in which viruses can replicate and spread to the wholeplant, and then persistently affect its development. Although morethan ten viruses and virus-like pathogens have been reported, Applechlorotic leaf spot virus (ACLSV), Apple stem pitting virus (ASPV) andApple stem grooving virus (ASGV) are the most commonly occurringviruses in commercially cultivated pear plants, and these virusesare also commonly present in apple plants. They have been includedin the lists of pathogens requiring tests in certification schemes inmany countries (EPPO, 1999). The mRT-PCR assay described aboveprovides a sensitive tool for the simultaneous detection of theseviruses in a large number of pear materials. For the effective appli-cation of mRT-PCR assay for virus detection in specific hosts, it isimportant to determine experimentally the specificity and sensi-tivity of the primers and to optimize the PCR conditions in thosehosts. In this study, serial PCRs were tried by using different anneal-ing temperatures, primer concentration ratios, and primer pairsto determine the best reaction program and primer combinationfor the mRT-PCR assay. Using the optimized mRT-PCR assay, theexpected fragments of 500 bp, 358 bp and 247 bp specific for ASGV,ACLSV and ASPV could be identified clearly in agarose gel, andthere was not the interference of un-specific products in the PCRproducts. The results indicated that the primer pairs selected werevirus-specific, and that the optimized PCR conditions were idealfor the rapid detection and discrimination of those three viruses inRNA preparations from pear plants.