The detection of Apple chlorotic leafspot virus (ACLSV), Apple mosaic virus (ApMV), Apple stem grooving virus (ASGV) and Apple stem pitting virus (ASPV) was investigated with infected apple mother trees over a period of 17 months. Relative virus concentrations were estimated once a month in leaf samples and in bark with buds from May to November and January to April, respectively, in a serial dilution series of sap extracts using the silica nucleic acid extraction method and detection by reverse transcription PCR (RT-PCR). The greatest dilution limits, indicat­ing the highest virus levels were found most frequently in the period from January to April, but these were different for the individual viruses. The values were highest for ApMV followed by ACLSV, ASPV and ASGV. In the remaining times a heterogeneous course of virus concentration was recorded for the viruses, also influenced by the year of sampling. Nevertheless, each virus was always detectable with the lowest dilution level of 1:10 (w/v) in plant extracts regardless if leaves or bark with buds were examined. From January to April all viruses were also reliably detected in dilutions up to 1:100, poten­tially enabling the use of pooled samples from different trees.