Plum pox virus (PPV) is the most important and devastating virus disease causing the highest economic impact in stone fruits worldwide (Cambra et al., 1994). The virus, classified as a quarantine pest (EPPO A2 list, EU 2000/29 EEC, annex II), was first detected in Bulgaria in 1900, and is now widespread in most European countries. The hosts of PPV are the fruit - producing species and wild and ornamental species of Prunus. Wild hosts can act as a natural reservoir for PPV. Eight PPV strains have been identified based on their biological, serological and molecular properties. One example for the biological relevance of this interspecific diversity is the PPV-M strain causing more severe symptoms in peaches than the PPV-D strain. For an effective control and surveillance strategy it is important to use PPV-free plant material, tolerant/resistant cultivars and eradicate infected plants. To reduce the spread of this disease, a reliable and sensitive detection method (to be used e.g. on propagation material) is necessary. CTAB and RNeasy nucleic acid extraction coupled with highly sensitive TaqMan real-time RT-PCR was demonstrated not reliable for stone fruit propagation material routine testing. Spot TaqMan real-time RT-PCR is recommended for that purpose. Researchers have previously observed that PPV infection produces modifications in some chlorophyll fluorescence parameters. The effect of the treatments with mandelonitrile (MD) or phenylalanine (Phe) on PPV symptomatology and in the chlorophyll fluorescence in peach GF-305 and the effect of PPV infection on cyanogenic glycoside turnover and salicylic acid (SA) biosynthesis were analysed. The results suggest that under stress conditions MD treatment, and hence the new SA biosynthetic pathway described, affect the SA-mediated H₂O₂ content. This finding supports the hypothesis that the SA biosynthesis from MD also occurred under stress conditions.