More than 600 Prunus samples were examined by using a nonradioactive digoxigenin‐labelled RNA probe specific for hop stunt viroid (HSVd). Prunus salicina and Prunus armeniaca appeared to be better hosts than Prunus persica. The weak viroid concentration in flowers and young leaves of peach trees growing in the field did not permit its detection in such samples. The diagnosis was more reliable (about 85%) with bark and leaves aged 4 months and more, from regrowths of GF 305 peach seedlings inoculated and kept in the greenhouse. Detection of HSVd in leaves and bark of apricot and Japanese plum plants aged 3 months or more also proved reliable (about 80% and 90%, respectively). HSVd could be transmitted in apricot, peach and plum nucleic acid preparations to GF 305 peach seedlings by repeated stem slashing, and to cherries (Prunus avium and Prunus serrulata) by approach grafting with an infected P. salicina source. The viroid was eliminated from 18% of the clones obtained after thermotherapy. In the course of this study, 25 selected Prunus accessions suspected to be infected by unusual diseases were analysed by hybridization with a HSVd‐specific probe and by indexing on GF 305 peach seedlings in the greenhouse. Fifteen of these accessions were found to be infected by HSVd, 19 induced reddish marbling, and four induced small blackish spots on the leaves aged about 4 months. Repeated assays showed that these foliar symptoms were not caused by the viroid. Peach red marbling (PRMa) has not been associated with any known virus and seems to be caused by an infectious agent not yet described. That could also be the case with the agent of peach sooty ringspot (PSRS). PRMa and PSRS symptoms were reproduced by grafting and indexing, and their causal agents eliminated by thermotherapy in a significant fraction of the treated plants. They behave like viral agents and can infect the different Prunus species studied.