Abstract: In top fruit production an infection with Candidatus Phytoplasma mali can causesevere damage and economic losses to the crop. Occurrence of both diseases, apple proliferationin apple and pear decline in pear, has been reported in several European countries. In Belgiumonly symptomatic trees of apple proliferation (AP) were observed sporadically and eradicated inthe past. In a recent research project a survey was carried out to obtain information on thepresence of AP in asymptomatic apple trees. Root samples of apparently healthy trees werecollected in different apple orchards (production and private) and analysed by real-time PCR.Results confirmed the presence of the pathogen in both symptomatic and asymptomatic trees. Assome Cacopsylla spp. are able to transmit Ca. P. mali from infected trees to healthy trees withinand between orchards, the dynamic of the psyllid population was studied in 4 apple orchards andin 5 hawthorn hedges from March till August. Several species were indentified: C. mali,C. melanoneura and C. picta in apple, C. melanoneura, C. peregrina, C. affinis and C. crataegiin hawthorn. PCR- analysis of individuals of C. mali and C. peregrina revealed their infectivitybut in transmission trials performed with C. mali under lab conditions no transfer of AP to anartificial feeding source could be obtained.