European elms (Ulmus glabra Huds., U. laevis Pall. and U. minor Mill.) have been damaged and are still threatened by an alien hypervirulent pathogen, Ophiostoma ulmi s.l., the agent of Dutch elm disease (DED). Therefore, several ex situ clone collections were established throughout Europe for breeding and conservation purposes. This thesis was carried out within the RESGEN CT96-78 project, which launched the EU-coordinated evaluation of these collections. The aim of this thesis was to analyse the variation in bud burst date and to acquire basic knowledge on the environmental control of this adaptive trait with regard to DED susceptibility. Bud burst date variation observed among collections and years in European elms was explained by a phenological model based on an inverse exponential relationship between thermal time and chilling to bud burst. According to the fitted curves, European elms have low dormancy and short chilling requirement for dormancy release. Bud burst date in U. minor was directly related to latitude and elevation. The order of bud burst was stable among years. The observed geographic trends were largely determined by difference in chilling requirement for dormancy release which increased with latitude and elevation. The effects of photoperiod and temperature on dormancy release in clones of European and Asian species were studied in partially controlled conditions in Italy. Dormancy was generally low and short in all clones. There was no evidence that photoperiod influenced dormancy release in these elms. Susceptibility to DED was assessed in the Italian clone collection. Susceptibility varied greatly among taxonomic groups and within the most represented species, U. minor. In this species, DED susceptibility was directly correlated with geographic origin and date of bud burst, southern and early flushing clones showing the least symptoms. The results suggest that earliness of bud burst represents a mechanism of disease avoidance owing to an asynchrony between the susceptible period in the ...