Avoidance by early flushing: a new perspective on Dutch elm disease research

European elms (Ulmus glabra Huds., Ulmus laevis Pall. and Ulmus minor Mill.) have been severely damaged and are still endangered by an alien hypervirulent pathogen, Ophiostoma ulmi s. l., agent of the Dutch elm disease (DED). Consequently, several ex situ clone collections have been established throughout Europe for elm breeding and conservation. In this paper we summarise the studies carried out within the EU project RESGEN CT96-78, which launched the EU-coordinated evaluation of these collections. Our aim was to analyse the variation in timing of bud burst and to acquire basic knowledge on the environmental control of this adaptive trait in European elms, under the hypothesis that DED susceptibility is related to spring phenology. The variation in the bud burst date of European elms among collections and years was explained by a phenological model assuming that the thermal time required for bud burst decreases exponentially with increasing chilling during winter down to a level where chilling requirement is fully met. According to the fitted curves, European elms have low dormancy and short chilling requirement for dormancy release. Although no simulation was performed, on the basis of the model applied we can hypothesise that under climate warming elms would flush earlier in most of Europe. The bud burst date was directly related to latitude and elevation in U. minor. The order of bud burst of clones from different origins was stable among years. The observed geographic trends were largely determined by differences in chilling requirements that increased with latitude and elevation. Susceptibility to DED varied greatly within U. minor and was directly correlated with geographic origin and bud burst date, southern and early flushing clones showing the least symptoms. Our results suggest that early flushing represents a mechanism of disease avoidance owing to asynchrony between host's susceptible period and time of natural infection by DED vectors (Scolytus). This escape mechanism, combined with true resistance and avoidance owing to unattractiveness for the vector's feeding, might be exploited for breeding DED-resistant clones of indigenous species.