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Resistance breeding of the native elms against Dutch elm disease, caused by the fungus Ophiostoma novo-ulmi Brasier, is a major objective in Europe for the conservation of this tree species. More than 2,500 cuttings of 324 elm clones (Ulmus minor Miller, U. glabra Huds., U. laevis Pall., U. pumila L., U. minor x U. glabra, and U. minor x U. pumila) from eight European countries, planted in several randomized two-block designed plots were inoculated with various O. novo-ulmi strains. Crown wilting and dieback were recorded during the first year after inoculation. The wilting of the control trees varied among the plots, making the results difficult to compare, but analysis of variance (ANOVA) within each plot showed significant differences in disease severity among the clones tested, allowing study of the variation of the response among elm taxa. Results showed that 19 European inoculated clones recovered from O. novo-ulmi attack, giving hope for the reestablishment of native elms in countryside hedges and forests.

The bud-burst dates of clones of Ulmus minor, Ulmus glabra, and Ulmus laevis were recorded in the field during the spring of 2000, 2001 and 2002 at six locations in five European countries. Meteorological data were obtained from stations close to the plots. Thermal time to bud-burst (T) and chilling (C) were calculated. A relationship on the form T = a + b erC was fitted for each species. In the three species, winter rest release seems to be controlled by two opposite quantitative effects of the temperature, which interrelate according to an inverse exponential relation. Besides, the bud-burst date was found to be stable between years characterised by different winter thermal trends. These three Elm species have small chilling requirements. U. minor has chilling requirements lower than those of the other two species, in agreement with the more southern distribution. Durante las primaveras de los años 2000, 2001 y 2002 se registró en campo, en seis localidades de cinco países europeos, la fecha de apertura de yemas en un número de clones pertenecientes a Ulmus minor, Ulmus glabra y Ulmus laevis. Simultáneamente se obtuvieron datos meteorológicos en estaciones próximas a las parcelas. Se calculó los grados día (T) y el requerimiento de frío (C). Para cada especie se estableció una relación de la forma T = a + b erC. En las tres especies, la interrupción de la quiescencia parece estar controlada por dos efectos termales cuantitativos opuestos, relacionados por una exponencial inversa. Se ha encontrados que la fecha de apertura de yemas ha sido estable incluso en años con características térmicas invernales considerablemente diferentes. Los requerimientos de frío son pequeños en las tres especies de olmo. U. minor requiere fríos menores que las otras dos especies, lo que es consistente con su distribución más meridional.