Seven Ulmus minor clones tolerant to Ophiostoma novo-ulmi registered as forest reproductive material in Spain

The Spanish elm programme began in 1986 in response to the devastating impact of Dutch elm disease on natural elm stands and urban trees. Its main objectives were to conserve remaining genetic resources and select and breed tolerant native elm genotypes. After 27 years of work conducting susceptibility trials on thousands of elm genotypes, the first seven tolerant Ulmus minor trees are now being registered by the Spanish Environmental Administration. This paper presents the results of the susceptibility tests on these clones and their distinctive genetic, morphological and phenological features. In all susceptibility trials the commercial Sapporo Autumn Gold clone, which is highly tolerant to O. novo-ulmi, was used as a control. The registered clones were named Ademuz, Dehesa de la Villa, Majadahonda, Toledo, Dehesa de Amaniel, Retiro and Fuente Umbria. The most tolerant clone was Dehesa de Amaniel, as its wilting values were below 5% during the two consecutive inoculation trials performed in Madrid. Fuente Umbria, tested over four consecutive years in Guadalajara and Palencia, was the Spanish clone with the most reliable tolerance level to O. novo-ulmi. The Ademuz and Majadahonda clones had the highest ornamental scores and are promising trees for use in urban environments and tree breeding for ornamental quality. These two genotypes showed a later bud burst phenology than the other U. minor clones, demonstrating suitability to areas with late frost events. The Spanish programme aims to substantially increase the range of tolerant native elms through new selections and crossings to gain a better understanding of the genetic basis of resistance.