Comparison of commercial elm cultivars and promising unreleased Dutch clones for resistance to Ophiostoma novo-ulmi

Abstract

© iForest -Biogeosciences and Forestry IntroductionIn the past centuries elms (Ulmus spp.) became a very important part of the landscape in large areas of the Netherlands. Elms have a combination of highly valued characteristics such as no specific requirements concerning soil type, the ability to resist (sea) winds, the very good recovery from mechanical damage, its adaptability to city conditions including its ability to endure de-icing salts and closed pavements and its much appreciated open crown. In fact, the elm is the ideal street and landscape tree in large parts of the Netherlands. This made the elm, and especially U. × hollandica cultivars, the dominant tree in the coastal areas and cities in the western part of the country at the beginning of the 20 th century. The spread of Dutch elm disease (DED), caused by Ophiostoma ulmi (Buisman) and later Ophiostoma novoulmi (Brasier) during the 20 th century however resulted in millions of trees being lost (Hiemstra et al. 2006). Since 1928, elm breeding for resistance to DED has resulted in the release of several more or less resistant cultivars to the European market. Especially in the Netherlands -where the first cultivar ("C. Buisman") was released in 1936 -and the USA, large and long-term selection programs have been performed resulting in a number of hybrid clones with moderate to very good resistance. Also, more recently, other European programs on elm breeding were set up, e.g., in Italy, Spain and France (Mittempergher & Santini 2004). Today a wide range of clones of different DED-resistance and different parentage is available on the market. However, the use of these new elm cultivars in the Netherlands is still limited. "Vegeta" and "Commelin" elms that were planted in very large numbers in the second part of the twentieth century because of their level of resistance to DED caused by O. ulmi were attacked by O. novo-ulmi. Apparently, the lasting problems with DED in these earlier released cultivars, such as the sanitation costs for removing and replacing diseased and dead trees, has led to a lack of confidence in the resistance of newly released cultivars among urban foresters and landscape managers. As a result and despite its glorious past, around the year 2000 the elm was not even listed in the top 20 of street trees produced by Dutch tree nurseries (Geurts & Hiemstra 2002). However, the outcome of a survey among Dutch trees nurseries was that there is no other species that can replace the elm and it was concluded that confidence in the elm should be restored. None of the substitute tree species has the same set of characteristics that makes the elm so well-suited as a street tree; therefore the best replacement for the classical Dutch elm is still an elm, provided it is resistant to DED (Hiemstra et al. 2006). This paper reports on a study aimed at restoring the posi-

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

“…japonica and U. wallichiana, have been the main sources of resistance in the Dutch, American and Italian elm breeding programmes (Heybroek 1993, Santini et al 2011, Buiteveld et al 2014. As a result of crossing these species with native elms, a wide range of hybrid clones of varying tolerance levels and genetic backgrounds is now available on the market.…”

“…The first elm breeding programme began in the Netherlands in 1928 (Heybroek 1993) and was followed by several programmes in the United States and various European countries Santini et al 2011, Buiteveld et al 2014. As a result of crossing these species with native elms, a wide range of hybrid clones of varying tolerance levels and genetic backgrounds is now available on the market.…”

“…The Spanish elm breeding and conservation programme began in 1986 as the result of an agreement between the Spanish Environmental Administration and the Technical University of Madrid School of Forestry Engineering. Its two main objectives were to conserve remaining elm genetic resources and to transmit their variability to future generations of tolerant elms obtained through breeding; i.e., hybridisation of selected progenitors (native or tolerant Asian elms) to obtain tolerant trees with the appearance of native elm species.The first elm breeding programme began in the Netherlands in 1928 (Heybroek 1993) and was followed by several programmes in the United States and various European countries Santini et al 2011, Buiteveld et al 2014. As a result of crossing these species with native elms, a wide range of hybrid clones of varying tolerance levels and genetic backgrounds is now available on the market.…”

Twenty-four years of Dutch Trig® application to control Dutch elm disease

“…Nowadays, elm cultivars with a high level of resistance to DED are available on the market (Buiteveld et al 2015;Martín et al 2015b). This is an important solution for the use of elm trees in the future, but it does not protect the existing valuable trees from infections.…”

“…The fact that elms grow in most soil types endure narrow root space and closed pavements make them an ideal tree for urban environments (Buiteveld et al 2015). Several elm varieties used in cities are appreciated for their attractive architecture and their light canopy giving shelter on hot days but without complete blocking of sun light.…”