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

Abstract

Elms, and especially Ulmus × hollandica have been dominant and very much appreciated trees in cities and rural landscape for centuries in the Netherlands. As a result of two Dutch Elm Disease (DED) epidemics in the 20th century these trees largely disappeared from the landscape. Despite the introduction of new cultivars with increased levels of DED-resistance, by the end of the 20th century the elm had disappeared from the top 20 list of trees produced by Dutch nurseries. New cultivars with increased resistance to DED are used to a limited extent only. Apparently the lasting problems with DED in old cultivars has led to a lack of confidence in the resistance of these latest released cultivars among urban foresters and landscape managers. This paper reports on a study that aims at restoring the position of the elm as a street tree in the Netherlands by providing information on resistance to O. novo-ulmi causing DED of the currently available cultivars. All elm cultivars currently on the Dutch market were compared in an inoculation test. In 2007 a field experiment of 18 cultivars, one species and 10 non-released clones from the Dutch elm breeding program was established. Two cultivars were used as reference clones: “Commelin” (relatively susceptible) and “Lobel” (relatively resistant). In 2008 and 2009 the elms were stem-inoculated with Ophiostoma novo-ulmi and disease development was assessed throughout the summer and the following year. Clear differences in resistance to O. novo-ulmi were found between the cultivars, with “Columella”, “Sapporo Autumn Gold”’ and “Rebella” being highly resistant and significantly different from “Lobel” and “Regal”, “Urban”, “Belgica”, “Den Haag” and the U. laevis seedlings being the most susceptible and comparable to “Commelin”. The non-released clones performed comparable to “Lobel’”or even better. The ranking of the cultivars based on their level of resistance to O. novo-ulmi in this field test corresponds well with experience in urban green practice. Our conclusion is that there is a wide range of cultivars available with a good to excellent level of resistance. The available cultivars have a broad genetic base due to different parentage and use of exotic germplasm in the crossings. This broad genetic background may contribute to the stability of resistance in case new forms of the disease appear. The non-released clones performed well compared to the released cultivars and give good opportunities to further broaden the current range of cultivars on the Dutch and European market.