Evaluation of Elm Clones for Tolerance to Dutch Elm Disease

Rooted cuttings of ten hybrid elm (Ulmus) clones, including one named 'Patriot', and all derived from various crosses of 'Homestead', 'Prospector', 'Urban', and Selection 970, were established along with a randomly selected American elm clones and three disease-tolerant cultivars ('Frontier', 'Homestead', and 'Prospector') in a replicated field plot. When the rooted cuttings were 3 years old, they were inoculated with a mixed spore suspension of Ophiostoma novo-ulmi and O. ulmi, the fungi that cause Dutch elm disease. Analyses of variance and regression showed significant variation among clones in crown dieback, survival, and height growth over a 7-year time period following inoculation. The American elm clone and selection 15-87 (an 'Urban' x 'Prospector' clone) showed the most crown dieback and lowest survival rate and were among the slowest in growth rate, 7 years after inoculation. Selections and cultivars showing the least dieback and highest survival were most hybrids from parentages of 'Urban' x 'Prospector', those from 'Homestead' x 'Prospector', and 'Homestead' x 970; and cultivars 'Patriot', 'Homestead', and 'Prospector'. Many of the same disease-tolerant clones and cultivars also showed the greatest height growth after inoculation. Significant variation in disease symptoms occurred among clones from the 'Urban' x 'Prospector' cross. Results of this study emphasize the importance of selection and testing of specific clones within full-sib families.