Variation in Growth and Response to Ophiostoma ulmi among Advanced-Generation Progenies and Clones of Elms

Abstract

Controlled pollinations between five disease-tolerant elm (Ulmus L.) clones (Number 970, ‘Urban’, and clones that were later named ‘Homestead’, ‘Pioneer’, and ‘Prospector’) yielded 686 seedlings. Various crosses produced from zero to over 90 seedlings. Only one of four female parents produced any viable selfed seedlings. At age four, all seedlings were inoculated with Ophiostoma ulmi, (Buism.) C. Nannf., the causal fungus for Dutch elm disease. A factorial analysis showed male parent, female parent, and male x female interaction influenced disease symptoms 4 and 8 weeks after inoculation. After a few years of further evaluation of the seedlings, 10 clones were selected for a combination of disease- and insect-tolerance and horticultural desirability. These clones were propagated and established along with four disease-tolerant cultivars and American elm seedlings in a replicated field plot. Three-year-old clonal plants inoculated with O. ulmi varied significantly in their disease symptoms 4 weeks, 1 year, and 2 years after inoculation. Even clones from the same full-sib family showed significant differences in disease tolerance. Results indicate that both specific and general combining ability are important in determining tolerance to Dutch elm disease.