Resistance of Hybrid Elm Progenies to Ceratocystis ulmi

Abstract

Five clones of Ulmus from the Netherlands, and with resistance to C. ulmi, when hybridized as male parents with U. pumila varied in their ability to transmit genes for Dutch elm disease resistance and growth rate. Significant differences in disease symptoms among the progenies occurred 15 and 56 wk but not 4 wk after inoculation. The progeny from 1 Dutch clone, N248 (U. wallichiana .times. U. carpinifolia), was more susceptible than were progenies from other clones. U. rubra .times. U. pumila as a female parent transmitted a slower growth rate and less disease resistance to its progeny than did U. pumila. Date of inoculation, interaction between inoculation date and progeny and site factors rarely influenced symptom expression. One C. ulmi isolate from Tennessee [USA] and labeled earlier as nonaggressive similarly caused less disease than did 6 other aggressive isolates when tested on U. pumila .times. N260 (U. pumila .times. U. hollandica).