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Response of Ulmus pumila and U. pumila × rubra Hybrids to Inoculation with Ceratocystis ulmi. D. T. Lester, Associate Professor of Forestry, University of Wisconsin, Madison 53706; E. B. Smalley, Professor of Plant Pathology, University of Wisconsin, Madison 53706. Phytopathology 62:848-852. Accepted for publication 15 February 1972. DOI: 10.1094/Phyto-62-848.

Forty-six full-sib elm progenies including parental, F1, F2, and backcross combinations were tested for response to artificial inoculation with Ceratocystis ulmi. Time of inoculation and interaction of progeny with time were statistically significant for three of six measures of disease response. Species combination was the major source of variation for all variables, although variation among progenies within species combination was present for five of six variables. Disease response was a function of the proportion of genes present from Ulmus rubra. Despite high susceptibility of progenies containing more than 25% U. rubra genes, symptomless individuals occurred in most inoculated progenies. Selection and vegetative propagation of resistant individuals, irrespective of average progeny performance, may still allow the elm breeder access to the ornamentally desirable features of U. rubra.

Additional keywords: Dutch elm disease, genetic variation.