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Response of Backcross Hybrids and Three-Species Combinations of Ulmus pumila, U. japonica, and U. rubra 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 62:845-848. Accepted for publication 15 February 1972. DOI: 10.1094/Phyto-62-845.

Thirty-two elm progenies, including half-sibs and full-sibs from parental, F2, and backcrosses were tested for response to artificial inoculation with Ceratocystis ulmi. Time of inoculation and species combination were the main sources of variation. Crosses among Ulmus pumila and a putative natural hybrid of U. pumila × japonica had low disease susceptibility. The introduction of germ plasm from U. rubra markedly increased disease susceptibility.

Additional keywords: Dutch elm disease, genetic variation.