Pathogenicity of Ceratocystis ulmi Isolates from Resistant Host Trees in California. T. E. Tidwell, Plant Pathologist, California Department of Food and Agriculture, Sacramento 95814. C. J. Krass, Plant Pathologist, and R. J. Sava, Environmental Hazards Specialist, California Department of Food and Agriculture, Sacramento 95814. Plant Dis. 67:511-513. Accepted for publication 8 October 1982. Copyright 1983 American Phytopathological Society. DOI: 10.1094/PD-67-511.

California isolates of Ceratocystis ulmi from naturally infected Ulmus americana, U. pumila, U. parvifolia, and Zelkova serrata were tested for pathogenicity to U. americana, U. parvifolia, and Z. serrata. Field symptoms of naturally infected U. parvifolia and Z. serrata were very limited. All C. ulmi isolates tested, including those from the resistant hosts, proved to be severely pathogenic to the susceptible indicator U. americana.