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Localization of Infection in American Elms Resistant to Ceratocystis ulmi. W. A. Sinclair, Associate Professor, Department of Plant Pathology, Cornell University, Ithaca, New York 14850; J. P. Zahand(2), and J. B. Melching(3). (2)(3)Former Research Assistants, Department of Plant Pathology, Cornell University, Ithaca, New York 14850. Phytopathology 65:129-133. Accepted for publication 9 August 1974. DOI: 10.1094/Phyto-65-129.

Branches of American elm trees previously identified as resistant or susceptible to Ceratocystis ulmi, and mainstems of 3-year-old ramets from these trees, were inoculated with bud-cell suspensions of the fungus. In branches of resistant trees, the interval from inoculation to appearance of foliar symptoms was longer, fewer shoots became symptomatic, and less extensive invasion of current-season shoots by C. ulmi occurred than in branches of susceptible trees. Resistant and susceptible trees differed in the proportion of shoots infected 4 days after inoculation; such differences increased during the growing season. When propagules of C. ulmi were flushed from segments of inoculated branches distal to points of inoculation, the number recovered from susceptible branches increased from 2 to 6 days after inoculation, while propagules from comparable resistant branches remained below detection levels. At 10 days after inoculation, 12 times as many propagules were flushed from susceptible, as from resistant, stem segments. Localization of natural infections in small branches and subsequent remission of symptoms were noted in four of the resistant trees. Differences in disease development and propagule recovery between ramets from resistant and susceptible trees were smaller than those between ortets. Young ramets of all clones were susceptible to C. ulmi.