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Anatomical Marker for Resistance of Ulmus americana 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, respectively, Department of Plant Pathology, Cornell University, Ithaca, New York 14850. Phytopathology 65:349-352. Accepted for publication 18 October 1974. DOI: 10.1094/Phyto-65-349.

Vessels in the second xylem ring outward from the pith of small branches of American elms resistant to Ceratocystis ulmi were found to have significantly smaller mean radial diameters than comparable vessels in branches of susceptible trees. This relationship was exploited by development of a vessel diameter index (VDI) that can be determined with few measurements. VDI = mean percentage of vessels of diameter ≥ 50 µm among total vessels intersected by arbitrary radii in the second and third xylem rings outward from the pith. American elms known to be resistant to C. ulmi and others found healthy among dense populations of elms killed by Dutch elm disease had lower VDI's than trees known to be susceptible. Severe Dutch elm disease symptoms were not observed in trees with VDI's of 12 or less. VDI may be useful for rapid assessment of possible resistance in American elms that survive Dutch elm disease epidemics.