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Aggressiveness in Ceratocystis ulmi. F. W. Holmes, Professor, Shade Tree Laboratories, Department of Plant Pathology, College of Agriculture, Agricultural Experiment Station, University of Massachusetts, Amherst, Mass. 01002; H. M. Heybroek(2), and J. N. Gibbs(3). (2)Head of Department of Elm Research, Forest Research Station “De Dorschkamp”, Wageningen, The Netherlands; (3)Pathologist, Forestry Commission, Forest Research Station, Alice Holt Lodge, Wrecclesham, Farnham, Surrey, England. Phytopathology 62:939-940. DOI: 10.1094/Phyto-62-939.

Ceratocystis ulmi isolates from two English elms in southern England, two Commelin elms in eastern Netherlands, and two Belgica elms in central Netherlands were compared by inoculation into Belgica (susceptible), Commelin (moderately resistant), and clone-496 (highly resistant) elms in The Netherlands. Both isolates from southern England caused more severe symptoms than the other four isolates in all three test clones of elm. Symptoms induced by the English isolates continued to increase in severity after symptoms induced by the other isolates had reached a maximum.