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Phloem Necrosis of Elms: Symptoms and Histopathological Observations in Tolerant Hosts. E. J. Braun, Graduate research assistant, Department of Plant Pathology, Cornell University, Ithaca, NY 14853, Current address of senior author: Department of Botany and Plant Pathology, Iowa State University, Ames 50011; W. A. Sinclair, professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853. Phytopathology 69:354-358. Accepted for publication 10 October 1978. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-354.

The agent of phloem necrosis (PN), a lethal disease of elms indigenous to North America, was transmitted among five elm species by grafting. Infected Ulmus americana and U. rubra died, U. laevis showed chlorosis and epinasty, and U. carpinifolia and U. parvifolia produced witches brooms. The PN agent was transmitted by dodder (Cuscuta epithymum) from U. americana and U. parvifolia to periwinkle (Catharanthus roseus), which became broomed, stunted, and chlorotic. Mycoplasmalike organisms (MLO), not found in control plants, occurred in phloem sieve tubes of symptomatic elms and periwinkle. MLO were more easily found in tolerant species than in those killed by PN and were most conspicuous in witches brooms. In a test of the hypothesis that PN-resistant roots could protect susceptible scions, rootstocks of U. pumila did not prevent death of graft-inoculated U. rubra. In histological comparisons of healthy and infected U. parvifolia, secondary phloem of infected main stems appeared normal except for MLO. In witches brooms, however, the midribs of leaves contained a thicker band of secondary phloem than was seen in leaves from healthy trees. Collapsed sieve elements, scattered throughout this phloem, were more numerous in infected than in healthy samples.