Phloem Necrosis of Elms: Symptoms and Histopathological Observations in Tolerant Hosts

Abstract

The agent of phloem necrosis (PN), a lethal disease of elms indigenous to North America, was transmitted among 5 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. Mycoplasma-like 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, 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.