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Viruslike Particles Associated with a Rhododendron Necrotic Ringspot Disease. D. L. Coyier, Research Plant Pathologist, Ornamental Plants Research Laboratory, Agricultural Research Service, United States Department of Agriculture, 3420 S. W. Orchard St., Corvallis, OR 97330; R. Stace-Smith(2), T. C. Allen(3), and Esther Leung(4). (2)Plant Pathologist, Research Station, Agriculture Canada, 6660 N. W. Marine Drive, Vancouver, British Columbia, V6T 1X2; (3)Professor, Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331; (4)Technician, Research Station, Agriculture Canada, Vancouver, B. C. Phytopathology 67:1090-1095. Accepted for publication 28 February 1977. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1090.

A disease characterized by concentric necrotic rings on cultivated rhododendron (Rhododendron sp.) leaves is described. A survey in Oregon and British Columbia revealed similar symptoms on numerous plants of at least 13 cultivars and seedlings of rhododendron and a Kalmia latifolia plant. The causal agent was graft-transmitted to a healthy K. latifolia plant, but attempts to transmit it mechanically to 21 herbaceous hosts failed to induce symptoms. Electron microscopy of ultrathin sections and leaf-dip preparations showed that flexuous viruslike rods 460-540 nm long, and about 13 nm wide were associated with infected rhododendron plants. The virus is presumably a member of the potato virus X group. In thin sections of leaves viruslike particles occurred in bundles of four. Although symptoms appeared only on 2-yr-old leaves, the viruslike particles were abundant in current seasonís growth. We believe this to be the best substantiation to date of the viral etiology of a disease in rhododendron.