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Top Necrosis and Cellular Changes in Soybean Doubly Infected by Soybean Mosaic and Bean Pod Mottle Viruses. Yih- Shyong Lee, Graduate Assistant, Department of Plant Pathology, North Carolina State University, Raleigh, North Carolina 27607; J. P. Ross, Plant Pathologist, Plant Science Research Division, ARS, USDA, P.O. Box 5397, Raleigh, North Carolina 27607. Phytopathology 62:839-845. Accepted for publication 15 February 1972. DOI: 10.1094/Phyto-62-839.

Soybean plants inoculated with soybean mosaic virus (SMV) 1 week prior to bean pod mottle virus (BPMV) inoculation often develop top necrosis. Conditions favorable for top necrosis of doubly infected plants included diurnally fluctuating (27-21 C) and constant (27 C) temperatures, inoculation of seedlings rather than older plants, and use of inoculum containing high SMV titers. Concentrations of SMV were significantly higher in doubly infected plants than those in singly infected plants 3 and 4 weeks after SMV inoculation as determined by virus particle counts and by serological and infectivity dilution end points. Growth of SMV-infected and doubly infected plants was not increased by applications of gibberellic acid, whereas growth of BPMV-infected and healthy plants was increased. No consistent differences in number or size of SMV inclusion bodies between singly and doubly infected plants were detected by either light or electron microscope observations. Inclusions of SMV and BPMV-like particles were found in the same cells. Starch granules were more numerous and larger in healthy than in singly or doubly infected plants. Osmiophilic globules appeared and increased in size in necrotic areas of doubly infected plants.