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Breakdown of Cross Protection Between Strains of Tobacco Mosaic Virus Due to Susceptibility of Dark Green Areas to Superinfection. J. A. M. Rezende, Former graduate research assistant, Department of Plant Pathology, Oklahoma State University, Stillwater 74078, Permanent address: Seção de Virologia, Instituto Agronômico de Campinas, Campinas, São Paulo, Brazil; J. L. Sherwood, Professor, Department of Plant Pathology, Oklahoma State University, Stillwater 74078. Phytopathology 81:1490-1496. Accepted for publication 28 June 1991. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-1490.

Reciprocal cross protection between the common strain of tobacco mosaic virus (TMV-C) and TMV-P (a necrotizing strain in Nicotiana sylvestris) was investigated in plants of N. tabacum ‘Samsun’ and ‘Xanthi’. When a concentration of challenge inoculum of 1 ?g/ml or higher of either TMV-P or TMV-C was used, there was superinfection of plants infected with the heterologous strain. The susceptibility to superinfection was associated with dark green areas of mosaic leaves, which were more susceptible to superinfection than the neighboring light green areas. Challenge inoculation with TMV-P RNA overcame the protection afforded by light green areas in TMV-C-infected Samsun and Xanthi. Systemic superinfection by the challenge strain occurred for all plants in which superinfection was detected in either dark or light green areas. Thus, once infection was initiated there was not extensive protection against the subsequent spread of the challenge virus. Protoplasts from dark and light green areas from N. sylvestris systemically infected with TMV-C were receptive to the attachment and/or uptake of the 32P-labeled TMV-P. Superinfection with virions or RNA from TMV-P was detected in protoplasts from both dark and light green areas, but virus accumulation was delayed and lessened in protoplasts from light green areas. The susceptibility of plants and protoplasts to superinfection may be a result of the uneven distribution of the protecting strain of TMV in infected plants.