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With 2 figures. It. var. vulgate H.)* and tobacco-necrosis virus (M. lethale H.) have about the same 10-minute thermal inactivation point, ca. 92 ~ C. 1~ 11 On the other hand, they react to aging in vitro at room temperature quite differently from each other; tobacco-mosaic virus remains active for years when stored at room temperature but tobacco-necrosis virus becomes inactive after about 20 days under the same conditions, la These facts suggested that the two viruses might have different temperature coefficients of inactivation. Studies were, therefore, undertaken to investigate this possibility. In a preliminary report 12, it was stated that the temperature coefficient, Qloo, of tobacco-necrosis virus is about 4, whereas th~t of tobacco-mosaic virus is of the order of 750. Actually, as in chemical reactions generally, Q10o is not a constant but varies with temperature so that the above figures represent averages for temperatures between 70 ~ and 95 ~ C. Because of the striking difference observed between the inactivation rates for tobacco-mosaic and tobacco-necrosis viruses, it seemed worth while to compare them with the inactivation rates of other plant viruses. Tobacco-mosaic virus (Marmor tabaciData on alfalfa-mosaic virus (M. medicaginis H.) and tobacco-ringspot virus (Annulus tabaci It. var. virginiensis H.) both of which have a thermal inactivation point of about 65 ~ C. 2, s, ~, is, are, therefore, included in the present paper. Materials and Methods.All four of the viruses used in this investigation were propagated in greenhouse-grown Turkish tobacco (Nicotiana tabacum L.) plants in 6-inch pots. Leaves bearing primary lesions were used as a source of tobacco-necrosis virus and systemically infected ]eaves as a source of the other 3 viruses. Diseased leaves were ground in a meat grinder and juice * Latin binomials used in this paper are taken from the Handbook of Phytopathogenic Viruses. a

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