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The Detection of Symptomless Virus-Infected Tissue in Inoculated Tobacco Leaves. J. A. Foster, Department of Plant Pathology, Cornell University, Ithaca, N.Y. 14850, Present address of senior author: Department of Plant Pathology, Ohio Agricultural Research and Development Center, Wooster 44691; A. F. Ross, Department of Plant Pathology, Cornell University, Ithaca, N.Y. 14850. Phytopathology 65:600-610. Accepted for publication 2 January 1975. DOI: 10.1094/Phyto-65-600.

Necrotic lesions were induced by three different procedures in Turkish tobacco (Nicotiana tabacum) leaves inoculated with tobacco mosaic virus (TMV), a virus that normally does not cause local necrosis in that host. The exposure of TMV-inoculated leaves to hot water at 50 C for 40 seconds 2-5 days after inoculation consistently collapsed necrotic rings or spots at the initial infection sites, except when tobacco plants were grown in the greenhouse during hot sunny summer days. The collapse of infected areas in inoculated leaves of cuttings during uptake of 0.5 M NH₄Cl starting 2 days after inoculation was consistent in leaves grown during summer, but rare in leaves which developed in winter. Despite some injury of the treated leaves, lesion development was also observed in TMV-inoculated leaves that had been grown at 20 C, then maintained at 50-54 C for 9 minutes, and immediately immersed in 20% ethanol at –4 C for 30 seconds. Each treatment induced lesion formation within 1-3 days but did not prevent systemic virus movement. Of these treatments, the hot water treatment proved the simplest and most dependable method of detecting the initial sites of TMV infection. Heat treatment also induced necrotic lesions in other host-virus combinations, such as potato virus X (PVX) or cucumber mosaic virus (CMV) in tobacco, that normally do not result in necrotic local lesions. That heat-induced necrosis of the virus-infected areas results from a rapid change in leaf temperature, rather than from the actual leaf temperature reached, is suggested by the facts that hot water (but not hot air at the same temperature) induced lesions, that cold liquid induced lesions only after plants were subjected to high temperatures, and that greenhouse conditions during hot summer days prevented complete heat collapse in the host-virus combinations tested.