The present paper deals with the induction of lesion formation by heat or cold treatment on TMV- or CMV-infected systemic hosts and also TMV-infected lesion hosts predisposed to systemic infection.
1) TMV-inoculated leaves of systemic hosts, N. tabacum Xanthi, N. tabacum Samsun, and Petunia hybrida, developed local lesions if they were immersed 2 days later in hot water (50°C) for 2min. CMV-inoculated leaves of systemic hosts, N. glutinosa and Petunia hybrida, also developed local lesions by the heat treatment.
2) The similar heat treatment resulted in local lesion formation on TMV-inoculated leaves of local lesion hosts, N. glutinosa, Xanthi nc and Samsun NN, during incubation at 30°C, a temperature at which no lesions appear and TMV spreads systemically.
3) TMV-inoculated leaves of local lesion hosts, N. glutinosa, Xanthi nc, and Samsun NN, developed local lesions if they were immersed 2 days later in ice water for 5 sec to 2min during incubation at 30°C.
4) The similar cold treatment occasionally resulted in local lesion formation on TMV-inoculated leaves of systemic hosts, Xanthi, Samsun, and Petunia hybrida.
5) Usually, heat treatment applied 2 days after inoculation induced dark necrotic rings which appeared in 15 to 20 hours after treatment, whereas cold treatment resulted in white necrotic rings which appeared in 1 to 2 hours. When leaves of N. glutinosa bearing lesions induced by heat treatment 2 days after inoculation with TMV were immersed in ice water 4 days after inoculation, dark necrotic rings developed around each original lesion during incubation at 30°C.