Effect of thermal shock treatments on symptom expression in test plants inoculated with potato aucuba mosaic virus

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Abstract
SUMMARY: Potato aucuba mosaic virus (PAMV) has few reliable local lesion assay hosts. However, lesions formed when PAMV‐inoculated leaves were exposed to thermal shock (dipping for 40 s in water at 50 or 2 °C). Leaves of Nicotiana tabacum (cv. Xanthi‐nc, Samsun or Samsun NN), Hyoscyamus niger and Datura metel consistently developed necrotic lesions, leaves of Chenopodium amaranticolor developed whitish rings, and leaves of N. glutinosa developed diffuse cream‐coloured rings and spots. In PAMV‐inoculated leaves of Xanthi‐nc tobacco, C. amaranticolor and D. metel, lesions formed only in areas exposed to light. Thermal shocks applied to systemically infected leaves of Xanthi‐nc tobacco induced necrotic vein banding patterns.In inoculated Xanthi‐nc tobacco leaves, PAMV seemed confined to local lesions. The rate of lesion enlargement was therefore a measure of rate of virus spread. Lesion size increased as the interval between inoculation and shock treatment increased. The mean rates of increase in lesion radius were 17 and 27 Cμm/h at 15 and 25°C respectively. ‘Target’ lesions, composed of concentric necrotic rings, formed when inoculated Xanthi‐nc tobacco leaves were given two or more 50°C shocks. The first of two 50°C treatments decreased subsequent rates of lesion enlargement.