Effects of heat shock treatment on symptom development in tobacco leaves inoculated with potato mop‐top virus

PMTV normally causes necrotic ringspot local lesions in Xanthi‐nc tobacco leaves at 15 °C but not at 22°. Dipping the leaves once in hot water at different intervals after inoculation induced necrotic reactions even at 22°. Successive concentric rings, one for each day of the interval between inoculation and treatment, were sometimes induced at 22°; these even formed in continuous lighting, suggesting an intrinsic diurnal rhythm. No lesions were induced at 22° by dipping inoculated leaves in ice‐cold water. Samsun tobacco inoculated with PMTV developed very few lesions at 15°. Many more were induced however, by dipping the leaves in hot water after inoculation. The results suggest some similarities between PMTV and TMV, and the two viruses have been reported as serologically related; but PMTV did not protect plants against TMV, for TMV challenge‐inoculations induced lesions within existing PMTV lesions.