The behaviour of some tobacco necrosis virus strains in plants

Strains of tobacco necrosis virus (TNV) differed in their behaviour when inoculated to French bean leaves which were exposed to different temperatures. At 20° strains A and D increased much faster than strain B. At 30° strain A increased at about the same rate as at 20°, but reached one-fourth the concentration attained at 20°, whereas strains B and D reached hardly detectable amounts at 30°. All strains infected less readily at 30° than at 20°, but the inhibition at 30° differed with different strains and was least with strain A. The strains were almost equally susceptible to inactivation by ultraviolet radiation in vitro. Tests in vivo were influenced greatly by the effect of UV on the capacity of the leaves to support virus multiplication. A method was devised that partly compensates for this effect. The corrected rate of inactivation of infective centres was similar to or a little less than the inactivation rate in vitro. The resistance of the infective centres of strains A, D, and E remained constant for 2 hours after inoculation and then increased, whereas that of infective centres of strain B decreased 1 hour after inoculation and then increased faster than with the other strains. The roots of some, but not all, species of plants growing in the same soils contained TNV. Strain A was the most prevalent, but as many as three strains were isolated from the roots of one plant. The leaves of normal-looking plants of some species also sometimes contain virus. The fungicide Captan delayed root infection of French bean and cucumber plants for 2–4 weeks. When virus was added to soil, more infections occurred in sandy than in compost soils and Captan did not prevent infection. The results suggest that there may be two modes of transmission: by a Captan-sensitive microorganism and by mechanical injury of root cells against soil particles.