STUDIES ON POTATO VIRUS DISEASES:VI. FURTHER EXPERIMENTS WITH THE VIRUS OF A POTATO MOSAIC UPON THE TOBACCO PLANT
Authors: SMITH KENNETH M. 
Published: February 2008

Summary.Further experiments with the virus of a potato mosaic upon the tobacco plant are described.

Marked increase in virulence has been shown to occur in ringspot by progressive needle inoculation through succeeding tobacco plants. Four strains of ringspot arising from four distinct mosaic potato plants (var. Arran Victory) have been so experimented with in White Burley and Virginia tobaccos.
Comparison of needle and aphis (M. persicae) inoculations of healthy potato plants from a tobacco plant affected with the virulent ringspot shows that although both methods of inoculation produce an intensified mosaic in the potato plant, that produced by the needle is often fatal to the potato.
Comparison of needle and aphis (M. persicae) inoculations of healthy tobacco plants from a tobacco plant affected with the virulent ringspot shows that while the needle reproduces the virulent disease, the aphis produces a mottle and dark green lines disease, apparently the same as that produced by aphis transmission from the ordinary type of ringspot.
An analysis is given of a large comparative series of aphis and needle inoculations with potato mosaic and tobacco ringspot. This indicates that the aphis‐induced disease is an infectious virus of constant charaeter. The experiments also show that the aphis fails to transmit the more lethal characters of the disease.
The response of different varieties of tobacco to the virulent virus was examined. Six varieties were used in the experiments: White Burley, Virginia, Kentucky, Maryland, Havana, Connecticut; Virginia was found to be the most resistant, and White Burley the most susceptible, while the other four varieties approximated to White Burley in susceptibility.
Large numbers of seeds were sown which had been derived from tobacco plants affected with ringspot and its various modifications. In no case was the disease transmitted through the seed.
Experiment showed that the virulent virus was present in the roots of an affected tobacco plant; and also that needle inoculation of the roots of healthy tobacco plants with the virulent virus produced the typical disease.
It was found that certain individual tobacco plants, both Virginia and White Burley, seem to possess an inherent resistance to the virulent virus which attenuates it. Needle inoculation of such a plant with the virulent virus produces faint symptoms only. When juice from this plant is passed on to other tobacco plants the virus still shows weak symptoms, as compared with those produced by the same strain of virus before its passage through such a resistant plant. This seems to indicate that certain plant individuals have the power of attenuating the virus.
A description is given of some preliminary attempts to produce experimentally in tobacco plants an immunity to the virulent virus. Plants were first inoculated with a suspension of “dead” virus, killed by heating, and three weeks later they were inoculated with the same strain of virus unheated. Experiments were also performed giving, first, inoculations with the virus artificially attenuated, and then inoculating with the virulent virus. Tobacco plants affected with the aphis‐produced disease were also needle‐inoculated with the virulent virus. A series of resistant individual plants which had developed only a mild disease after inoculation with the virulent virus, was inoculated a second time, with the same strain of virus in some cases and in others with a different strain. In all the above experiments the normal virulent disease developed, no immunity having been conferred.
Tubers, from potato plants infected the previous year with tobacco ringspot, were grown in order to study the behaviour of the virus after a winter spent in the resting tuber. Plants were produced exhibiting all the symptoms of the intensified mosaic characteristic of infection with tobacco ringspot which were exhibited by the parent plants the year before. This seems to indicate that the change induced in the virus of a potato mosaic by passage through tobacco may be of a permanent nature.