Virus multiplication in plants doubly infected by potato viruses X and Y

The ratio of virus X in juice from tobacco plants simultaneously inoculated with potato viruses X and Y to that in juice from comparable plants inoculated only with virus X varied with the stage of the disease. Ratios up to 10:1 were obtained with leaves invaded during expansion. With leaves that expanded after the growing tip had been invaded, increases up to threefold were obtained; with inoculated leaves increases were usually twofold. There was a direct relationship between severity of symptoms and virus X concentration of doubly infected plants. In leaves invaded during expansion, the relative increase in virus X concentration of XY-infected leaves over that of X-infected leaves was greater in veinal tissue than in interveinal tissue. With singly infected leaves the concentration of virus X in juice from interveinal tissue was about six times that in juice from veinal tissue, whereas with XY-infected leaves juice from interveinal tissue contained about twice as much virus X as did juice from veinal tissue. When X-infected plants were inoculated with virus Y, there occurred increases in virus X similar to those observed in plants inoculated simultaneously with the two viruses. When Y-infected plants were inoculated with virus X, large relative increases of virus X did not occur and the usual severe symptoms did not develop. The magnitude of the increase in the concentration of virus X in juice from XY-infected leaves over that in juice from X-infected leaves varied considerably among experiments. Tests indicated that this variation was not caused by changes in either virus or in nutrition of the host plants. The variation was seasonal, with the greatest ratios occurring in the spring when greenhouse temperatures favor multiplication of virus Y during the day and that of virus X during the night. In both singly (Y) and doubly (X + Y) infected leaves, the concentration of virus Y in interveinal tissue was about twice that in veinal tissue. Neither the concentration nor distribution of virus Y was affected by the presence of virus X.