ANNUAL REPORT, 1939 17 



ten years previous to its use for this purpose. Five plots in all were used. One 

 plot, separated from the others by approximately one hundred feet of turf land, 

 was used to grow tobacco for control purposes during the course of the project. 

 Of the four other plots, two were kept fallow, and two were planted to tobacco 

 the first year, whereas all four plots were planted to tobacco the second year, 

 in accordance with plans for the experiment. One plot was" used the first year 

 to grow tobacco which was inoculated artificially and completely with mosaic 

 virus a:, soon as possible after planting, in order to permit the disease to become 

 fully systemic, to provide suitable material for study of the first objective of the 

 project. This plot from which the tobacco was cut and removed and one of the 

 fallow plots on which this same tobacco was placed, were plowed at once and then 

 again the next spring, and were used the second year to grow tobacco for the 

 further study of the first objective. As a result, there were provided foi this 

 purpose the second year, one ploi which contained only the stumps and roots 

 of systemically diseased plants, and one plot which contained only the tops of 

 the same tobacco. One other plot was used the first year to grow tobacco which 

 had five percent of its piants inoculated artificially with mosaic virus as soon as 

 possible after planting, to provide a suitable setup for the study of the second 

 objective, which was made the first year. However, because of the prevalence of 

 mosaic which developed in this tobacco the first yea.-, this plot from which the 

 tobacco was cut and removed and one of the fallow plots on which the same 

 tobacco was placed were plowed at once and then again the next spring, and were 

 used the second year to supplement the study of the first objective of the project. 



One strain of Havana Seed tobacco was used for all purposes of the experiment. 

 AH the tobacco received like treatments, except for the special treatments which 

 have been explained. 



At the time of plowing for tobacco the second year, samples of upturned stumps 

 and roots and of the upturned tops of the systemically diseased tobacco plants, 

 were taken to determine whether this overwintered tobacco material contained 

 virus capable of infecting greenhouse tobacco plants. Duplicate tests using ten 

 plants each were made for each kind of material collected. All the plants inocu- 

 lated by hand with water extractions of this material developed mosaic, while 

 duplicate controls consisting of ten plants each developed no mosaic when treated 

 to the same inoculating technique except that sterile water was used instead of 

 water extractions of the upturned tobacco material. 



In field tests made the third year of the experiment to ascertain to what extent 

 the mosaic virus overwintered in such manner in soil might constitute a source 

 of inoculum for mosaic in the second succeeding crop of tobacco, special care was 

 taken not co let any cultivating tools touch the plants and possibly thereby dis- 

 seminate inoculum from plant to plant. And when the tobacco had grown so 

 large that no further cultivating could be done without brushing the plants, 

 counts of the mosaic-diseased plants were made, and were assumed to represent 

 the extent to which the overwintered mosaic virus had served as a source of 

 inoculum in this case. This assumption seemed to be justifiable, inasmuch as the 

 control tobacco which had been cultivated and treated in like manner had 

 developed no mosaic. 



In the case of the tobacco grown on the plots where only systemically mosaic- 

 diseased tobacco had been grown the previous year, it was found that out of 

 1894 plants grown on the plot which contained only the stumps and roots of the 

 preceding tobacco, 16, or .84 of one percent of all the plants, had developed 

 mosaic; and out of 1125 plants grown on the plot which contained only the tops 

 of the preceding tobacco, 9, or .80 of one percent of all the plants, had mosaic. 

 And in the case of the tobacco grown on the plots where the previous crop of 

 tobacco had been completely but probably not all systemically infected with 



