16 BULLETIN 40, U. S. DEPARTMENT OF AGBICULTURE. 



were arranged in 2 rows of 7 each and inoculated with the virus of 

 mosaic tobacco plants. The first mosaic symptoms were showing in 

 these plants on July 23. On July 28, the entire 14 plants showed pro- 

 nounced mosaic symptoms. With the exception of a single plant, the 

 remaining 14 plants which had been inoculated with pokeweed mosaic 

 on June 13 were still healthy. These plants came into bloom on 

 August 13 and still remained free from the disease, with the exception 

 of one or two plants. 



On June 14, 14 similar plants were inoculated with tobacco-mosaic 

 virus. The first mosaic symptoms showed in nearly every plant June 

 23 and 24, 9 to 10 days later. On June 14, 20 similar plants were 

 inoculated with pokeweed mosaic virus. The disease did not develop 

 in these plants. 



On August 13, when all the plants were just coming into blossom, 

 2 rows of 7 plants each were selected from the plants treated on June 

 13 and 14 with the virus of mosaic pokeweed. One row of 7 plants 

 was then inoculated with the mosaic virus of tobacco. The remaining 

 row of plants was inoculated with the mosaic virus of pokeweed. On 

 August 26, 13 days after inoculation, 6 of the 7 plants receiving the 

 virue of tobacco mosaic had developed the disease. The entire lot of 

 7 inoculated with the pokeweed mosaic remained free from the disease 

 as late as October 9, when seed pods were fully ripened. 



In October, inoculation experiments were again made with tobacco, 

 using the virus of mosaic pokeweed and of mosaic tobacco plants. 

 On October 10, 20 small Connecticut Broaclleaf plants in pots were 

 inoculated with the juice of mosaic pokeweed plants. Using the same 

 mortar for extracting the juice, 20 similar plants were inoculated 

 with the virus of mosaic tobacco. The first mosaic symptoms began 

 to appear in these plants on October 21, 11 days later. On October 

 27, mosaic symptoms had developed in 17 of the 20 plants treated with 

 the virus of mosaic tobacco. On the other hand, not a single case of 

 the disease appeared in the series of 20 plants treated with pokeweed 

 mosaic, although these plants were kept under observation until 

 February 1, 1913, at which time they had reached complete maturity. 



INCUBATION PERIOD OF THE DISEASE. 



Following inoculation a rather variable period precedes the appear- 

 ance of the first visible symptoms. The length of this period depends 

 to a considerable extent upon the kind of plant, the age of the plant 

 when infected, and such external factors as may retard or accelerate 

 growth. 



So far as the writer has been able to learn, eight days, reported by 

 Woods, 1 is the shortest period previously observed between the time 



1 Woods, A. F. Observations on the mosaic disease of tobacco. U. S. Department of 

 Agriculture, Bureau of Plant Industry, Bulletin 18, 1902, p. 17. 



