18 BULLETIX 40, U. S. DEPARTMENT OF AGRICULTURE. 



Numerous experiments have shown that the disease develops very 

 readily in 10 or 12 days in large, vigorous plants inoculated just 

 prior to blossoming or even after all blossoms have ceased to appear 

 in the flower head of the main stalk. The progress of the disease is 

 then coincident with the development of suckers at one point or an- 

 other on the plant. 



DISTRIBUTION OF VIRUS IN DISEASED PLANTS. 



As already pointed out. obvious symptoms of the mosaic disease 

 do not necessarily appear simultaneously in all the leaves and branches 

 of an affected plant. If nearly mature plants become affected with 

 the disease, the topmost leaves and suckers usually develop unmis- 

 takable symptoms, while the lowermost mature leaves always remain 

 perfectly normal in appearance. In many instances a single sucker 

 at the base of a plant becomes typically diseased, while at the same 

 time others growing beside it show no symptoms whatever. Although 

 it has been proved experimentally many times that the sap of mosaic 

 leaves contains an infectious virus, it has not been definitely shown 

 that this virus may reside in all parts of the affected plants. 



Many inoculation experiments made by the writer have shown con- 

 clusively that the apparently healthy lower leaves, the roots, and even 

 the corollas of mosaic plants contain the virus of the disease. 



On December 26, 1911, 27 young, healthy Connecticut Broadleaf 

 plants growing in 1-inch pots were arranged in 3 series of 9 each. 

 One series was inoculated with the sap of the apparently healthy 

 lower leaves of a plant indicating mosaic disease in only the small 

 leaves at the top. The second series was inoculated with the sap of the 

 topmost mosaic leaves of this same plant. The third series, which 

 served as a control, was inoculated with the sap of a healthy plant. 

 The inoculations in each plant of the three series were made in iden- 

 lically the same manner by puncturing the leaf blade and midrib of 

 one or two leaves. "When examined on January 6, 12 days after inoc- 

 ulation, all 9 plants of the series treated with the sap of the topmost 

 mosaic leaves had developed the disease. Of the 9 plants in the series 

 inoculated with the sap of the apparently healthy lower leaves of the 

 same plant 6 had developed the disease. The 9 controls were per- 

 fectly healthy. When examined about a week later each series re- 

 mained as before. 



On January 5 this experiment was repeated, using 1 series of 8 

 or 9 plants each. All plants were inoculated alike at two points by 

 placing the virus upon the stub of a leaf which had been cut off and 

 by puncturing the leaf blade of a second leaf, as follows: 



Nine plants were inoculated with the sap of the mosaic topmost leaves of 

 a diseased plant. 



Eight plants were inoculated with the sap of the apparently healthy lower- 

 most leaves of a diseased plant. 



