178 Maiue Agricultural Experiment Station. 1920. 



all the healthy hills next to mosaic hills in the row did not cause 

 any reduction of infection as compared with the removal of 

 only the mosaic hills. 



It seems from the facts so far secured that contact infection 

 can not be very common or important, especially when consider- 

 ing the results obtained from artificial inoculations and with 

 aphids. However, some infection by contact possibly may occur 

 too late to be evident during the season in which it occurs. Ex- 

 periments on this point are not completed but stocks are avail- 

 able for growing the second generation in 1920 to test this pos- 

 sibility. Of course any measures necessary to prevent contact 

 would be included in those necessary to avoid aphid transmis- 

 sion, which obviously often is made easier by contact although 

 apparently frequent without contact. 



SOIL. 



The causes of many potato diseases are carried both from 

 place to place and from season to season by means of soil. It 

 is maintained that 80 per cent of field infections of tobacco mo- 

 saic originate in contaminated soil 16 . Hence it has seemed neces- 

 sary to disclose any soil harboring of potato mosaic that may 

 occur. 



In the greenhouse twelve Green Mountain tubers were split. 

 Half of each was planted in steam-sterilized soil and the other 

 half in soil from which a full-grown mosaic plant had been re- 

 moved either a day or a fortnight previously. The 24 plants 

 were all healthy, as was also the second generation of the same 

 stock. 



While soil transmission was favored in this greenhouse test 

 by the shortness of time between the growth of succeeding crops 

 in the same soil, there were lacking certain factors in the pos- 

 sible soil-harboring in fields, namely, old stalks, volunteer potato 

 plants, and hibernating insects. Therefore three rows of Green 

 Mountain rogued stock were planted in 1919 across the sites of 

 a 1918 twenty-per cent mosaic plot and a wholly diseased one, 

 both of Green Mountains. All mosaic hills were dug and the 

 seed pieces examined to determine the volunteers. Disregarding 



"Chapman, G. H. Op. cit. See p. 80. 



