168 Maine Agricultural Experiment Station. 1920. 



for transmission. The absence of symptoms previous to that 

 time was not the reason inasmuch as the virulent juice used for 

 successful inoculations sometimes came from diseased ' stock 

 that had not yet shown mottling. 



Variations of the preceding experimentation were made in 

 other series of inoculations in 1919. Inoculations were made in 

 Green Mountain plants protected by insect cages, with similar 

 results. In the cages, and also in the open both within and be- 

 tween varieties, single inoculations were made for comparison 

 with the usual method of repeating the inoculation several times, 

 with the same results even when made as late as July 20.. In 

 four series the juice for inoculation was transferred from plants 

 in the worst stage of the disease, both within each variety and 

 from Mountains to Cobblers, with the resultant appearance of 

 the same sort of symptoms whereas the induced mosaic was 

 often but not always of the worst type in the other inoculations. 



Artificial transmission of potato mosaic by means of graft- 

 ing and juice inoculation thus demonstrated the infectious na- 

 ture of the disease and made necessary the discovery of a nat- 

 ural means of transmission. This was found to be a certain 

 type of insects, as will be explained in the next section. 



Insects as Carriers, 

 effect of using insect cages in the field. 



Simultaneously with the first experiments with grafts and 

 juice inoculations, preliminary tests were made regarding trans- 

 mission by insects. Potato plants were grown throughout the 

 season of 191 7 under cheesecloth cages. These cages were not 

 entirely insect-proof but their use resulted in a reduction of 

 mosaic that season. Only 5 per cent of the tubers from the 

 healthy caged plants produced mosaic progeny the next season, 

 while other healthy plants, grown outside the cages, produced 

 progeny the following year with much greater percentages of 

 mosaic. 



Again in 191 8 healthy plants were grown under cages. On 

 account of the poor quality of the cheesecloth obtainable and 

 the abundance of aphids in the field, many of these insects were 

 found on some of the caged plants. However, all the tubers 



