Interspecific Transmission of Mosaic Diseases of Plants 29 



TABLE 21 (concluded) 



INTERPRETATION OF RESULTS 



Liability to error 



Reasonable precautions were taken to prevent errors. In spite of 

 these, there are evidences in the data that errors did occur. Errors in 

 diagnosis were reduced to a minimum by keeping the plants growing 

 vigorously as much as possible, and discarding doubtful cases. It is 

 very likely that some of the species recorded as not susceptible were 

 carriers without showing symptoms. It was not always found possible 

 to record the type of mosaic present. Errors due to accidental contam- 

 ination did occur, especially during the early part of the experimental 

 work when the hands were applied to the plants in making inoculations. 

 After this practice was discontinued, very little contamination occurred. 

 Some of the earlier experiments were omitted where there was evidence 

 of contamination. Aphids occasionally became fairly abundant in the 

 greenhouses in spite of frequent fumigation, but apparently they did 

 not cause much natural spread. White flies, thrips, and red spiders 

 were sometimes annoying, but there was no indication that they were 

 ever instrumental in transmitting mosaic. 



While accidental infections were not frequent, they occasionally did 

 occur. If, therefore, only three or four plants showed positive results 

 from a given transfer, this cannot be taken as proof that the species is 

 susceptible to that particular virus. 



