32 Karl Hermann Fernow 



nounced types of potato mosaic may be masked under some field con- 

 ditions. Is it not reasonable to suppose that there might be a fourth 

 type of potato mosaic so obscure as to be masked even under the most 

 favorable conditions? Such a mosaic would presumably not have much 

 effed oh the yield, and, since it could not be detected by ordinary inspec- 

 tion methods, it would not tend to be eliminated, but, on the other hand, 

 would tend to become universal in a tuber-propagated crop such as the 

 potato. Fields of beans are sometimes seen showing an almost 100-per- 

 cent infection, even though bean mosaic is not 100-per-cent seed-trans- 

 mitted and though bean mosaic materially reduces the yield. 



General results 



The most significant feature of the experiments is not a determination 

 of species susceptible to any particular virus, but the fact that different 

 mosaics on the same species exhibit symptoms which are sufficiently 

 different to be easily distinguishable. This is not the first time such 

 a phenomenon has been observed, it having been previously reported 

 by Allard (1916), Jagger (1918), Schultz and Folsom (1923), and Johnson 

 (1925). It is rather surprising that such differences in symptoms have 

 not been reported by other authors who have done extensive cross- 

 inoculation work. 



CONCLUSIONS 



A comparison of the results of cross-inoculation experiments reported 

 in literature reveals many cases in which the results are in conflict. A 

 review of the literature discloses also the fact that most authors have 

 omitted consideration of a factor here shown to be important, namely, 

 the identity of the mosaic concerned. It has been shown here that a 

 single species may be affected by two or more distinct mosaics, which 

 differ not only in their symptoms but also in their suscept range and 

 probably in other characters as well. It may be that this failure to iden- 

 tify the mosaic concerned accounts for many of the discrepancies found 

 in the literature. 



As in the case of diseases caused by bacteria, fungi, and so forth, it is 

 found that some viri are able to produce disease on a wide variety of 

 species distributed in different families, and are at the same time unable 

 to infect species in the same genus as their usual suscepts; while other 

 viri seem to be more restricted in their range of suscepts. 



SUMMARY 



Inoculation experiments were conducted with mosaics on nineteen 

 species of plants, of which fifteen were Solanaceae. All but one of the 

 species tried were shown to be susceptible to one or more of the mosaics 

 used. It appears that the eight mosaics, which for convenience have 

 been designated by letters A to H, are distinct, with the possible exception 



