Interspecific Transmission of Mosaic Diseases of Plants 11 



are dropped, leaving only a small crown. These symptoms closely resemble 

 those described for potato streak by Schultz and Folsom (1923). 



Mosaic B was obtained whenever potatoes were used as a source of 

 inoculum on a suitable suscept. This was the case whether the potatoes 

 used showed symptoms of mild, leaf-rolling, or rugose mosaic (Schultz 

 and Folsom, 1923), mosaic A, or some other virus disease. Later the 

 same symptoms were obtained when apparently healthy potatoes, were 

 used as sources of inoculum. One is therefore led to the conclusion that 

 the symptoms of the various mosaics mentioned are due not to mosaic 

 B but to other viri, ami that the virus of mosaic B produces no visible 

 symptoms on potato. 2 



Time required for the production of symptoms 



The length of time after inoculation and before symptoms appeared 

 varied greatly with the suscept, the inoculum, and the environmental 

 conditions. In general, the symptoms appeared most quickly on plants 

 that were growing rapidly. The shortest time recorded was four days; 

 this was for Nicotiana rustica inoculated with mosaic A. Other plants 

 showed the first detectable symptoms seven weeks after inoculation, 

 and with potatoes the symptoms were sometimes detected only in the 

 second generation, seven months or more after inoculation. It may be 

 said that the longer periods usually indicate conditions unfavorable for 

 the detection of mosaic, such as slow-growing pot-bound plants or red- 

 spider or thrips injury. Under favorable conditions of temperature and 

 other factors, plants will usually show symptoms within three weeks. 

 In any experiment, the individual plants are likely to show mosaic at 

 about the same time. Plants were therefore generally kept for only a 

 week or so after the first symptoms began to appear. 



One would naturally expect to find some difference in the length of 

 time between inoculation and the appearance of symptoms, according to 

 the suscept and the inoculum. It is evident that so many other factors 

 were involved, besides these two, that conclusions drawn from the experi- 

 ments here reported can be regarded only as approximations. How- 

 ever, the general impression gained from experience is that in most cases 

 the period varies more with the suscept than with the inoculum used, 

 and that there is a considerable personal equation involved; that is to 

 say, one person will recognize the first symptoms more readily than will 

 another, and the same person will recognize early symptoms more readily 

 after some experience with a given mosaic and suscept than at an earlier 

 date in his experience. 



When inoculated with mosaic A, Nicotiana rustica, appears to show 

 Symptoms soon, tobacco and Solatium aculeatissimum somewhat later, 



'This phenomenon is discussed at great ••' 1 • igth on page 31. 



