84 Diseases of Truck Crops 



from two to three weeks old, but more often when the 

 plants have attained full growth. Often the trouble 

 is so serious and the curling so pronounced that the 

 plants thus affected cannot make any headway and 

 remain dwarfed. An attempt is made by the curled 

 plants to produce blossoms, but the latter, too, are 

 distorted and abnormal. Frequently, however, the 

 affected plants outgrow the disease entirely, and thus 

 a distinct line of demarcation is observed between the 

 previously diseased part and the healthy part of the 

 new growth. In rare cases, affected plants seem to 

 thrive in spite of the disease. Such plants should be 

 selected for the purpose of breeding resistant strains. 

 Cause of Mosaic. The recent works of Allard 1 

 and Freiberg 2 have shown that the cause of mosaic 

 is as yet a disputed question. Allard claims that 

 mosaic is caused by an ultra-microscopic pathogen, 

 that is, a parasitic organism which cannot be de- 

 tected by our present technique in microscopy. 

 Freiberg claims that the cause of mosaic is physio- 

 logical. The following is a summary of the claims 

 advanced by these two investigators. 



Allard Freiberg 



i. The virus is not inhibited i. The virus is not inhibitel 



by concentrations of one part of by formaldehyde, 

 formaldehyde in ioo, 200, 400, 

 600, 800, 1000, 1200, and 1500 

 parts of virus solution. 



'Allard, H. A., "Some properties of the virus of the mosaic 

 disease of tobacco," Jour. Agr. Research, 6 : 649-674, 1916. 



2 Freiberg, G. W., "Studies in the mosaic, diseases of plants," 

 Ann. Missouri Bot. Gard., 4 : 175-232, 191 7. 



