160 Maine Agricultural Experiment Station. 1920. 



ascertained only by growing and examining the progeny of the 

 tubers. While these differences have in the past been puzzling 

 to those who tried to demonstrate similarity between the mosaic 

 of tobacco and that of potato, it now seems that the two are 

 essentially similar maladies and that the principles discovered 

 regarding the former may be safely assumed in general to govern 

 the latter where they have not already been proved to do so, as 

 described in this bulletin. This similarity should now facilitate 

 progress in the study of potato mosaic. 



Appearance of the Diseased Plants. 



The leaves of mosaic potato plants are mottled with light 

 green or yellowish green spots which vary greatly in abundance, 

 location, and shape. They may be merely scattered sparingly, 

 occurring on any part of the leaf, or may be numerous over the 

 whole leaf. They often include or join parts of the larger veins 

 or ribs, but may not come in contact with them. They may ap- 

 pear as dots or circles, but usually are irregular and often are 

 elongate. They may be distinctly set off from the healthy green 

 parts or may fade out gradually. They seldom are more than 

 a quarter of an inch across, altho they may follow a vein for a 

 longer distance. Usually the foliage is wrinkled or ruffled. In 

 fact, this wrinkling ordinarily will be apparent to an observer 

 before the mottling or spotting is, especially if the sunlight is 

 allowed to strike the plants that are being looked at. Diseased 

 plants are frequently dwarfed. Figures 28-30 show diseased 

 leaves and plants. 



The preceding description is most characteristic of the dis- 

 ease on the Green Mountain and Bliss Triumph varieties. In 

 some cases with these varieties, and nearly always with Irish 

 Cobblers, the spotting is absent and only extreme dwarfing and 

 wrinkling indicate the presence of the disease. The Rural types 

 seem to display the disease only as dwarfing and leaf wrinkling 

 and curling, if at all. Some other varieties, as indicated later, 

 are as yet not known to have mosaic even when they have been 

 exposed to infection. 



The dwarfing and the wrinkling and downward curling of 

 the leaves that may be characteristic of mosaic plants should not 

 lead to confusion with leaf roll, which is characterized by an 



