204 The Potato 



Mosaic 



This disease is characterized by a crinkhng of the 

 fohage and by a mottled condition due to numerous small 

 areas of the leaf-blade having a lighter green color than 

 is usual. In some plants this is not especially notice- 

 able, for there are all gradations from healthy plants to 

 badly diseased ones. The disease is more commonly 

 found on white sprout varieties and is said to reduce the 

 yield materially. Since, like curly dwarf, it is transmitted 

 by means of the tuber, the same selection should be prac- 

 ticed. 



Leaf-roll 



Leaf-roll is the name given to a non-parasitic disease 

 of the plant and is characterized by a dwarfing and a 

 yellowing of the vines and by an upward rolling of the 

 leaflets about their midrib. The leaves have a tendency 

 to point upward and are rather stiff to the touch. There 

 are, perhaps, several types of the disease. In one type 

 the lower leaves at first show a tendency to roll and be 

 stiff though the plant is otherwise normal in appearance. 

 In another type the foliage has a yellowish cast. The 

 leaflets of the upper and younger leaves point upward 

 and roll and often show a rose or light purple color at the 

 lower margins. There are all gradations of the disease, 

 and the well-marked t^-pes show great reduction of growth 

 and yield. The disease is transmitted by means of the 

 tubers, so that rigid selection in the field is necessary to 

 avoid it. 



The cause of curly-dwarf, mosaic and leaf-roll is un- 

 known. Many theories have been advanced to explain 

 them, but it is unnecessary to discuss them here. All 



