Vegetable and Field Crops 



233 



and is, therefore, of greatest importance to growers of seed 

 potatoes. Seed should be carefully selected, and then dis- 

 infected. 



Leaf-roll -^' -^ '^^^^ ■'^'^' -'-^^ (Phloem-necrosis) and Mosaic 

 (Leaf-curl or Curly-dwarf) , two closely related diseases of wide 

 distribution, cause serious crop loss. In Bermuda, for exam- 

 ple, the loss from leaf-roll is estimated to be well above 50 



Fig, 130. — Potato leaf-roll. After Orton. 



per cent of the crop. The two diseases, formerly regarded as 

 three, are difficult to distinguish accurately without the use 

 of the microscope, but in general in leaf-roll the segments of 

 the lower leaves are pale, permanently rolled upward and 

 rigid, the terminal segments becoming spoon-shaped; while in 

 mosaic, generally, the leaf segments are distorted, the veins 

 unduly prominent, and the blades mottled with light areas, 

 and in severe cases the plants tlwarfed by shortened inter- 

 nodes, giving rise to the designation "Curly-dwarf." Plants 

 affected by either of these diseases show retarded tuber 

 development and reduced yield. 



