242 DISEASES OF ECONOMIC PLANTS 



ent upon horse-radish, but does not usually become 

 serious. 



Leaf spot, cercosporose (Cercospora Artnor acioe Sacc). — 

 This spot is much like that of ramulariose. 



LETTUCE 



Sclerotiniose, drop, wilt (Sclerotinia Libertiana Fckl.). — 

 The disease begins with wilting of the outer lower leaves, 



Fig. 110. — Sclerotia of Sclerotinia from lettuce germinating. 



which droop and fall flat upon the ground; a similar fate 

 rapidly overtakes the inner leaves, and so on over the whole 

 plant, until within a few days the entire plant is dead, 

 lying flat upon the ground, appearing much as though 

 scalded by a dash of hot water. The disease progresses 

 with wonderful rapidity, the plants appearing to die in a 

 day. 



Upon the undersides and at the bases of the leaves occur, 



