196 Diseases of Economic Plants 



Leaf-spots {Ramularia armoracice Fcl. and Cercospora 

 armoracice Sacc.)- — These spots are much like those of leaf- 

 blight except that no pycnidia are present. 



Black-mold (Macrosporiurn herculeum E. & M.). — In 

 late summer circular, blanched, and later black, moldy leaf- 

 spots are prevalent. The injury is not sufficient to warrant 

 treatment. 



White-rust {Albugo Candida) and Downy-mildew {Peronos- 

 pom). — These diseases are the same as upon cabbage. 

 They are often present but usually insignificant upon horse- 

 radish. Root-rot due to Thielavia also occurs. 



KA.LE and KOHL-RABI. See cabbage. 



LETTUCE 251 



Drop -^^ {Sclerotinia lihertiana Fcl.). — The outer lower 

 leaves wilt, 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 

 hot water. The disease progresses with wonderful rapidity, 

 the plants appearing to die in a day. 



Upon the under sides and at the bases of the leaves occur, 

 at this time, dense, cotton-like growths of mycelium, and in 

 later stages, small, irregular black bodies, the sclerotia, which 

 vary from 3-10 mm. long, somewhat less in thickness. The 

 sclerotia and the mycelium are unmistakable signs of this 

 disease. 



Drop was first definitely recognized in America in 1900, 

 though it doubtless did much damage before then. It has 

 since then followed the rapidly extending lettuce industry 

 into many states and is particularly destructive in the 

 South Atlantic seaboard states from Virginia to Florida, 

 where its inroads upon the crop grown under cover are es- 

 pecially damaging. The causal fungus may persist from 

 year to year, as sclerotia, in beds once infested — beds cost- 



