258 



Diseases of Economic Plants 



prophylaxis are discussed under damping-off, p. 19. Seed 



beds known to be diseased 

 should be avoided unless 

 previously disinfected. 



Drop ^^^ {Sclerotinia liber- 

 tiana). — Damping-off due 

 to Sclerotinia is reported as 

 a common trouble in seed 

 beds. The injury to the 

 plant is similar to that of the 

 usual damping-off while the 

 fungvis presents the charac- 

 ters described under lettuce- 

 drop. Young plants are 

 killed ; older plants may sur- 

 vive, and if set in the field, 

 develop poorlv. 



Mosaic. ^^- ^'^ — Wher- 

 ever tobacco is grown the 

 mosaic is very widely dis- 

 tributed and destructive. 

 Depreciation of $50,000 in 

 quality was attributed to it 

 in Ohio in 1905. It pro- 

 duces a mottled appearance 

 of the leaves due to varia- 

 tion in texture and green- 

 ness ; parts of the leaf show- 

 ing full green and normal 

 thickness, other spots pale 

 oi- yellowish green, and thin. 

 Accompanying these signs 

 are distortions due to un- 

 equal growth, wrinkled or 

 curled leaves, and leaves of 



one-sided growth. Slightly affected leaves are depreciated in 



value ; badly diseased leaves are worthless. 



Fig. 141. — Tobacco leaf showing 

 leaf-spot. After Conn. Agr. 

 Exp. Sta. 



