38 



BULLETIN 1250, IT. S. DEPARTMENT OF AGRICULTURE. 



are not extremely high, although it is at times quite an important 

 factor in crop production (fig. 21). 



Cause. — The actual cause of frenching is not yet known. Al- 

 though a number of predisposing factors have been observed, the 

 production of this disease at will, as far as known, has not been 

 repeatedly accomplished. Frenching is apparently not infectious; 

 consequently there is no reason to believe that any foreign organism 



Fig. 20. — A frenched tobacco plant. The numerous shoestrin.ay leave:- 

 This disease is often confused with mosaic. 



are typical. 



is associated with it. The disease is frequently found on soils ex- 

 posed in one way or another to excessive water. This, together with 

 its occurrence at times on portions of fields in poor tilth, indicates 

 a relation to soil aeration. On the other hand, frenching seems to 

 have considerable relation to soil fertility, or at least to a deficiency 

 of certain fertilizing elements, but in combination with these factors 

 certain weather conditions must apparently exist to bring about its 

 expression. 



