306 DISEASES OF ECONOMIC PLANTS 



chief symptom consists in small blackened areas upon the 

 leaf, limited at first to the regions near the vines. These 

 spots may enlarge within two days to cover whole leaves, 

 and the contents of the entire barn may become worthless, 

 the leaf having become so wet and soft as to readily fall 

 apart. 



The disease is known in Connecticut, Virginia, Kentucky, 

 etc. In some sections it occurs yearly in more or less de- 

 structive form. Its cause is not certainly known; indeed 

 there may be several separate organisms, each capable of 

 inducing the rot. 



The trouble can be largely controlled by proper regula- 

 tion of the moisture and temperature conditions, i.e., by 

 building such barns that the air of outdoors, when too 

 damp, can be kept out, and yet with proper provision for 

 ventilation to carry off the moisture from the drying tobacco. 



Mosaic, calico, frenching. — The tobacco mosaic is very 

 widely distributed and destructive. Depreciation of 

 $50,000 in quality was attributed to it in Ohio in 1905. 

 It causes a mottled appearance of the leaves due to va- 

 riation in texture and greenness ; parts of the leaf show- 

 ing full green and normal thickness, other spots pale or 

 yellowish green, and thin. Accompanying these signs are 

 distortions due to unequal growth, wrinkled or curled 

 leaves, and leaves of one-sided growth. Slightly affected 

 leaves are depreciated in value; badly diseased leaves are 

 worthless. 



This disease is not perfectly understood, but it seems to 

 be communicable from plant to plant by first touching a 

 diseased plant and later touching a healthy one. Thus 

 the disease is spread largely by topping. It is known also 



