MOSAIC DISEASE OF TOBACCO. 



81 



Economic Importance. 



• 



It is very difficult to estimate the loss to growers due to mosaic disease, 

 as the prevalence in different localities varies greatly, as also does the 

 intensity of the attack in different seasons. The damage resulting from 

 mosaic disease is twofold: first the plants when severely attacked are 

 smaller and the leaves poorer in quality; secondly, the buyer, if he sees 

 much mosaic in a field, will invariably cut the price a few cents a pound, 

 as the leaves affected do not in many cases make a valuable wrapper and 

 are much poorer in quality. The writer has observed certain fields where 

 the loss would run into hundreds of dollars from this cause alone. The 

 amount of damage done by late mild attacks when the plants are maturing, 

 or appearing on the sucker growth after topping, is practically negligible, 

 and, so far as can be learned, does not in any way injure the commercial 

 leaf. It is always well to clean off the diseased suckers, however, as they 

 present a very ragged appearance, and might injure the sale of the crop 

 to a certain extent. There is no question but that during certain seasons 

 the loss due to mosaic is quite large, but an exact estimate of this loss is 

 difficult to obtain, owing to the many other factors involved. 



Infectious Nature of the Disease. 



That the mosaic disease is very infectious is well known, and a discus- 

 sion of the detailed experiments on this point is not necessary. Experi- 

 mentally it has been repeatedly shown that the juice from all parts of a 

 diseased plant is capable of transmitting the disease, although it should 

 be stated that the percentage of infection obtained from the root extract 

 is considerably lower than that obtained from the leaves. A few of the 

 results obtained are given in the foUomng table, however: — 



Table I. — Infectivity of the Juice from Different Parts of Diseased Plants, 

 August, 1909. 



Part of Diseased Plant used 

 (Plants from Field). 



Number of Healthy Seedlings 

 inoculated. 



Number of 

 Plants Dis- 

 eased Three 

 Weeks after 

 Inoculation. 



Leaves showing disease, 



Control, 



Leaves showing disease, 



Control, 



Basal leaves (not showing disease) 



Control, 



Roots 



Control, 



Roots 



10 (juice; needle pricks), 



10 (distilled water; needle pricks), 



10 (insertion of tissue into veins), 



6 (insertion of healthy tissue into veins) 

 12 (juice; needle pricks), 

 5 (distilled water; needle pricks), 

 21 (juice; needle pricks), 



7 (distilled water; needle pricks), 

 16 (insertion of tissue into veins), 



