﻿26 BULLETIN 16, U. S. DEPARTMENT OP AGRICULTURE. 



the only way to free a field from the pest is by absolutely clean fal- 

 low cultivation for a year or two or by growing only immune crops 

 for two or three years so as to starve them out. 1 



The Granville wilt, first observed in the eighties of the past century, 

 is a bacterial disease communicated through the soil. In the flue-cured 

 district the infested area so far as known is largely confined to one 

 soil type in the southern part of Granville County, N. C. This soil 

 naturally produces a very fine type of wrapper tobacco. The disease 

 is spreading quite rapidly locally and now occupies a considerable 

 area in that section, embracing one of the very best bright-tobacco 

 areas which we have. Once the soil is infected, no practical means 

 have yet been devised for controlling the disease, and it is difficult 

 to prevent it from gradually spreading to other adjoining areas. 

 While the disease is confined to a comparatively restricted area, it is, 

 nevertheless, a very ruinous one in that section. 



The mosaic disease, frequently spoken of as calico or "mottling," 

 probably is the most widespread of all the tobacco diseases. Until 

 recently it has been quite generally supposed to be simply a mani- 

 festation of malnutrition, caused by unfavorable growing condi- 

 tions. It has long been known to be infectious, however. It can be 

 spread, for example, by rubbing the leaves of a diseased plant and 

 then likewise rubbing the leaves of healthy plants. Recent tests by 

 Mr. H. A. Allard, of the Office of Tobacco and Plant- Nutrition 

 Investigations, Bureau of Plant Industry, go to show that the dis- 

 ease is a specific infection and in the absence of such infection can 

 not arise from impaired nutrition. It has been discovered that cer- 

 tain aphids or plant lice are largely responsible for the dissemina- 

 tion of the disease. Other names, such as trenching, Walloon, etc., 

 are applied to a group of diseases resembling true mosaic more or 

 less. Such diseases as the so-called "sore-shin" and "rotten-stalk" 

 are sporadic and occasional in their appearance, and are thought to be 

 due primarily to some mechanical injury which may admit disease 

 germs that attack the tissues locally. 



The occurrence of so-called " dead spots " here and there over a 

 field, particularly in the Old Belt section, in which the plants, with- 

 out apparent cause, fail to make any growth, is a phenomenon fre- 

 quently observed, especially in a dry summer following a very wet 

 spring. The soil of these spots generally appears to be in as pro- 

 ductive a condition in all respects as other parts of the field. The 

 roots of the plants show no apparent injuries of any kind or evi- 

 dences of disease. No fully satisfactory explanation of the cause or 

 means of remedying the trouble are known to the writer. 



1 For a full discussion of nematodes and methods of eradication, see " Root-Knot 

 and Its Control," U. S. Department of Agriculture, Bureau of Plant Industry, Bulletin 

 217, 1911. 



