TOBACCO DISEASES AND THEIR CONTROL. 27 



In many districts mild cases of wild-fire are likely to be mistaken 

 for rust or firing of a less serious kind. Many other forms of leaf 

 spots or rust occur, as will be described later, but with close observa- 

 tion the grower, in most cases, can learn to distinguish wild-fire from 

 other diseases. When found it behooves the grower to use all possible 

 precautions to prevent its further spread. 



Cause. — Wild-fire is caused by an organism belonging to the bac- 

 terial group and known as Bacterium tdbacv/m. This organism is 

 capable of entering tobacco leaves, especially through wounds, and 

 when once started proceeds to multiply to enormous numbers, so that 

 hundreds of thousands of bacteria eventually may be present in a 

 single spot and serve as a source of infection on the same or other 

 leaves as soon as conditions become favorable. 



Under unfavorable conditions for the growth of the bacteria, how- 

 ever, they are usually short lived. While the great majority of 

 them die during the season without producing any infection, a few 

 manage in some way to live over winter and become the source of 

 seed-bed infection the next season. Under just what conditions 

 and on what materials used in or about seed beds the bacteria over- 

 winter is not yet definitely known in all cases. While strong sus- 

 picion rests on dried leaves, seed, soil, seed-bed covers, and so forth, 

 yet all observed cases can not be explained on these grounds. Never- 

 theless, our present basis of control must rest on the supposition 

 that some of the bacteria do in some manner live over winter on one 

 or more of the materials or equipment used in growing plants. It 

 now seems, however, that tobacco trash from infected crops, trans- 

 ferred in some manner to seed beds in the spring, is the most likely 

 source of infection. 



Once infection has occurred in the seed oed or field, subsequent 

 spread is readily explained. In addition to being spread through 

 the air by rain and wind, it may be carried by men, animals, tools, 

 or any other equipment in various ways from leaf to leaf, plant to 

 plant, and farm to farm unless proper precautions are observed. 



Conditions favoring the disease. — Beating rainstorms are well 

 known to be especially favorable to the spread of wild-fire in the 

 fields or in unprotected seed beds. This follows from the fact that 

 such storms not only spread broadcast the organisms causing the dis- 

 ease, but also wound the leaves sufficiently to* markedly aid infec- 

 tion, at the same time furnishing the necessary moisture. Few new 

 infections occur during dry weather, although heavy dews remaining 

 on the leaves for some time permit some infection. To wound the 

 leaf tissue is not necessary for infection in all cases, but it enhances 

 infection markedly. Rainy periods without accompanying winds 

 may result therefore in a very considerable spread of the disease. 

 As far as temperature is concerned, wild- fire seems to work over a 

 wide range, although some difference occurs in the incubation period 

 and the type of injury produced. Some infection will take/ place 

 with the temperature as low as 60°, but the best infection results when 

 it ranges between 80° and 90° F. 



It is also fairly evident that rapidly growing tobacco, as a rule, 

 is more likely to become heavily infected than stunted tobacco. It 

 also seems that some relation exists between the type of soil or the 

 fertilizer applied and the extent of infection, but this relation is not 

 yet fully understood. 



