TOBACCO WILDFIRE. 71 



for two or three daj's, this confirms our conclusion as to the length of the 

 period. 



According to Wolf and Foster (5) the bacteria at first propagate only 

 between the cells of the leaf. The cells soon begin to collapse, and one 

 finds not only the intercellular spaces, but the cells themselves filled with 

 dense masses of bacteria. Enzymes secreted by the bacteria apparently 

 break down the cell contents, and some of the decomposition products 

 are used as food for further multipUcation of the parasites. As the tissue 

 collapses the bacteria either ooze out to the surface or are exposed by 

 rupture of the epidermis. 



Dissemination. 



This disease was called "wildfire" because of the extreme rapidity with 

 which it spreads. It has been noted by all investigators of the disease and 

 by tobacco growers that rapid spread invariably follows heavy rains. 

 When the rain drops fall on the diseased spots, the bacteria float out into 

 the water and successive drops splash them to other leaves of the same 

 plant or to neighboring plants. If the rain is accompanied by wind the 

 drops are carried farther and the spread is greater to the windward of 

 diseased plants. A number of cases have been observed by the writers 

 where the spread from a single diseased plant or diseased row resulting 

 from wind-driven rain has been carefully followed. Invariably the area 

 of new infection has been from two to ten times as great to windward as 

 to leeward. These two agents (wind and rain) are undoubtedly the most 

 potent of all the factors involved in dissemination. A number of experi- 

 ments with various agents suspected of disseminating the disease have 

 been conducted at this station and are summarized below. 



Splashing Rain. — In order to corroborate field observations, dropping 

 water from a rose nozzle in the greenhouse was allowed to fall onto dis- 

 eased leaves and then splash to healthy young plants. The splashing was 

 continued for five hours. Wind, insects and all other agents were ex- 

 cluded. Within five days lesions were observed on the plants where the 

 water had splashed. Check plants in the same bed, which were separated 

 from the splashed plants only by a glass partition and which had been 

 splashed with uncontaminated tap water at the same time, remained 

 entirely healthy. The results confirm in every way the conclusion from 

 field observations that this is a very important agent of dissemination. 



Wind. — That wind may be important in carrying the infested rain 

 drops to greater distances has been previously mentioned and needs no 

 further demonstration. But on the other hand, it seemed to be important 

 to determine whether wind alone, without rain, could carry the bacteria 

 from a diseased to a healthy plant. Therefore five diseased plants in pots 

 were placed before an electric fan, and twelve healthy potted plants set 

 at distances ranging from three to twent5'-four inches, so placed that the 

 air current passed from the diseased to the healthy plants. The fan was 

 turned on for three hours on four successive days, the plants were sprinkled 



