Feb. i8, 1918 Tobacco Wildfire 453 



in the field of diseased tobacco. Microscopic examination, furthermore, 

 showed that the dead tissues were filled with bacterial organisms. 



The lesions, both naturally and artificially produced, are believed to 

 have originated around punctures made by leaf-hoppers, which were 

 abundantly present on these plants throughout the season. The wild- 

 fire organism is capable of multiplying within the cells weakened as a 

 result of the withdrawal of their contents by the feeding of these insects, 

 but is not able to parasitize normal cells. Drops of moisture laden with 

 bacteria certainly dripped from the diseased tobacco plants to the cowpeas 

 beneath them, and could thus have supplied the inoculum which caused 

 the cowpea foliage to become spotted. This explanation is supported 

 by the observation that the lesions on cowpeas did not increase in size 

 beyond pinpoint-like dead areas, indicating that Bact. tabacum can not 

 adapt itself to invade healthy tissues, and by the further fact that no 

 new spots developed subsequently on the naturally and artificially 

 inoculated plants. Furthermore, spots never developed on cowpeas 

 growing at a distance from diseased tobacco plants — that is, where they 

 could not be infected through the agency of water dripping from diseased 

 tobacco plants. Bact. tabacum, therefore, is not parasitic upon cowpeas, 

 and its chance occurrence upon this crop indicates that conclusions as to 

 the pathogenicity of bacteria when judged from inoculation experiments 

 in which the inoculum is introduced through wounds are not entirely 

 convincing. In view, therefore, of the fact that tobacco is not grown in 

 the vicinity of West Raleigh, where the inoculation experiments were 

 conducted, and that all of the uninoculated plants within the greenhouse 

 grew to maturity without any manifestation of wildfire, there is no doubt 

 that all of the infections which were secured resulted from inoculations 

 with the organism in hand. When judged by the readiness with which 

 infection occurs, Bact. tabacum is to be regarded as a very vigorous 

 pathogene. 



Aside from the inoculations upon cowpeas, only two other host species, 

 bell peppers (Capsicum, annuum,) and Jimson weed {Datura tatula), were 

 employed, with negative results. 



PATHOLOGICAL ANATOMY 



Affected tissues were fixed in 95 per cent alcohol, embedded in paraffin, 

 sectioned, and stained with carbol-fuchsin. The presence of a crystal- 

 line substance whose nature is described in a recent paper by Ridgway (5) 

 interfered seriously with the cutting of suitable sections. In tissues in 

 which the cells had not yet become dry and collapsed, bacteria are abund- 

 antly present within the intercellular spaces (fig. i). In mature lesions, 

 however, they occur also within the cells. The contents of such cells 

 appear to have been completely destroyed, whereas the walls have 

 undergone little disintegration. The complete disintegration of diseased 

 tissues, which occurs in the presence of excessive moisture, results, it 



