456 Journal of Agricultural Research voi. xii, No. 7 



DISSEMINATION OF WILDFIRE 



The fact that the disease appeared in epidemic form twice in one 

 season and that each epidemic followed a rainy period, with little or 

 no new infection in the interim suggests that dissemination is primarily 

 influenced by moisture. This is in accord with observations on the 

 dissemination of certain other plant diseases of bacterial origin, as 

 angular leafspot of cotton (4) and Citrus canker (6). 



Very striking evidence was found in two instances that wind is also 

 a potent factor in the spread of wildfire. In one instance a field was 

 observed before and several days following a rain which was accom- 

 panied by a high wind. The disease had advanced in consequence 

 over a distance involving 1 6 to 18 rows lying parallel to diseased tobacco. 

 The disease terminated rather abruptly beyond this distance. In 

 another locality no disease occurred, except in the case of a field of 

 approximately an acre in area, the plants for which had been brought 

 for a distance of several miles. Here, again, the disease spread, fol- 

 lowing the same storm into an adjacent field to the leeward, was most 

 abundant near the field of diseased plants, and gradually became less 

 in the direction away from the diseased field. The organism had very 

 evidently been spread by wind-blown rain, a phenomenon in accord 

 with observations by Faulwetter (j) upon the angular leafspot of cotton. 



When the first epidemic was prevalent in the vicinity of Oxford, N. C, 

 thrips were abundantly present upon tobacco and were popularly sus- 

 pected of being responsible for the spread of the disease. Accordingly, 

 diseased leaves bearing numbers of these insects were collected, and 

 the thrips were liberated upon healthy plants in the greenhouse at 

 West Raleigh. Careful watch was kept, but no evidence of wildfire 

 developed upon any of the plants. In the same season only a few 

 thrips were found in diseased fields near Wendell, N. C, during an 

 entire afternoon's search. For these reasons it is improbable that 

 thrips are to be regarded as agents of dissemination. 



Following the first outbreak of wildfire, opportunity was afforded to 

 make numerous observations upon the origin of the disease. In every 

 instance where the disease occurred in the field it has been possible to find 

 that plants in the seed beds or "plant beds" were also affected. It was 

 adjudged, therefore, that the disease must have been introduced into the 

 plant beds either through the use of infected seed or through the agency 

 of fertilizers. One large seed farm upon which diseased plants occurred 

 was visited in searching for wildfire lesions upon seed pods, and, further- 

 more, affected pods were carefully sought lor in many other diseased 

 fields with negative results. 



Since the disease is already so widely spread, which suggests that it 

 must have had some common agency of dissemination, and since tobacco 

 stems were incorporated in certain fertilizers as the source of potash, 

 inquiry was directed to determine the possibility of the introduction and 



