TOBACCO WILDFIRE. 69 



on a single leaf (Plate I, Fig. C), and when numerous they usually run 

 together to produce large irregular dead areas. Frequently, when a leaf 

 is attacked while it is rapidly developing, the affected part becomes dis- 

 torted and uneven (Plate I, Fig. B). Spots may be located on any part 

 of the leaf, but a great number of them are marginal with a semi-circular 

 halo. During dry weather the dead areas remain intact, but in stormy 

 weather they may be broken out and result in a ragged appearance of 

 the leaf. Severely attacked plants, especially in the seedling stage, may 

 die, but more often the plant continues to grow and is only stunted by 

 the injury or loss of a part of its leaves. In the field the lower leaves are 

 most affected. No lesions are found at the top of the plant on the very 

 young leaves which are just unfolding. Badly affected leaves are prac- 

 tically worthless on the market, as they cannot be used for wrappers or 

 binders. 



In the very early seedling stage, when the plants are no larger than the 

 thumb nail, the symptoms may be atypical and not readily recognized 

 as wildfire by the inexperienced. Superficially, affected plants have much 

 the appearance of being attacked by ordinary dam ping-off . The leaves 

 are usually affected from the margins inward and the lesions are more 

 typical of a wet rot, and in this condition the water-soaked, translucent 

 line is particularly noticeable between the living and dead tissue. Very 

 often the entire leaf is withered, and nothing but the dried midrib is ob- 

 servable. In such plants the stem, however, is usually not affected, and 

 this character differentiates the trouble from the ordinary damping-off, 

 for in the latter disease infection starts in the stem and the entire plant 

 rots down. 



CAUSE OF WILDFIRE. 



When the disease first came under investigation in North Carolina, 

 Wolf and Foster (4) demonstrated by isolations and inoculations that it 

 is produced by a parasitic species of bacteria which they named J5oc- 

 terium tohacum. In Massachusetts the writers have made numerous isola- 

 tions from all the types of lesions described above, and have invariably 

 obtained pure cultures of an organism which gave the same cultural 

 tests as described by Wolf and Foster (5) . The same organism has never 

 failed to produce the typical disease when healthy plants were inoculated 

 with it from pure cultures. 



An individual bacterium of this species is so microscopically small 

 (3.3 X 1.2/i) that over 7,500 of them placed end to end would form a 

 chain only one inch long. Its body is cylindrical with rounded ends, and 

 two or three times as long as broad. In fresh cultures the organisms 

 may be seen under the microscope actively darting about, the motion 

 being produced by vibrating the long tail-like fiagella (1 to 4 in number) 

 which are attached to one end of the body. The body increases in length 

 and divides into two individuals by the process of fission. After repeated 

 division short chains of bacteria may be observed before they break apart. 



