90 Report of the Botanist of the 



From l)lackenc(l jjundles resulting from these artificial inocula- 

 tions and at considerable distances from the point of infection, 

 in tissue which had been formed subsecjuent to the inoculation, 

 pure cultures of P. campestris were obtained. This completes the 

 proof considered necessary to establish the causal relation of an 

 organism to a given disease. 



MODE OF INFECTION. 



There are at least three avenues through which these germs 

 may gain access to the plants: 



(i) At the time of transplanting into the field some of the roots 

 are broken, exposing the ends of the fibro-vascular bundles to 

 the attack' of P. campestris in the soil. The plants which wilt 

 badly within a few weeks often have many black fibro-vascular 

 bundles in the stem when there is no evidence of the disease in 

 the leaves. (See page 103.) Tliis mode of infection seems to be 

 most active during the early life of the plants. 



(2) Insects, by eating the leaf tissue, expose the cut ends of the 

 fibro-vascular bundles and either infect these directly by their 

 jaws or leave the surfaces in condition to be infected from other 

 sources. This avenue of infection appears to be more important 

 with cauliflower than with cabbage, but in either case is of sec- 

 ondary importance. 



(3) Under favorable atmospheric conditions the water pores 

 at the margin of the leaves exude liquid. Any germs which find 

 their way into these drops after swimming back through the 

 opening of the water pore find themselves at the termination 

 of a fibro-vascular bundle. A great majority of the infections 

 occurring during the month of August can be traced to this 

 source. Sometimes as many as a hundred cases of this form 

 of infection may be seen on a single large plant. 



MODE OF DISSEMINATION. 



The natural habitat of P. campestris and the ways in which it 

 is distributed from plant to plant have not been satisfactorily 

 worked out. Observations made in connection with root infec- 

 tion makes it probable that P. campestris is able to live in the 



