26 



THE FUNGI WHICH CAUSE PLANT DISEASE 



sealed with wax. The plants showed rot in a few days and the 

 actual causal relation of the organism was thus established. Con- 

 firmatory evidence was gained by Russell " from puncture inocula- 

 tions in cabbage and 

 cauliflower petioles. It 

 was further shown by 

 E. F. Smith ^ that the 

 cabbage and turnip or- 

 ganisms are identical 

 and that the bacteria, 

 by solution of the cellu- 

 lose, produce pits and 

 holes through the walls 

 of the host cells re- 

 sulting eventually in 

 large cavities. 



Infection was shown 

 by Russell ^' and by 

 Smith ^- 39 to be chiefly 

 through the water pores 

 or through wounds 

 made by insects; the 

 bacteria being air or 

 insect borne and de- 

 rived largely from in- 

 fected soil. After en- 

 tering the plant the 

 bacteria multiply rap- 

 idly, and migrate in 

 every direction by 

 means of the veins. 



Fig. 



13. — ^Ps. campestris; cross-section of a turnip 

 root. After Smith. 



Studies of Harding, Stewart and Prucha *" (see also) *^ showed 

 that it can survive the winter on the seed and thus infect seedlings. 



Ps. destructans Potter ^^ is described as an uniflagellate organism 

 causing a destructive soft rot of turnips and beets in England. 

 Doubt has been thrown upon its identity by the work of Harding 

 and Morse '^ and of Jones ^* who found supposedly- authentic 

 cultures to bear peretrichiate rather than polar flagella. See p. 42. 



