30 



Black Dry Rot in Swkdes. 



above ground, and the roots, being uninjured, continue 

 to supply the "bulb" with water; hence the cells are 

 turgid, and when dead become reduced to a sloppy mess. 



An examination, by the microscope, of the small rough 

 patches on the external surface of the rind of the infected 

 swedes proved them to be due to local developments of cork, 

 but the cells beneath them were quite healthy, and I am 

 unable to suggest any satisfactory explanation of this special 

 cork formation. In places, here and there, traces of the 

 original purple colour still remained. 



Microscopical examination of the internal diseased tissues 

 showed that the cells were dead, the walls discoloured and 

 brown, slightly swollen, and easily separable from one 

 another. The protoplasm was also much discoloured. No 

 trace of any hyphae could be discovered, but innumerable 

 bacteria swarmed in the cells and intercellular spaces. These 

 bacteria were the only organisms present which might 

 account for the appearances described. The presence of 

 bacteria, however, afforded no proof that they were the cause 

 of the disease ; it was only by obtaining pure cultures of the 

 bacteria, and observing their action upon the cells of healthy 

 swedes, under special sterile conditions, that their degree of 

 responsibility could be determined. 



Minute portions of the diseased tissues, in an early stage 

 of attack, were employed for a series of cultivations ; and 

 by means of cultures in^a neutral turnip broth, rendered solid 

 by gelatine, a bacterium was ultimately isolated which, 

 when sown upon sterile blocks of swede procured from an 

 entirely fresh source, gave rise to this particular disease. 

 The rot commenced at the point of inoculation and gradually 

 spread throughout the block. Its development, at first, was 

 extremely slow; thus, in a set of ten blocks it was not until 

 the fifth day after inoculation that the black-brown colour 

 appeared, and the entire block was not completely rotten in 

 seven weeks. But these experiments were carried out in an 

 incubator at the constant temperature of 20° C. (68° F.) This 

 temperature was too high, for whenVeduced to 15 0 C. (6o c F.), 

 the progress of the parasite was much more rapid. 



