78 Internal Rust ^iH)t of Potato Tuber 



ring. In some of the first potatoes examined the patches had a greenish- 

 black water-soaked appearance and seemed to have arisen from the 

 vascular bundles suggesting P. solanarearum as the causal organism; 

 it is, however, probable that the water-logging is secondary being due 

 to the presence in the diseased tissue of certain saprophytic bacteria. 



The size of the patches varies enormously from a small island of 

 brown tissue of the size of a pin's head to an irregular-shaped patch 

 spreading through several centimetres of the tissue. In certain cases 

 the large patches can be traced to lenticels which show the " scabbed " 

 appearance described above, indicating that the exciting cause has found 

 entry into the tuber through the lenticel and that "the scabbing" is 

 the result of its action on the surrounding cells. 



Attempts to prepare diseased tissue for microtome sectioning have, 

 so far, been unsuccessful, strong lignification of the walls of the attacked 

 cells making them very difficult of penetration by fixing and other fluids. 

 Hand sections through a small patch show a group of normal cells packed 

 with starch grains and surrounded by a ring of brown tissue. In the 

 brown zone the cells have thickened walls which stain red with phloro- 

 glucin and this coupled with their resistance to cutting has led to the 

 conclusion that they are strongly lignified. As contents, these cells show 

 brown protoplasm in which starch grains may be embedded or the starch 

 may be replaced by globules of various shape and size which appear to 

 be of oil or possibly of tannin. In certain of the cells there may be 

 recognised masses of small bacteria intermingled with the starch grains. 

 In some sections, however, bacteria could not be distinguished, either by 

 motility or shape, from minute globules of the fat-like substance. The 

 brown ring which gives the characteristic appearance to the patch is 

 surrounded by a zone of colourless cells in which the starch has entirely 

 disappeared and given place to more or less colourless oily globules. 

 Frequently a small patch is found to be cut off from this zone of starch- 

 free cells by the formation of a typical cork callus. 



Sections through one of the larger patches show essentially the same 

 cell characters as described above except that the inner cells are much 

 disorganised, or they may have dried up and broken down leaving a 

 cavity surrounded by cells with the characteristic brown and lignified 

 walls. 



By inoculations of diseased tissue into sterile potato l)r()th an organism 

 has with difficulty been obtained and shown to be the cause of the disease. 

 Inoculations of sterile potato slices using pure cultures of the organism 

 have rejjeatedly given successful infections, and from the diseased area, 



