4 BULLETIN 82, U. S. DEPARTMENT OF AGRICULTURE. 



It seems established that the amoebae pass from one cell to another 

 when cell division takes place, although it is claimed by Massee (1908) 

 that the amoebae invade new cells by boring through the cell walls. 

 On the other hand, Osborn (1911), who has made an extensive cytolog- 

 ical study of this organism, holds that "on the division of the host 

 cell * * * it is a purely fortuitous circumstance whether each 

 resulting cell shall contain an amceba, and so be infected or not. 

 * * * I have never seen any signs of the migration of an amoeba 

 to a neighboring cell nor any continuity of protoplasm, such as 

 Massee has described." Osborn' s contention as to the method of 

 migration of the amoebae has been confirmed by Home (1911). The 

 abnormal local increase of cells causes a swelling and a faint discolor- 

 ation of the skin, which latter becomes a wartlike outgrowth. The 

 fungus present in this tissue consumes largely the contents of the 

 ceHs (PI. I, C, D), after which the amoebae coalesce (PI. I, C, D) and 

 form one or more large spongy masses in each cell, known as plasmodia 

 (PI. I, D). These latter divide into many small spores, each of which 

 takes on a heavy yellowish brown wall (PI. I, E). Instead of these 

 spores separating, they remain attached, forming a "spongelike body," 

 according to Johnson (1908), and not a hollow sphere, as reported by 

 Berkeley (1846) and Massee (1910). 



Since the contents of the attacked host cells are used up in form- 

 ing spore balls, the infected area becomes a pit filled with a yellow- 

 ish brown dust consisting chiefly of spore balls (PI. II, C). These 

 pits, or son, at maturity are bordered by the torn skin of the tuber. 

 The torn skin standing up on the periphery of the sorus is one of the 

 characteristics of powdery scab, which often enables one to dis- 

 tinguish it from the Oospora scab macroscopically. The powdery con- 

 tents of the sori in this stage of the disease doubtless suggested to 

 Johnson the name "powdery scab," as has already been noted. It 

 has been observed that in storage shriveling and shrinkage take place 

 about these pits, or sori. How generally this shriveling occurs and its 

 significance are not known up to the present writing, nor have these 

 matters been emphasized in the literature, to the writer's knowledge. 

 The cause of this shrinkage is not known, but it may possibly be due 

 to insufficient cork deposition in the bottom of the sori which afford 

 an avenue for storage rots to attack the infected tuber. 



When conditions are highly favorable for the fungus, it may eat 

 large cavities in the immature tubers. Besides consuming part of 

 the tubers, it stunts their further growth and produces malformed 

 tubers, such as are shown in Plate III. The nature and extent of the 

 depressions caused are shown in Plate III, C. This stage of the dis- 

 has been called the cankerous stage (Home, 1911) and is the 

 one that causes the greatest loss. 



