272 Journal of Agricultural Research voi. iv, no. 3 



Spongospora, as well as in the other genera of the Plasmodiophoraceae, 

 Plasmodium formation takes place only a short time before spores are 

 produced. 



OBSERVATIONS ON THE SECONDARY INFECTION OF TISSUE AROUND 



THE OLD SORI 



One of the most serious aspects of the powdery scab as it appears in 

 the United States is the dry rot that often sets in around the sori during 

 the fall and winter while the tubers are in storage. The dry rot is familiar 

 to all who are well acquainted with the powdery scab in this country. 

 Both Melhus (12) and Morse (13) have made mention of it in their bul- 

 letins on that disease. 



The demonstration of the existence of a saprophytic plasmodial stage 

 in the life history of S. subterranea as given below at once suggested the 

 possibility that this might be responsible for the so-called dry rot around 

 the old sori. Pustules showing the rot in various stages were fixed in 

 Flemming's stronger solution and later sectioned and stained with the 

 triple stain. Sections from this material plainly show plasmodia in the 

 shrunken areas around the old sori. Here they can be seen feeding on 

 the potato cells and killing them. Plate XXXIX, figure 9, shows a 

 Plasmodium pushing down between the walls of mature cells in tissue 

 near an old sorus. It causes the walls to become slightly swollen in 

 much the same way as was described for the infecting plasmodium in 

 the tissue of the young tuber. The walls appear to be softened and 

 more or less gelatinous. They are not swollen as much as in the case of 

 the cells in young tubers. The plasmodium can be seen pushing into the 

 cells through the softened cell walls. Sometimes the openings thus made 

 are quite small, but often a large portion of the wall is broken down. 

 Once through the wall, the plasmodium seems to distribute itself through- 

 out the protoplasm of the host. The dififerent stages of this process have 

 not been carefully studied, and it is not known in this case again just 

 how the Plasmodium gets through the limiting membrane of the host 

 cell. The cells are quickly killed, and the cytoplasm and nucleus disin- 

 tegrate, leaving only the starch grains. The starch seems to be acted 

 on very little by the plasmodium, which is rather surprising in view of its 

 action on the cell walls. As soon as one cell is killed, the plasmodium 

 passes on to the next, and in this way cell after cell is destroyed. In front 

 of the Plasmodium can be seen the healthy tissue, while behind it is left 

 a disorganized mass of broken-down cell walls, starch grains, and other 

 debris. It seems that the plasmodium has no harmful effect in advance 

 of the cells actually attacked, as would be the case if some poisonous 

 substance were secreted, but that the cells are killed by actually becom- 

 ing engulfed in it. The line between healthy and diseased tissue is very 

 sharp. 



