juneis. I9IS Spongospora Subterranea 273 



Although the dry rot usually extends in all directions from the old sorus, 

 the Plasmodium is generally to be found only on one side. Nevertheless, the 

 broken-down cells throughout the dry-rot area show the path that it has 

 taken, indicating that the plasmodium moves about as it feeds on the 

 tissue around and beneath the old sorus. It is not common for the rot 

 to extend very deep, but plasmodia may occasionally be found as much 

 as 6 or 8 mm. beneath the surface of the tuber. They usually feed in the 

 tissue immediately beneath the epidermis, and whether or not they may 

 go deeper into the tuber after it has been planted is a question that 

 needs investigation. When infected potatoes are used as seed, the 

 mother tubers undoubtedly harbor the plasmodia during the growing 

 season. 



In sections through some of the sori that are just beginning to show 

 the dry rot very young plasmodia can be seen. Many of the spore balls 

 in the base of the old sorus show germination, each cell of a spore ball 

 producing a single uninucleate amoeba. This method of germination 

 agrees with that observed when the spore balls are placed in culture 

 media. Sometimes, however, the walls of an entire spore ball disinte- 

 grate, leaving the amoebae in such close contact with each other that they 

 seem to fuse and can not be distinguished as separate bodies. Thus, a 

 single spore ball may give rise to a baby plasmodium, several of which 

 coming together form much larger ones. 



The Plasmodia in these secondary infections have much the same 

 appearance as those which infect the young tubers. In favorable sec- 

 tions nuclei and the characteristic deeply stained globular bodies can 

 be seen. The nuclei are rather poorly fixed, but a nuclear membrane 

 and nucleolus can be distinguished. 



It is interesting to compare the effect of the plasmodium on the growing 

 cells in the young tubers with its action on the mature cells in the tissue 

 around the old sori. The growing cells are not killed, but are stimulated 

 to increased growth and division; and it is this growth and proliferation 

 of cells that produces the raised sorus. The mature cells, on the other 

 hand, are quickly killed, the tissue is destroyed, and shrunken, discolored 

 areas result. The sorus is somewhat definite as regards shape, size, and 

 number of cells attacked. Although the dry-rot areas are usually not 

 very large, there seems to be no limit to the number of cells that may 

 be destroyed when secondary infection occurs. The writer is of the 

 opinion that the dry rot may be considered a mild form of the canker 

 stage of 5. subterranea. The canker stage by which deep holes are eaten 

 into the tuber is, in all probability, an especially virulent form of the 

 plasmodial stage in secondary infections. It is frequently mentioned in 

 the European literature and is considered a serious form of the disease. 



Other types of dry rot following 5. subterranea and associated with the 

 presence of Fusarium spp., Phoma spp., and other wound parasites also 

 occur. 



