Soil Sickness Due to Parasites 43 



usually begins in spots in the seed bed or in the field 

 and then may spread in every direction. 



The Organism. Pythium de Baryanum was first 

 named and described by Hesse in 1874. Ward 1 

 found it to be a very prevalent parasite in the garden 

 soils of Europe. In America the fungus was first 

 recognized by Atkinson 2 as of great economic im- 

 portance. Pythium de Baryanum, when examined 

 under a compound microscope, is seen to be made 

 up of coarse, non-septate, highly granular, irregular 

 branched hyaline vegetative threads or mycelium 

 (fig. 5 a). The younger threads are more finely 

 granular, the oldest ones are coarsely granular or 

 more often empty. These threads penetrate the 

 cells of the host, where they obtain food. 



Pythium de Baryanum does not often fruit freely 

 on the dead host. The fruiting is better observed 

 when it is grown in a pure culture. Under normal 

 conditions the fungus produces two forms of spores, 

 conidia (fig. 5 b) and oogonia (fig. 5 d, e). The 

 summer spores, or conidia, are swellings formed at 

 the tip of the hyphae. These swellings readily break 

 off from the mother threads and germinate by send- 

 ing out a slender tube (fig. 5 c) . This tube penetrates 

 the seedling tissue, where it grows and develops and 

 after due incubation reproduces the disease. The 

 oospore or sexual spore is the stage which is most 

 commonly found. The female oogonium first devel- 



1 Ward, M., Quart. Jour. Micros. Soc., New Ser. 22 : 487, 1883. 

 a Atkinson, G. F., New York (Cornell) Agr. Expt. Sta. Bui. 94 : 

 233-272, 1895. 



