104 DISEASES OF CULTIVATED PLANTS 



PYTHIUM (PRINGSH.) 



Mycelium branched, septa rare; zoosporangia usually ter- 

 minal, wall thin; resting conidia present in most species, 

 terminal or interstitial ; oogonium containing one spore ; 

 wall of oospore colourless. 



Differs from Saprokgnia in the contents of the zoospor- 

 angia escaping before the cilia of the zoospores are formed, 

 whereas in Saprolegnia the zoospores are fully developed in 

 the zoosporangium. 



Parasitic or saprophytic on plants or animals, mostly in 

 water or very damp places. 



All forms of reproduction in this genus are produced on 

 the surface, never in the substance of the host. 



' Damping off.' This well-known term is used by gardeners 

 to indicate the sudden collapse of seedling plants, evidently 

 due to the shrivelling of the stem just above the ground line. 

 The injury to the stem is caused by a very minute para- 

 sitic fungus called Pythium de baryamim (Hesse). 



Seedlings of crucifers are more especially liable to attack, 

 but are by no means the only ones victimised, maize, clover, 

 millet, cucumber, and probably all seedlings are liable to 

 attack when conditions are favourable. Even the prothallus 

 or youngest stage of such cryptogams as club-mosses (Lyco- 

 podium) and horse-tail (Equisetum) are attacked. Finally, 

 stored potatoes, where 'sweating' has occurred, have been 

 attacked. 



Zoosporangia globose or broadly elliptical usually papillate, 

 terminal or intercalary ; resting conidial globose, terminal or 

 intercalary, 20-25 f- diam. Oospores globose, colourless, 

 smooth, 15-18 /A diam., producing a germ-tube on germina- 

 tion. 



Although this disease is considered as a scourge by almost 

 every gardener, yet in truth its occurrence is invariably due 

 to neglect. It can only possibly exist where seedlings are 

 too densely crowded, or where seed-beds are located in damp, 

 stuffy localities. The fungus cannot exist in open ground 

 exposed to wind and sunshine. 



Soil that has produced a diseased crop should be sterilised. 



De Bary, Bot. Ztg., (1881) p. 357. 



Marshall Ward, Quart. Journ. Micr. Soc.^ 23, p. 487 (1883). 



