i8 Diseases of Greenhouse Crops 



greenhouse plants may become subject to damping 

 off by Pythium. When examined under a compound 

 microscope, Fythium de Baryanum is seen to be made 

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

 ly branched hyaline vegetative threads or mycelium. 

 The younger growing threads are more finely granu- 

 lar. The oldest are coarsely granular or more often 

 empty. These threads penetrate the cells of the 

 host, where they obtain its food. 



Pythium de Baryanum does not often fruit on 

 the dead seedlings. The fmiting is better observed 

 when the fungus is grown in pure culture. Under 

 normal conditions it produces two forms of spores, 

 conidia and oogonia. The summer spores, or co- 

 nidia, are swellings formed at the tip of the hyphse 

 (fig. 3, a.). These swellings readily break off from 

 the mother threads and germinate by sending out 

 a slender tube. 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 organ (oogonium) first devel- 

 ops as a terminal enlargement which is cut off by a 

 septum from the mother thread. Next or adjacent 

 to it a slender tube is cut off from the mycelium by 

 a septum. This tube performs the function of the 

 male sexual organ and is known as antheridium. The 

 latter then comes into close contact and empties all 

 its content into the female oogonium (fig. 3, b and 

 c). Fertilization thus takes place, and a mature 

 egg, or oospore, or winter resting spore is formed. 



