210 Diseases of Truck Crops 



In severe cases the entire stand may be wiped out. 

 On carefully pulling out a diseased plant, we shall 

 find the side rootlets blackened, shriveled, and dead 

 (fig. 35 a). Frequently the plant attempts to pro- 

 duce new roots above the diseased area. In this 

 case, however, there is only partial recovery. The 

 disease is most prevalent in the heavy soils. 



The Organism. In general characters, the organ- 

 ism may be mistaken for Pythium de Baryanum, but 

 it differs from the latter in its asexual fruiting bodies. 

 The mycelium of Rheosporangium aphanidermatum 

 is hyaline, non -septate (fig. 35 c), and grows profusely 

 on solid- media. Mycelium of cultures one or two 

 days old exhibits considerable streaming of proto- 

 plasm which seems always directed toward the tip 

 end of the hyphae. This protoplasmic streaming 

 results in the final accumulation in protoplasmic 

 material, and in consequence of a considerable en- 

 largement of the tip of the thread. Finally a cell 

 wall is laid down which cuts off the swollen portion 

 from the rest of the mycelium. This swollen body 

 which Edson named presoporangium (fig. 35 b) has 

 the appearance of a zoosporangium but in reality 

 it differs from it since it gives rise not to zoospores, 

 as might be expected, but to an independent body 

 which later gives rise to zoospores. The pre- 

 soporangium now absorbs water and its outer wall 

 ruptures, from which is seen to flow out a mass of 

 protoplasm enclosed in a thin cell wall. This es- 

 caped mass is really the young zoosporangium, the 

 cytoplasm of which finally cleaves into zoospores. 



