THE FUNGI WHICH CAUSE PLANT DISEASE 



77 



Fig. 47. — ^P. citriophora; de- 

 velopment of swarmsporea 

 from sporangia. After 

 Smith and Smith. 



and the contents of the antheridium are carried over to the egg 

 by a fertilizing tube. Members of the 

 genus are aggressively parasitic only under 

 most favorable environmental conditions 

 of heat an(f moisture. 



Some sixteen species are known. 



P. de baryanum Hesse, is most com- 

 mon ''"'' as the cause of "Damping Off." 



Zoosporangia or "conidia" globose to 

 eUiptic, usually papillate, 20-25 n; gemmae 

 similar in form and size; oospores globose, 

 hyaline, smooth, 15-18 /i. 



P. intermedium de Bary, causes a 

 "damping off" of fern prothalia,'^ P. 

 gracile Schenck, a rot of ginger; ^* P. 

 palmivorum Butler, a palm disease in India.'"^' ''^ 



Pithiacystis, Smith & Smith (p. 75) 



The sporangiophore is delicate; septate; and bears numerous 

 sporangia sympodially. These produce many biciliate zoospores 



internally. No oospores have been 



seen. Only one species is known. 



P. citriophora Sm. & Sm.''- '» 



Parasitic on lemons, the sterile 



myceliimi inhabiting the rind; 



spores normally formed in the soil 



near infected fruits; sporangia ovate 



X. or lemon-shaped, papillate, 20-60 x 



48.— Sporangiophores and spo- SQ-QO u, averaging 35 X 50 JU, bomc 

 ngia of Pythiacystis. After ""; ° f m ic 



Smith and Smith. sympodially; zoospores lU-io n, 



at first elongate, becoming rounded and bearing two lateral cilia. 



This was first noted by Smith and Smith ''• '" on rotting lemons 

 in California. Infection by pure cultures proved that the fungus 

 was the true cause of the rot. 



Peronosporales (p. 66) 

 These fungi constitute an order characterized by a richly 

 developed, branching, non-septate, usually coarse, mycelium of 



