THE FUNGI WHICH CAUSE PLANT DISEASE 



87 



oval, flattened biciliate zoospores which emerge from the conidia, 

 swim about, come to rest, develop a wall, then produce a germ 

 tube. Direct germination by a germ tube also occurs rarely. In- 



FiG. 59. — P. infestans; 1, section showing conidiophores and conidia- 

 formation; 5, germination of a conidia. After Scribner. 



fection is brought about by the germ tube, either by penetrating 

 through stomata or directly through the epidermis. 



The walls and contents of parasitized cells are browned. When 

 this fungus is alone on the tubers dry rot is induced, but invasion 

 of numerous saprophytic fungi and bacteria usually turns this 

 into a disagreeable wet rot. Tuber infection occurs largely from 



