ORDER PERONOSPORALES 



139 



Fig. 46. Peronosporales, Family Peronosporaceae. Rhysotheca viticola (B. & C.) 

 G. W. Wilson. (A) Germination of oospore to form one large conidium. (B) Zoospores 

 with flagella, and encysted and germinating. (C) Infection of host tissue through a 

 stoma. (After Gregory: Phytopathology, 2(6):235-249.) 



jacent host cells. Like those of the Albuginaceae the haiistoria of this 

 ^ family may be knob-like but in many species they are filamentous or 

 B finger-like. The conidiophores are external to the host and produce the 

 ^conidia singly on the ends of the branches. These conidia are plurinucleate 

 ^Band germinate in most cases by the formation of zoospores, as in Albugo. 

 ^■In Peronospora and Bremia the typical mode of germination is by means 

 ^■of a stout germ tube without the formation of zoospores. In Rhysotheca 

 ■ (included in Plasmopara by many authors) the conidia germinate by the 

 Bformation of zoospores but in Plasmopara (in the narrower sense) the 

 whole protoplasmic contents of the conidium escape as a naked pluri- 

 nucleate but nonflagellate mass which quickly rounds up and encysts 

 and then germinates by a germ tube. This difference in the mode of ger- 

 mination is the basis for Wilson's (1907) division of Plasmopara into the 

 two genera. In nearly all genera in which germination by means of zoo- 

 spores is typical the conidia may, under special conditions, germinate 

 directly by germ tubes. A. de Bary (1876), Melhus (1915), Jones and 

 Torrie (1946), have shown that as in Albugo Candida, so also in this 

 family, particularly in the genus Peronospora, the mycelium can live over 

 winter in the tissues of a biennial, winter annual, or perennial host and 

 thus infect the new plants in the spring without the aid of conidia or 

 oospores. Sexual reproduction is like that in those species of Albugo in 

 which the mature oogone contains a uninucleate egg. Mostly the oospores 



