148 FUNGOUS DISEASES OF PLANTS 



latter case the conidium becomes a zoosporangium. Oogonia 

 and antheridia are also present, and these are produced within 

 the tissues of the host. The oospores upon germination give 

 rise to numerous zoospores or to a single germ tube. Some 

 members of both families (Albuginaceae and Peronosporaceae) of 

 this order deserve careful study. To the genus Phytophthora of 

 the Peronosporaceae Pythium and Pythiacystis are perhaps closely 

 related. 



Albuginaceae. In this family the conidia are borne in chains ; 

 the conidiophores arise in the form of large cushions which 

 might be termed sori. They are developed beneath the epi- 

 dermis, but the latter is finally ruptured, and the conidia are 

 exposed. The oospore germinates by the production of zoo- 

 spores. There is one genus, Cystopus. 



Peronosporaceae. The members of this family are distinguished 

 from the preceding largely by the conidiophores, which are from 

 the beginning aerial. The conidia are also produced singly. This 

 is properly the family of the downy mildews. The mycelium, 

 which is commonly intercellular, develops either branched or 

 knob-like haustoria. The oospore germinates by means of a 

 germ tube. 



The following generic key includes most of the genera together 

 with the salient generic characteristics, and is, in this family, more 

 practical than a description of each genus : 



A. Conidiophore fully developed prior to the formation of conidia. 



1. Conidium on germination becoming a swarm sporangium (zoo- 



sporangium). Oospore free from the wall of the oogonium. 

 ".''; . '. ''"' . . . . Plasmopara 



2. Conidium becoming a swarm sporangium, conidiophore short, 



irregular in form and diameter, oospore filling oogonium, with 

 closely adherent walls. ... . . , ._ .. . Sclerospora 



3. Conidium germinating by means of a germ tube. 



a, Conidium provided with a terminal papilla from which 



the germ tube proceeds. Fertile tips arising from a 

 disk-like swelling Bremia 



b. Conidium without papilla. Fertile tips ordinarily branch- 



like Peronospora 



B. Conidiophore incomplete when first conidia produced. Fertile tips 



swelling and continuing growth as successive conidia are formed. 

 . . . , Phytophthora 



