66 Journal of Agricultural Research voi.v, no. a 



The remaining five of these seven infected plants were carefully dug 

 up, the stems cut off at their junction with the rhizomes, washed very 

 clean with a brush, and disinfected in corrosive sublimate for five min- 

 utes. After this they were planted in steam-sterilized soil in the green- 

 house, in which there had never been any Plasmopara halstedii. On 

 May 23 two shoots broke through the ground; and three days later, when 

 one was i inch and the other 2 inches high, they were covered with jelly 

 glasses in order to keep the atmosphere moist. On this date the initial 

 leaves appeared chlorotic, but no spores of Plasmopara halstedii could be 

 found. The next day the lower surfaces of the leaves were almost cov- 

 ered with a gUstening white coat of conidiophores and spores, which 

 on microscopic examination were found to be the conidia of Plasmopara 

 halstedii. Of the three remaining rhizomes, two failed to come up, while 

 the third sent up a spindly shoot on June 5. This shoot was treated in 

 the manner already described and the fungus fruited in the same way. 



This experiment showed that the diseased plants grown in the green- 

 house manifested the same symptoms as those grown in the open. It 

 also showed that the mycelium of Plasmopara halstedii may be present 

 in the rhizome of Helianthus diversicatus, and this, coupled with the obser- 

 vations described, strongly suggests that Plasmopara Imlstedii is peren- 

 nial in the rhizomes of Helianthus diversicatus. 



CONCLUSIONS 



As seen from these investigations, several species of the Peronospora- 

 ceae live over from one growing season to another by at least two means : 

 Resting spores and perennial mycelium. As is well known from the 

 excellent studies of De Bary (2), the oospores germinate after a rest 

 period either by zoospores or germ tubes and cause the infection of plant 

 tissues. Because of their extremely ephemeral nature, the conidia 

 hardly merit consideration as resting organs, but, nevertheless, they may 

 under certain conditions function as such. If a fungus has two or more 

 annual host plants, it may spread to one or more by conidia after pri- 

 mary infection has resulted from oospores on one; or the fungus may be 

 perennial in one host and spread to another by conidia borne on the 

 former — e. g., Phytophthora infestans on the potato and tomato. 



The species of Peronosporaceae known to have perennial mycelium 

 are given in Table II. 



