6o Journal of Agricultural Research voi. v. No. a 



role in bridging over the winter. This paper gives the results of experi- 

 ments and observations which show that in the Northern States species 

 of the Peronosporaceae which have perennial mycelium are common 

 and that the mycelium may live from one growing season to another in 

 the living diseased host tissues. 



In several of these experiments the locality where infected plants were 

 growing was marked in the autumn and the plants collected from time 

 to time during the winter and eariy spring, after which they were allowed 

 to revive in the greenhouse and a careful watch kept for any evidence of 

 fruit of the fungus. In other cases the underground parts of infected 

 plants were taken in the spring and planted in steam-sterilized soil in the 

 greenhouse, and when the shoots came through the ground conditions 

 were made favorable for the sporulation of the fungus. In still other 

 cases the presence of the mycelium in perennial parts of the host was 

 determined microscopically. 



PERONOSPORA PARASITICA 



Late in the fall of 1910 and 191 1 it was observed that young plants of 

 Lepidium virginicum in the vicinity of Madison, Wis., were very generally 

 infected with Peronospora parasitica and that the tissues of these plants 

 contained few or no oospores, although they were produced in abundance 

 in the summer when the host tissues were dying. Plants of Lepidium sp. 

 always form a rosette of leaves in the late fall, and some of these remain 

 alive through the winter. 



In the fall of 191 1 two patches of Lepidium plants, about 50 per cent 

 of which were infected with Peronospora parasitica, were marked so that 

 they might be easily found during the winter. One was on the side of a 

 short incline made by dumping several loads of soil in a heap and the 

 other on the parking of a city drive in Madison. Both patches were 

 well exposed during the winter of 1911-12, which was unusually severe, 

 there being no covering of snow on the former at any time and the 

 latter being covered only a part of the time. 



After the first killing frost, which, according to the Weather Bureau, 

 occurred on October 24, infected plants of Lepidium virginicum were 

 collected at various times during the winter. Beginning on October 30, a 

 test was made of the germination of the conidia of Peronospora parasitica 

 growing on Lepidium virginicum. Although when alive the conidia of this 

 fungus usually germinate profusely within 2 to 3 hours and always within 24 

 hours, no germination occurred in this test, although exposed to favorable 

 conditions for 48 hours. This coincides with what is known of the behavior 

 of the spores of other species — e. g., Cystopus candidus (Melhus, 10) — and 

 excludes the possibility of these conidia becoming a source of further 

 infection. A careful search for oospores was made after October 30 in a 

 large number of infected plantlets, but none was found. 



