Oct. II, 191S Perennial Mycelium in Species of Peronosporaceae 63 



two additional plants showed white pustules of this fungus and also 

 spores of Peronospora parasitica, the number of pustules increasing on 

 the lower side of the leaves until many were well spotted. Two plants 

 in the collection made on February 22 bore white pustules within three 

 days after they were taken into the greenhouse, showing that they were 

 infected with Cystopus candidus and that the fungus was alive in the 

 tissues in late winter (PI. Ill, fig. i). Again, in the collection made on 

 March 25 one plant developed pustules of Cystopus candidus and conidio- 

 phores and spores of Peronospora parasitica four days after being trans- 

 ferred to the greenhouse. 



Cystopus candidus is also a very common parasite on Capsella bursa 

 pastoris, a plant that may become a winter annual. In the fall of 191 1 

 a patch of plants of Capsella bursa pastoris, many of which were infected 

 with Cystopus candidus, was marked; and on March 30, 1912, 25 plants 

 were collected and treated in the same way as the plants of Lepidium 

 virginicum infected wuth Peronospora parasitica. After two days the 

 plants began to show signs of life; and at this time they were covered 

 with a small glass house. Three days later white pustules were dis- 

 covered on one leaf; and the following day, or six days after the plants 

 were brought in, white pustules developed on other leaves of the same 

 plant. 



On April 5, 1912, just as the ground thawed out, another collection, 

 consisting of 76 plants, was made. Four days after, or on April 9, there 

 were white pustules on four of the plants. Except in the.case of one large 

 leaf, which was probably produced early the preceding fall, the pustules 

 were all on the youngest leaves, which indicates that the mycehum can 

 winter over in leaves of plants of Capsella bursa pastoris that live through 

 the winter. The fact that the youngest leaves were infected suggested 

 crown infection; and later this proved to be the case, all of the leaves 

 growing from certain plants being infected as soon as they appeared, 

 while the leaves growing from certain others remained free from infec- 

 tion. On April 10 white pustules appeared on two other plants, making 

 a total of six infected plants in the second collection. As soon as the 

 plants of Capsella bursa pastoris in the marked patch started to grow in 

 the spring some of them showed infection with Cystopus candidus, which 

 developed like the infections studied in the greenhouse. From these 

 experiments it will be seen that the mycelium of Cystopus candidus in 

 the tissues of the host remains alive through the winter. 



In the fall of the year Cystopus candidus becomes systemic in the 

 tissues of Sisymbrium officinale and Brassica nigra also. So far these 

 two host plants have not been followed through from fall to spring, but, 

 like the plants of Lepidium virginicum, and Capsula bursa pastoris, both 

 may become winter annuals, as is well known. 



