82 



renders it all the more objectionable and difficult to com- 

 bat. When wilted plants are pulled up, the parts of the 

 stem a few inches above the level of the ground are found 

 to be blackened and often decaying; these symptoms have 

 given the disease its popular name. 



The parasitic fungus which causes the aster disease, 

 unlike Phytophthora mfestans of the potato, never attacks 

 the leaves directly but enters the plant from the soil 

 through the root of the seedling. Once withm the young 

 plant it may, either grow slowly not seriously interfering 

 with the work of the vital parts"^of the seedling, or it may 

 extend rapidly through these and cause immediate col- 

 lapse. Alicroscopic examination of the blackened portion 

 of the stem shows the distribution of the fungus m the 

 tissues. The cells of the rind as well as the spaces betw^een 

 them are occupied by filaments of the fungus. The 

 former are not so rapidly killed by the fungus as are 

 those of the potato plant by Phytophthora injestans. 

 The ultimate collapse of the plant, however, is brought 

 about b}' the extension of the fungus to the vascular 

 cylinder of the stem, and the consequent reduction of the 

 supply of water to the leaves. 



In moist weather the fungus gives rise to conidia on the 

 diseased stem. Unlike those of Phytophthora injestans 

 they are only produced under water, and do not become 

 detached from the filaments bearing them. A few hours 

 after their first appearance they burst at the apex liberat- 

 ing about fourteen motile spores which, after swimming 

 about for a short time, come to rest and are able to infect 

 other seedlings. 



In studying this disease the writer has up to the present 

 failed to find any resting spores of the Phytophthora in 

 the tissues of diseased asters, and attempts to obtain such 

 spores by artificial cultivation have been equally unsuc- 

 cessful. Experience however has shown quite conclusively 

 that the disease originates each season from the presence 

 of the fungus in the soil, especially of the seed bed ; and 

 the ease with which it mav be cultivated on dead organic 

 substances suggests that it may be able to persist in a 

 vegetative condition in the rich soil used for seed-beds. 

 On the other hand, further research may prove the exist- 

 ence of resting spores. 



It is often stated that this disease is caused by a species 

 of Fiisariuvi, which is frequently found on the decaying 



