Minnesota Plant Diseases. 383 



The evidence of the presence of the disease is seen in the falling 

 over of the seedlings and their subsequent death and decay. The 

 fungus requires a great deal of moisture and attacks the stem of 

 the seedling at the surface of the ground. It is very destructive 

 in its attack and kills off the tissues at this point, causing the fall 

 of the seedling. The spores are formed only in the presence of 

 moisture and the plant is able to live in a saprophytic manner. 

 It is a primitive, but directly destructive, parasite. Under favor- 

 able conditions the fungus will spread very rapidly and cause 

 great destruction to beds of seedlings. The summer spores are 

 of two kinds. One germinates usually very soon after forma- 

 tion, producing directly an infection tube. The other is some- 

 what like the so-called summer spores of downy mildew, but un- 

 dergoes a resting period, after which it breaks up into swimming 

 spores. Each of the latter is provided with two lash-like pro- 

 cesses, which aid in its swimming about in the water. When 

 these swimming spores come to rest they germinate into an in- 

 fection tube which attacks new seedlings or builds up an aquatic 

 mycelium. The winter spore is formed by a breeding act and 

 is provided with a very thick wall so that it can undergo long 

 periods of rest. When it develops further it produces swimming 

 spores, just as do some of the summer spores. 



Plenty of sunshine and good drainage will keep the fungus 

 in check. Soil which contains many winter sjxjres, or resting 

 summer spores, should be avoided. (Fig. 34.) 



The damping-off of prothallia(f ythium intermedium DeBy.}. 

 This fungus is a very close relative of the damping-off of seed- 

 lings and its behavior is somewhat similar. The host plant is. 

 however, the sexual plant of the ferns, commonly known as the 

 prothallium. The fungus is a common enemy of fern culture in 

 greenhouses, when ferns are raised from spores. The fungus 

 permeates the tissues of the prothallium and the latter wilts. In- 

 comes dark in color, dies and decays. The fungus produces sum- 

 mer spores in a manner, in general, similar to that of the damp- 

 ing-off of seedlings. The so-called spores form, under proper 

 conditions of moisture, numerous swimming spores, and these 

 behave in a peculiar manner: for. when they cease to swim, they 

 move in an amoeboid manner until they finally come to rest and 

 germinate. 



