Minnesota Plant Diseases. 



45 



only injure them so that the fungus can gain entrance, but ex- 

 uding juices will often furnish nutrition where the fungus can 

 build up a strong mycelium from which the attack upon the 

 uninjured parts may be better carried on. 



Damping-off fungi. Another half-saprophytic habit is that 

 of the damping-off fungi. These fungi are usually found under 

 very moist conditions and feed upon plant and animal debris 

 in the water. One of the most common of these fungi (Pythi- 

 um debaryanum) requires a very considerable amount of water 

 for its development. When it comes into contact with seed- 

 lings, especially of the mustard family, it is capable of attacking 

 the young plant at about the surface of the ground and kills 

 the tissues. The plant falls over and dies. In this way whole 

 beds of seedlings may be destroyed in a few days. This fungus 

 is never able to attack old plants. The weakness of the seed- 

 lings lies in the fact that the living substance is not yet pro- 

 tected by thick coats of cork and cuticle as it is in the older 

 plants. When the seedlings have been killed, the damping-off 

 fungus continues to live as a saprophyte. The fungus is not a 

 highly proficient professional in its parasitism for two reasons : 

 First, just as do the mold fruit rots, it exercises no particular 

 selection for special kinds of hosts, but will attack almost any 

 plant in the seedling stage ; and, second, it kills its host as soon 

 as it penetrates, thus preventing any further service which the 

 host might pay to its parasite. Rusts and smuts, as we shall 

 see later, are much more proficient in this habit of life than is 

 the damping-off fungus. 



Wound parasites. Perhaps the most important of all half- 

 saprophytes are those which have already been mentioned un- 

 der wood-inhabiting fungi as wound parasites. These fungi 

 are usually found on dead wood. The bark of trees ordinarily 

 refuses to them entrance to the tree trunk, since the fungus 

 threads are incapable of forcing their way through layers of 

 cork. When, however, a wound occurs which lays bare the 

 wood, this difficulty is overcome and the fungus thrives in the 

 heart-wood. After building up considerable mycelial strength 

 in its saprophytic life it proceeds to attack the growing zone 

 of the trunk, i. e., where the sap wood joins the bark and where 

 the living substance is protected by very thin walls. The host 



