136 Minnesota Plant Diseases. 



The sacs are long cylinders and contain eight spores, each 

 of which is made up of one large and one small cell. The sac 

 spores are shed in the spring and can infect new branches or 

 other trees. Almost all of our plums and cherries are subject 

 to the attack of this very dangerous fungus and in the wild 

 plants it is often found in very great abundance. It has proved 

 a dangerous pest to cultivated plants, and in many places in the 

 United States has ruined whole orchards and rendered aid by 

 state laws necessary for protection. 



Another very common disease which is a close relative of 

 the black-knot is the fungus producing black spots on a great 

 many grasses both wild and cultivated. These spots are formed 

 on the leaves and are often mistaken for black rusts, but they 

 can easily be distinguished from the latter diseases by the fact 

 that the skin of the host is not split open in long lines and the 

 spores are not produced in the way well known for rusts. The 

 spores are formed in sacs borne in small spherical cavities or 

 capsules. These appear in clusters at the surface of the black 

 spots just as they are found in black-knot. The fungus is often 

 very abundant on grasses. In such cases they undoubtedly 

 levy a considerable tax on the starch-making apparatus of the 

 host and thus impoverish the latter. 



Similar spots are produced on the leaves of elm trees. The 

 leaves of the common white elm are often found almost com- 

 pletely covered with such spots. The capsules and sacs are 

 produced in a manner somewhat similar to that in the black 

 spots of grasses and, though never seriously threatening the 

 life of the elms, undoubtedly steal much nourishment from 

 them. These black spots must not be confused with the tar 

 spots of maple and willow which are different diseases. The 

 latter are also sac fungi but belong to the cup-fungus group. 

 Superficially these fungi resemble each other ; in the black spots 

 of grasses and elms, however, the sacs are found in pored cap- 

 sules while in the tar spots they occur in cups. (Figs. 19, 59, 

 191.) 



Dung fungi and their allies (Pyrcnomycetinece (in part) 

 including Sordariacece and Ch&tomiacea). If horse dung be 

 placed in a moist closed chamber and allowed to remain un- 

 disturbed for a week or two there will almost invariably arise 



