78 Minnesota Plant Diseases. 



Immediate destruction. The immediate destruction of 

 plants or plant parts has been characterized as an amateur 

 method. It is admittedly not as clever a method as is the de- 

 layed destruction preceded by stimulation. But the perform- 

 ance of even such amateur methods can be carried to a high 

 degree of proficiency and that is what many fungi have done. 

 The green-mold fruit-rot effects its parasitism not only accord- 

 ing to amateurish ways, but is a beginner in this work. This 

 is not so with the potato blight, which is an adept at its meth- 

 ods of killing and feeding on the potato plant. There are 

 many special methods to be found among these parasites but 

 they may be brought under these heads : the destroyers of 

 small areas of plants, the destroyers of whole organs, and the 

 destroyers of whole plants. 



The destroyers of small areas. Among the simplest of the 

 algal fungi one finds certain kinds which possess a very small 

 mycelium so that they occupy only a single cell of the host. 

 This cell the parasite may immediately destroy without effect- 

 ing any change in the surrounding cells. In some cases, how- 

 ever, it may affect neighboring cells and these may grow ab- 

 normally large. Such growth results in the formation of galls. 

 Galls vary in size from that of a pin head, or even smaller, to 

 walnut size, and some galls are known to be even very much 

 larger. Not all plant galls are of a fungus nature ; for by far 

 the great majority are caused by the sting of insects when the 

 latter deposit their eggs in the plant tissues. Such are the very 

 common galls formed so abundantly on leaves or branches of 

 oaks, as in the nut galls of commerce. 



Most small-area-destroying fungi do not confine their at- 

 tacks to one cell but prey on a large group of cells. Typical 

 examples of these fungi are found among the leaf spots. 

 These fungi are exceedingly abundant parasites and are usually 

 characterized by the destruction of limited, often circular, spots 

 of the leaf which they inhabit. These spots usually turn brown 

 and are sometimes fringed with a red or white band. The 

 spots of strawberry leaves which are so destructive to certain 

 varieties in our state^ are excellent examples of leaf spots. 

 Many fungi of this class are very injurious if they occur in 

 abundance, while others do not perceptibly affect the general 



