6o 



Minnesota Plant Diseases. 



cases of rusts all of the spore forms are found on the same kind 

 of host plant and it is now a well-known fact that the rusts are 

 extreme specialists in the selection of their hosts. So exact 

 has this selction become that certain rusts will attack one grain 

 plant and are unable to attack other even closely related 

 grasses. In this respect they are among the most proficient 

 of all known parasitic fungi. Still further complications may, 

 however, arise. A rust fungus may increase its distribution 

 by selecting in the spring time another earlier plant for a host, 

 and produce upon this plant its spring spores. This migration 

 is the expression of one phase of the education of the most 

 highly educated plant parasites known to botanists of today. 

 In view of these and other accomplishments of these fungi one 

 has little hesitation in pronouncing them the most proficient 

 parasites amongst the fungi. 



The modes of life of parasitic fungi. In general, there are two 

 methods of life. The fungus may live on the surface or it may 

 live within the tissues of the host plant. The powdery mildew 

 of lilacs lives on the surface of the leaves while the smut of 



oats lives inside the tis- 



sues of the oat plant. 

 Those fungi living in the 

 tissues of their host, how- 

 ever, come to the surface 

 when they are about to 

 produce spores. The sur- 

 face-dwelling fungus may 

 derive its nutrition in one 

 of two ways : it may 

 send special threads into 

 the living substance of 

 the host and through 

 these sucker-threads 



draw nourishment, or 

 may merely attach itself 

 to the surface of the 

 plant and never send 

 threads into the living 

 substance. It is clear 



FIG. 28. An endophytic (internal) mycelium be- 

 tween the cells of a grass grain. (Fungus of 

 Lolium temulentum.) Highly magnified. By 

 the author. 



