Minnesota Plant Diseases. 79 



health of the host. Certam smuts and powdery mildews may 

 also be confined to small and limited areas. In the former case 

 the area is converted into a smut heap while in the latter it be- 

 comes mildewed and later dotted with • the very small spore 

 cases. Most smuts and mildews, however, are not restricted to 

 small areas. The mildews are seldom confined to small areas 



Fig. 35. — Strawberry leaf-spot. The fungus (Sphaerella fragariae) destroys small areas of 



the leaf. Original. 



of the leaf surface or even to the leaf itself, though usually this 

 is its favorite habitation. The smuts, although often occupy- 

 ing a sharply delimited area, more often occupy whole plant 

 organs, as fruits or stamens. Moreover, the smut mycelium 

 always dwells inside of the leaf while the mildew is external in 

 its habits, except in its sucker-threads. Rusts, though often 

 confined to certain organs of the host, are not usually restricted 

 to particular or limited areas. 



Methods of killing tissues. There seems to be two ways by 

 which tissues of the host plant are killed. The nutrient sub- 

 stance of the host tissues can be directly absorbed through the 

 membranes of the fungus thread wall and also, in some cases, 

 through the wall of the host cell. The substances are with- 



