124 



Minnesota Plant Diseases. 



Pozi'dery mildezt's (Erysiphacecc). The mildews constitute 

 the simplest group of black fungi. The mycelium is usually 

 to be seen on the surface of leaves as a white, moldy covering. 

 The threads send branches into the skin cells of the host and 

 there absorb their food and live parasitically, btit the main my- 

 celium of the fungus never lives inside of the host. In the 



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Fig. 50.— a powdery mildew on common plantain leaf. The powdery coat of tlie threads 

 and the small black fruiting bodies can be clearly seen. Original. 



summer, spores are produced in enormous numbers and form a 

 dust-like covering over the leaf, whence the common name of 

 powdery mildew for this group of fungi. These spores are 

 pinched off in rows from upright threads, which thus become 

 converted into chains of spores. Towards fall there arise, on the 



