Minnesota Plant Diseases. 



109 



below it, killing the tissues and the seedling tumbles over and is 

 further appropriated by the fungus. In this way the fungus gains 

 strength and large numbers of seedlings may fall. Not only 

 mustards but clover and beet and other plants are subject to at- 

 tack. In the slight preference of host, the damping-off fungus 

 shows a slight indication of advance in parasitic methods, but 

 the latter are still primitive and success only attends such favor- 

 able conditions as excessive moisture and crowded host plants. 

 (Fig- 34-) 



FIG. 44. Downy mildews. 1. Downy mildew of seedlings (Phytophthora omnivora) ; 

 formation of egg-spores by breeding act; a male cell and egg-cell case. 2. Potato 

 blight (Phytophthora infestans), thread with spore-like swimming-spore-cases (s). 3. 

 (Peronospora alsinearum.) Formation of egg-spore by breeding act in another mil- 

 dew; letters as in 1. 4. Egg-spore formation in still another mildew; letters as in 1. 

 5. Thread of a downy mildew (Peronospora leptosperma) bearing spore-like swimming- 

 spore-cases (s). All highly magnified. After DeBary. 



Downy mildews and their allies (Peronosporinece in part). 

 Long ages ago, when the fungi had developed the saprophytic 

 and parasitic habits, as one sees in the damping-off fungi, it 

 became advisable to abandon the aquatic life because more op- 

 portunities are presented in aerial conditions. The damping-off 

 fungus shows some such tendency but the blights or downy 

 mildews show it still more clearly, for most of these fungi are 



