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 mildew*. 1. Ilowny mil. lew of seedlings ( I'hyto|>hthora omnivon) ; 

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

 blight ( I'hyt.'i'hthora infentans). tlirrad with >; n- like swimming spore-case* (). 3. 

 (Pcronospora alsinearum.) Formation of epg-itpore by breeding act in another mil- 

 dew; letters as in 1. 4. Kgg s;orc formation in still another mildew; letters as in 1. 

 5. Thread of a downy mildew ( f'eronos;ora leitosperma) he.iriiiR spore-like swimming- 

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



Downy mildews and their allies (Fcronosporinc<c in part). 

 Long ages ago, when the fungi had developed the saprophytic 

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

 became advisable to abandon the aquatic life because more op- 

 portunities are presented in aerial conditions. The damping-ofT 

 fungus shows some such tendency but the blights or downy 

 mildews show it still more clearlv. for most of these fungi are 



