Minnesota Plant Diseases. 



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favorable. Then it shows itself to be a spore-case and forms 

 internally numerous swimming spores, which escape and spread 

 the disease. Some of the downy mildews, however, have 

 learned still more thoroughly the terrestrial habit and have al- 

 most entirely forgotten the ways of aquatic fungi. In these 

 the spore-like body really acts as a spore, grows out directly 

 into a thread and does not develop swimming spores at all, 

 although in some forms it starts out as though it were going to 

 form them and then abandons the attempt. We have here an 

 excellent example of the persistence of a habit even after it is 



FIG. 45. A Downy mildew with the aspect of a white rust. Under surface of burweed 

 elder (Iva xanthiifolia) showing dense clusters of spore-like spore-cases. Original. 



ill adapted to the plant's new methods of life. Egg-spores 

 are also formed throughout this group, though in a few cases, 

 as in the common potato-blight, they have not yet been ob- 

 served. As in the damping-off fungus, the breeding act of 

 many forms has actually been demonstrated. The egg spores 

 are typically resting spores and, as in the damping-off fungus, 

 serve to tide the plant over winter or other unfavorable sea- 

 sons. They are usually found in abnormally swollen parts of 

 the host. The plants of this group are all parasites and most 

 of the known forms attack cultivated garden plants. They live 



