v CHAPTER XIX 

 The Downy Mildews 



THE group of fungi commonly called the Downy Mil- 

 dews includes several of the most destructive fungous 

 enemies of cultivated crops. These parasites grow at the 

 expense of the vital tissues of the host plants, penetrating 

 between the cells in all directions and sending into the 

 cells curious projections that absorb their contents. They 

 produce summer spores in untold billions, to be widely 

 scattered so quickly that whole fields may seem to succumb 

 in a single day. The technical name of the great group to 

 which they belong is Phycomycetes. 



POTATO BLIGHT 



In America the Downy Mildew of the Potato probably 

 causes the greatest loss of any of these parasites, espe- 

 cially in the Eastern states. During recent years this 

 disease has been generally called the Late Blight to dis- 

 tinguish it from the Early Blight of Potatoes due to a 

 different fungus. This Late Blight causes not only the 

 death of leaves and stems, but also the decay of the tubers. 

 It destroys millions of dollars worth of potatoes. 



To get an idea of what happens in a potato field when it 

 seems suddenly to blight, let us assume that we have be- 

 fore us a green and healthy plant in August. Its leaves 

 are busily engaged absorbing the sunshine and converting 

 the unorganized substances from the soil, moisture, and air 

 into highly organized materials. These it is sending down 



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