PRINCIPLES OF FARM PRACTICE 



the cool, wet weather which frequently prevails in the North- 

 ern States, and sweeps across whole fields in a very short time. 

 Not only does it destroy the foliage and thus reduce the yield 

 of tubers, but also causes a dry rot of the outer tissues of the 

 tubers and exposes them to infection by the dry rot of pota- 

 toes mentioned above. 



Wilt. The name indicates the character of the injury. 

 The roots or parts of the stem are injured, thus cutting off 



the water supply from the leaves 

 and upper parts of the plant, caus- 

 ing them to wilt. Examples: 



Cabbage yellows. This is the most 

 destructive disease of cabbage in the 

 United States. It is caused by a 

 soil fungus which invades the roots 

 and grows up through the water- 

 conducting system of the plant so 

 as to make the vegetables worthless 

 from the commercial point of view. Sometimes the plants 

 wilt, but ordinarily the most striking symptom is the yellow 

 discoloration and dropping off of the lower leaves, and the 

 failure of the cabbage to develop a marketable head. Yellows- 

 resistant varieties of cabbage have been developed. 



Other examples of typical wilt diseases are potato wilt, 

 tomato wilt, and watermelon wilt caused by soil fungi, and 

 the bacterial wilt of cucumbers and muskmelons which is 

 carried from plant to plant by insects. 



Mildew. There are two entirely different types of mildew 

 among plant diseases. One of these types, called powdery 

 mildew, in which the mycelium or vegetative part of the 

 fungus lies on the surface of the host, has been described 

 in a previous paragraph. It is illustrated by such diseases 



Germinating spores of early 



blight of potato. 

 (Wisconsin Agricultural 

 Experiment Station.) 



