PLANT DISEASES 169 



Blight. This name is applied to diseases which result in 

 a rather sudden death of the host plant or portions of it, such as 

 leaves or branches. Examples: 



Fire blight. This is a disease caused by bacteria. Al- 

 though bacteria are not true fungi, they are like the fungi in 

 their food requirements, 

 and some cause injuries 

 similar to those made by 

 parasitic fungi. Fire 

 blight attacks the leaves 

 and twigs of apple and 

 pear trees, causing them 

 to shrivel and blacken as 

 if burned. The leaves of Spores o ^ blight rf potato _ 

 the diseased twigs do not A Various stages of development of 

 fall as do the leaves of spores, 

 healthy twigs. This fact B - Mature s P res - (Wisconsin Agri- 



cultural Experiment Station.) 



makes it possible to 



recognize the blight in winter. In the spring the diseased 



twigs may be recognized by their dead, black leaves. 



Bean bacterial blight. This is a destructive disease of the 

 bean crop throughout the country. It produces large, brown, 

 parchment-like spots on the leaves, small, sunken, water- 

 soaked spots on the pods, and in addition may kill the plant 

 outright by forming a canker on the stem or by clogging the 

 water tubes. 



Late blight of potato. This is the most destructive disease 

 of the potato crop in the Northeastern States. It is character- 

 ized by large, dark brown spots on the leaves. These spots 

 enlarge rapidly involving the entire leaf surface. The tuber 

 is also affected, a dry, brown rot appearing in its outer tissues. 

 Late blight is caused by a fungus which spreads rapidly during 



