DISEASES OF CROPS 939 



to understand when, why and how to give the necessary treatments. In 

 recent years it has been found possible to overcome some diseases by 

 growing plants that are disease-resistant and, therefore, do not need 

 treatments. 



In this chapter only the most common and important plant diseases 

 in the United States and Canada are considered. Brief descriptions and 

 condensed directions for treatment are given. 



Farmers should always report the presence of disease on crops to 

 the agricultural experiment station of the state in which they reside, and 

 ask advice as to treatment. The treatment of some diseases will vary 

 somewhat, dependent upon the part of the country in which it occurs. 



APPLE 



Bitter Rot or Ripe Rot (Glomerella rufomaculans [Berk.], Spaul and 

 von Schrenk). — This rot is not confined to ripe apples and is not always 

 bitter. It attacks both fruit and twig and occurs in orchard and in storage. 

 On the fruit it appears as a brown, sometimes black, circular spot which 

 gradually enlarges. It may be soft and wet or dry and corky, depending 

 on variety of the fruit and age of the infection. The spore pustules start 

 from the center of the spot and gradually spread over the surface, usually 

 forming rather definite circles. They are pinkish in color and watery and 

 spread the disease from fruit to fruit. Large spots become depressed and 

 wrinkled and the entire fruit eventually becomes rotten, then dry and 

 shrunken, and is finally known as a "mummy." 



The disease may be carried from year to year on these mummies and 

 also on the stems. On the twigs and branches it causes rough spots known 

 as cankers. These cankers are rough and vary in size with age. The 

 fungous spores from these infect the growing crop. 



Treatment. — Remove and burn the mummied fruit and twig cankers. 

 Spray with lime-sulphur before the buds open. After the petals fall, spray 

 with self-boiled lime-sulphur or Bordeaux mixture. (See spray table on 

 page 943.) 



Black Rot (Sphceropsis malorum, Peck). — The rotten spot on the fruit 

 is usually blacker and drier than the bitter rot spot and can be readily 

 distinguished by the numerous black dots or papillae from which masses of 

 black spores emerge. 



It also causes a stem canker in which the twigs become swollen, 

 rough and black. On the trunk and larger branches it causes peculiar 

 cankers. On young trees it causes a blight which is somewhat similar to 

 the fire blight of the pear, but which can be readily distinguished by the 

 presence of numerous small black dots. It also attacks the leaves, causing 

 peculiar spots frequently spoken of as "frog eye." 



Treatment. — Same as for bitter rot. 



Brown Rot. — Usually not severe on the apple. (See Peach.) 



Storage Rots. — The rots which occur in storage may be due to the 



