PLANT DISEASES 443 



brown, and becomes sunken through rotting of the tissue 

 (Fig. 92) . These spots may be recognized from other kinds of 

 spots by their sunken appearance, their bitter taste, and their 

 ringed border. This disease was observed for many years 

 before the method of infection was discovered. Now it is 

 known that the parasite is a wound parasite that develops 

 cankers on the twigs (Fig. 93). Abundant spores are formed 

 in these cankers and are washed down by rains on the fruit 



FIG. 92. Bitter rot of apple. After CLINTON. 



below. It had long been noticed that a sudden attack of 

 bitter rot was brought about by a few rainy days. 



142. Diseases of the second group. In this group of 

 diseases the parasite does not kill living cells, but lives on 

 their products and often induces them to develop unusual 

 structures. 



Rust. This is a very conspicuous disease of cereals, in 

 which the parasite and host live peaceably together for a 

 time, but in which there is usually no abnormal growth. 

 The " disease," therefore, consists in the gradual weakening 

 of the living cells by the drain upon their food supplies, 

 until finally they can work no more and are destroyed by 



