/ 



[Chap. XLV PLANT DISEASES 569 



blights, leaf spots, and wilts. These symptoms are just as "normal" to an 

 infected plant as their absence is to an uninfected plant. Indeed, a well- 

 trained pathologist can often tell which pathogen is present in a host by 

 the characteristic appearance of the symptom. 



One mav wish to use the term "plant disease" to in- 

 clude physiological conditions that result from a de- 

 ficiency of food, water, oxygen, light, or inorganic 

 salts. Such diseases have been called "deficiency dis- 

 eases." The observable symptoms when such deficien- 

 cies exist are a characteristic, or "normal," development 

 of the plant under such conditions. They are often suf- 

 ficiently specific that by studying their appearance one 

 may learn to decide correctly which particular condi- 

 tion is deficient ( Chapter XXX ) . 



Likewise, wounding results in plant development 

 peculiar to wounding but unusual in an unwounded 

 plant. Yet the effects of many kinds of wounds are not 

 considered to be diseases. Gnawing insects and graz- 

 ing animals, as well as lawn mowers and pruning 

 knives used bv man, injure plant tissues, but such 

 wounds in themselves are not diseases. 



Economic aspects of plant diseases. One needs onh 

 to examine the records of modern history to realize 

 the extent of human misery and distress, as well as the 

 starvation of millions of other animals, brought about 

 bv plant diseases. When rye is infected by the ergot 

 fungus, large purple to black bodies replace many of 

 the rye grains (Fig. 257). These structures are poison- 

 ous to man and other animals. From the 17th to the 

 19th centurv there were about 45 epidemics of ergot 

 poisoning in Germanv and al:)out 20 in France and 

 Spain. The Irish famine was brought about by the 

 destruction of most of the potato crop by a blight fungus during the 

 vears 1843 to 1846. This blight resulted in the death of a quarter of a 

 million people and the migration of a million and a half persons from 

 Ireland to America. 



The increasing occurrence of plant diseases is undoubtedly the result 

 of increased concentration of crops, continuitv of areas devoted to single 

 crops, more speedv svstems of transportation, and greater transfer of 



Fig. 257. Head 

 of rye in which 

 two large black 

 ergot sclerotia 

 have developed. 

 From Luerssen. 



