12 PROTECTION OF PLANTS^1922-23 



insects and plants. Moreover, his Annual Reports and Bulletins were widely- 

 read. 



Plant Pathology is the newest of the sciences arising out of Botany. Its 

 special field is not yet quite clear to many persons — even to many botanists. 

 Mycology is not Plant Pathology, for the former deals with fungi, the lattd- 

 with the diseases caused by fungi. The study of fungi as causative agents of 

 disease should be carefully distinguished from the study of the diseases caused 

 by the physiological reactions and responses of the plants to the parasites. Such 

 pathological conditions form the field of study of the plant pathologist. To 

 know the cause and its effect is not enough: We should know how the cause 

 produces the effect. Then only will the treatment of plant diseases become 

 rational and not empirical. 



The plant pathologist is concerned chiefly with the diseases of plants, conse- 

 quently he classifies diseases according to the relations existing between the 

 parasite and the host. Roughly, then, he distinguishes three main types of 

 diseases: 



1. Those diseases where the parasite destroys the hpst and lives upon the 



dead or dying protoplasts:, such as pear blight, many leaf spots 

 and rots; 



2. Those disea,S(ps where the parasites do not destroy the host, but continue 



to live ^mbiotically with the living cells, such as black knot, peach 

 leaf curl, crown galls, etc.; 



3. Those diseases produced by the invasion of the xylem vessels by the 



myceliuni of the parasite, ther'eby cutting off the supply of nutrient 

 solution, such as the "wilts", and many timber bracket-fimgi. 



It is evident that plant pathology as a science had to wait for the develop- 

 ment of normal plant physiology and of a technique such as the bacteriologist 

 has at his command. Moreover, a knowledge of biochemistry, systematic and 

 morphological botany, histology and cytology is a fundamental acquirement 

 of the plant pathologist. 



Three outstanding factors contributed to the great development of our 

 knowledge of plant diseases and methods of control : 



1. The epochal investigations of European botanists on the causal organism 



and their relation to such plant diseases as smut (1853), the potato-rot 

 disease (1861), and wheat rust (1865). 



2. The work of Ameiiican investigators in the seventies and eighties, such 



as Burrill, Farlojw, Arthur ,Bessey, Halstead, Earle and others whose 

 contributions added substajntially to our knowledge of fungi and of 

 fungous and bacterial diseases. 



