CONTROL OF PLANT DISEASES 469 



PEOGEESS IN CONTROL OF PLANT DISEASES 



By Dr. F. L. STEVENS 



PROFESSOR OP BOTANY AND VEGETABLE PATHOLOGY IN THE N. C. COLLEGE OF AGRICULTURE 

 AND MECHANIC ARTS AND AGRICULTURAL EXPERIMENT STATION 



PLANTS are subject to disease. As in the human being these 

 diseases decrease vigor and productiveness of the organism or 

 cause death. An attack upon valuable plant products such as ripe 

 fruit, tubers and root crops, arid mature timber may result in depre- 

 ciation in value or even entire loss of the product. The rot of apples 

 upon the tree or in the bin, the common blue mold seen upon lemons 

 and oranges, the wide-spread blight of pear and apple twigs (Eig. 1) 

 are familiar examples of plant disease. 



In the early years of American history these afflictions were re- 

 garded as natural and inevitable, but during the last three or four 

 decades scientific study has shown that rots, blights, molds, mildews, 

 rusts, smuts, etc., are true diseases; that they do not constitute any 

 part of the normal life stages of the plant affected. That they are 

 caused by living parasites and, moreover, that they are often prevent- 

 able. 



Plant diseases have increased largely in number and destructiveness 

 during recent years. This is due in part to the migration of disease 

 from county to county and state to state; in part to the cultivation of 

 weak or susceptible varieties and in part to long continued cropping in 

 a given region, thus affording opportunity for the plant pests, which 

 may at first have been weak and unimportant, to become thoroughly 

 established and aggressive. 



The asparagus rust, which has in some states nearly prohibited 

 asparagus culture, offers an excellent example of migration in its pro- 

 gressive westward march across our continent. This invasion seems to 

 have occupied the years between 1896 and 1902 since the rust was first 

 noted at New Jersey in 1896, South Carolina 1897, Michigan 1898, 

 Illinois 1899, Dakota, Nebraska and Texas 1900, California 1901 or 

 1902. 



The destructive pear blight, our worst pear disease, made a similar 

 journey, starting from the neighborhood of the Hudson valley, near the 

 beginning of the last centun r , reached the Rockies sometime after 1886 

 and arrived in California about. 1895. 



Diseases have come to America from Europe. Grape anthracnose, 

 cabbage club root, potato wart ( Fig. 2 ) , are noteworthy examples ; sim- 



