DIVISION VI* 

 Microbial Diseases of Plants 



Introduction 



Although the earliest study of bacterial diseases in plants antedates 

 the isolation of the tubercle bacterium and the cholera spirillum, this 

 branch of bacteriology has not been marked by the progress which has 

 characterized the investigation of animal diseases. The loss of a human 

 life or of a valuable domestic animal has appealed to the student of 

 disease more strongly than the blighting of a pear tree, or the wilting of 

 a potato vine, and, quite naturally, he has directed his efforts along those 

 Unes which have offered the greater inducements, and which have 

 demanded immediate attention. 



However, with the introduction of new plants, foreign seeds, and 

 strange nursery stock, many previously unheard-of plant diseases have 

 made their appearance. As the farming communities have become 

 more thickly populated, with less uncultivated land between the fields, 

 these diseases have spread from farm to farm more rapidly than in the 

 earlier days, and the losses from these causes have been so heavy during 

 the past decade that the farmers, gardeners and orchardists have come 

 to the Agricultural Experiment Stations all over the country for advice 

 and assistance in combating their troubles. This has stimulated an 

 increased interest in plant diseases, especially along bacteriological 

 lines, with the result that to-day some forty bacterial diseases of plants 

 have been described. 



It is a matter of not infrequent observation that closely related 

 species of plants, as well as animals, exhibit a marked difference in their 

 susceptibility to the same disease-producing agents. The Bartlett 

 pear, for example, suffers more severely from blight than the Kieffer, 

 and, among apples, the Toleman Sweet more than the Rome; the small- 

 leaf, stemmy varieties of tobacco seem to be more resistant to the Gran- 

 ville wilt than the large-leaf kinds. Resistance of this sort, which ap- 



* Prepared by W. G. Sackett, except a-protozoal disease "Fingers and Toes" by J. L. Todd, 

 revised by E. E. Tyzzer. 



S88 



