Article IV. — A Preliminary Report on the Occurrence and Distri- 

 bution of the Common Bacterial and Fungous Diseases of Crop Plants 

 in Illinois. Bv L. R. Tehon. . 



The common diseases of crop plants, caused by the attack of para- 

 sitic fungi and bacteria, take an annual toll of the crops of Illinois that 

 runs into values the magnitude of which is seldom realized ; and the fact 

 that Illinois stands second, among the United States, to none but Texas and 

 Iowa in the value of its agricultural products lends an especial significance 

 to their destructiveness. 



As early as 1881 the State Laboratory of Natural History, since 

 merged in the State Natural History Survey, undertook the study of the 

 parasitic fungi of the state, and in that and the following year A. B. Sey- 

 mour was employed to make an extensive collection of diseased plants. 

 Studies based upon Seymour's collection, but confined to identification 

 and classification, were made later by T. J. Burrill on the rust-producing 

 fungi, and by Burrill and F. S. Earle on the powdery mildew-producing 

 fungi, of the state. 



In the forty years which have elapsed since Seymour made his col- 

 lection, the classification, the structure, and the life histories of the bac- 

 teria and fungi which cause the diseases of crop plants have been the 

 subject of extensive and exhaustive investigation. As a consequence, 

 little of economic significance now remains to be done in that direction. 



The next important step in the study of plant diseases is concerned 

 with epidemiology — the fluctuation in severity and abundance from year 

 to year exhibited by all diseases. An explanation of the causes under- 

 lying these fluctuations promises things of the greatest importance to 

 the growers of our crops in the improvement of control measures and the 

 regulation of times and methods of their application. 



A necessary preliminary step is the accumulation of a complete cata- 

 log of the kinds of diseases present, and of accurate information concern- 

 ing their distribution and severity in our state. With this purpose in 

 mind the Natural History Survey began in the summer of 1021, and con- 

 tinued through the summers of 1!)22 and 1923, an examination of the 

 crops of the state' the results of which are incorporated in these pages. 



'The work has been carried on under the Keneral super\ Islnn of Pr. F. I.. 

 Stevens. Professor of Plant Pathology In the University of Illinois, and under the 

 direction of the writer. The rapid propress made Is due In a large measure to the 

 enthu.siastic and capable assistance In the Held of P. A. Young during the summ. rs 

 3f 1922 and 1923. and of O. A. Plunkett, C. L. Porter, and C. O. Peake. during the 

 summer of 1922. 



