174 



The reader will find that the diseases discussed in this preliminary 

 report are grouped according to the crops which are subject to them. 

 Because of the great importance of the cereal crops their diseases are 

 discussed first. Following them are the diseases of forage crops ; the 

 diseases of fruits, including small fruits ; the diseases of vegetable and 

 field crops ; and the important diseases of a few commonly grown orna- 

 mental plants. The plan of the discussion has been to give in each case 

 a brief description of the disease, a short summary of its history in Illi- 

 nois, a statement of its distribution as now known, some indication of the 

 crop losses for which it is held responsible, and, finally, a brief statement 

 of the usual methods of control. 



As a basis for the interpretation of the discussions, available infor- 

 mation on the distribution and value of each crop has been summarized 

 in a preliminary paragraph or two accompanied by an illustrative map.' 

 This information has been taken from the reports of the State Agricul- 

 tural Statistician as printed in the "Illinois Crop Reports" issued by the 

 federal Bureau of Markets and Crop Estimates, and from the reports for 

 1920 of the Bureau of the Census. 



Accompanying the discussion of each disease is a map showing its 

 known distribution in Illinois. For the purpose of the report the county 

 has been selected as the unit for expressing occurrence since it would be 

 obviously impracticable to show on so small a map the actual place at 

 which the disease was found. The county serves as a practical unit for 

 the additional reason that diseases found in one field or in one orchard 

 usually may be found in the fields and orchards throughout a considerable 

 area of neighboring territory. 



It is not to be assumed that at this stage of the survey the maps will 

 show the entire range or the only places of occurrence for any disease; 

 actually, they show only those counties in which the disease has been 

 found during our three seasons of work; but even with this limitation 

 they emphasize the almost universal prevalence of many diseases and the 

 limited occurrence of others. 



In discussing the damage which diseases do to crops, use has been 

 made of estimated crop reductions, which are summarized in the text and 

 given in detail in tables at the end. The estimated reductions represent 

 the eiTect which each disease has had, in the judgment of experienced and 

 competent workers, upon the yield of the crop in any one season. Esti- 

 mates are commonly expressed in percentages ; and the equivalent loss, 

 in terms applicable to any crop, is arrived at in the manner illustrated by 

 the following example : 



' In order to have tlie maps iUu.strate clearly the relative importanoe of crops 

 In the several parts ot the .state it ha.s been necessary to choose an arbitrary unit 

 in eacli case, such as 1,000 acres or 2,000 bushels, to be represenli-d by a sinfjle dot : 

 but, when a county grows an amount which involves less than the arbitrary luiiiiber, 

 a dot has been used to indicate that the crop is jrrown there. As a consequence, .when 

 a county contains a single dot, the reader may interpret it to mean that less than the 

 amount indicated as equivalent to a dot on that map is grown there. 



