. Bulletin. Natural history; Natural history. 74 Illinois Natural History Survey Bulletin heavy or light on all of the five cereals. The closest approach to such a condition occurred in 1925, the total prevalence of disease in that year being, for each of the five crops, less than the year before. Begin- ning with the wide range of total prevalences displayed in the season of 1922, the trend was downward for rye, corn, and oats diseases, biit upward for those of barley and wheat in 1923; down for wheat and rye diseases but up for those of oats, barley, and corn in 1924; down for the diseases o
. Bulletin. Natural history; Natural history. 74 Illinois Natural History Survey Bulletin heavy or light on all of the five cereals. The closest approach to such a condition occurred in 1925, the total prevalence of disease in that year being, for each of the five crops, less than the year before. Begin- ning with the wide range of total prevalences displayed in the season of 1922, the trend was downward for rye, corn, and oats diseases, biit upward for those of barley and wheat in 1923; down for wheat and rye diseases but up for those of oats, barley, and corn in 1924; down for the diseases of all five crops in 1925 ; and in 1926 down for corn and rye, but up to relatively high points for wheat, oats, and barley diseases. The average total prevalence over the period of years covered by the survey is remarkably characteristic for each of the five crops. This fact is illustrated in Figure 88, in which the five-year index averages given in the foregoing five tables are shown graphically. The high prev- alence of infection on wheat and oats is indicative of the fact that these crops suffer most from diseases of their above-ground parts. The. Oa/j iVheat 40 60 80 /oo ao Preya/ence /nde^ Fig. 88. Average disease prevai,ence for each crop The indexes of prevalence given in the text for the diseases of each crop have been added and averaged in order to show the normal prevalence of dis- eases on each crop in Illinois. lesser prevalence shown for barley and rye result, in part at least, from the limited range of culture of the first crop and from the scattering cultivation accorded the latter. In the case of corn, however, the very low average prevalence may be considered as directly correlated with the relatively minor ranks held by smut and rust' among corn diseases. It is not possible to gain a clear-cut conception of the destructive- ness of diseases through the simple process of adding the indexes, as has been done for prevalence. The types of injury caused by the vari
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