. Bulletin. Natural history; Natural history. Diseases of Fruit Trees, 1922-1928 493 of disease, when the separate kinds of disease are recognized. An equally concrete, perhaps a more easily understood statement of the same fact is that the individual tree was attacked on an average by diseases. The sums of prevalence indexes, when regarded in this way, are an ex- pression of the number of cases of disease per 100 trees, and by dividing this by 100 we obtain a number which we may very properly term the average incidence of disease per tree. The average incidence of disease per tree, becaus
. Bulletin. Natural history; Natural history. Diseases of Fruit Trees, 1922-1928 493 of disease, when the separate kinds of disease are recognized. An equally concrete, perhaps a more easily understood statement of the same fact is that the individual tree was attacked on an average by diseases. The sums of prevalence indexes, when regarded in this way, are an ex- pression of the number of cases of disease per 100 trees, and by dividing this by 100 we obtain a number which we may very properly term the average incidence of disease per tree. The average incidence of disease per tree, because of the manner in which it is derived, expresses briefly and accurately the prevalence of disease in any year or, by an average of years, for any desired longer period of time. In Table XXII the average incidence of disease per tree, derived by totaling the prevalence indexes given previously for the several dis- 5 r ^4 ^3 :« r w ^ Wk Peach \P/um U Cherry WtPear. 1922 /SZ3 /S24 /925 /926 /927 /928 Fig. 29. Comparison of the annual incidence of disease per tree for each of the five tree fruits Apples are most subject to disease attack every year and peaches are next. Plums and cherries are nearly equally subjected to attack, and pears less than any. This ranking on the basis of disease prevalence would not, however, correspond to a ranking on the basis of commercial injury. Table XXII Average Incidence of Diseases Per Tree Kind of tree 1922 1923 1924 1925 1926 1927 1928 Apple .20 .96 .71 .45 .14 .81 .31 Pear .. .05 Peach . Cherry Plum Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Illinois. Natural History Survey Division. Urbana, State of Illinois, Dept. of Registration and Educati
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