. Bulletin. Natural history; Natural history. Diseases of Fruit Trees, 1922-1928 483 leaf-curl its up^-ard trend, with the result that, in spite of the steep fall of leaf-curl, the trend of average incidence was turned distinctly- upward. Partly through lack of data for two diseases, but more evidently because the year was very unfavorable to peach diseases, as is indicated by the steep falls of the brown-rot and shot-hole trend lines from 1924 to 1926, the average incidence curve was bent down very sharply in 1925. With all four diseases tending to increase their prevalence in 1926, the avera
. Bulletin. Natural history; Natural history. Diseases of Fruit Trees, 1922-1928 483 leaf-curl its up^-ard trend, with the result that, in spite of the steep fall of leaf-curl, the trend of average incidence was turned distinctly- upward. Partly through lack of data for two diseases, but more evidently because the year was very unfavorable to peach diseases, as is indicated by the steep falls of the brown-rot and shot-hole trend lines from 1924 to 1926, the average incidence curve was bent down very sharply in 1925. With all four diseases tending to increase their prevalence in 1926, the average incidence line returned again to nearly a normal level; and with the continued rise in the trends of brown-rot, leaf-ciirl, and shot-hole, the incidence curve rose a trifle higher in 1927 and was held to a level slightly below that of the 1922 only by the sharp decline of scab. Between 1927 and 1928 all trends were downward, those of brown-rot and leaf-curl very distinctly so, those of scab and shot-hole perceptibly so, with the result that the trend of average incidence moved downward also. I J I L O /O 20 /f//e. /r7c/c/<z/7ce. ScaS 66!53\ ^^^^^^^^^^^^^^^^ ZO 30 4iO so so 70 8o so /oo Fig. 23. Comparison of the average annual prevalence of peach diseases Brown-rot is prevalent to the least extent and bacterial shot-hole to the greatest, while leaf-curl and scab are intermediate. These four diseases, by their combined attack, give an average annual incidence of disease of per tree. There apj)ears to be but little agreement among peach as to their prevalence reaction in ordinary years. While certain ones may move together in one year they are apt to move in opposite directions in another year. Each disease appears to be influenced to a marked degree, either favorably or unfavorably, by changes in some one or more factors in the environment provided from year to year. It is notable, hoAvever, that in spite of the great fluctuations exhibited by the in- d
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