. Bulletin. Natural history; Natural history. May, 1974 CuiTjaj & Cartek: Phloem Necrosis and Dutch Elm Disease for phloem necrosis than for Dutch elm disease. Phloem necrosis-affected elms do not show foliage symptoms for at least 1 >'ear following infection, while many Dutch elm disease-affected elms show foliage symptoms in the year when infection occurs. Drought conditions that prevailed from 1952 through 1955, especially in 1953 and 1954, may ha\e caused a re- duction in the elm leafhopper popula- tion, since this insect is adversely af- fected by such conditions. Precipitation for
. Bulletin. Natural history; Natural history. May, 1974 CuiTjaj & Cartek: Phloem Necrosis and Dutch Elm Disease for phloem necrosis than for Dutch elm disease. Phloem necrosis-affected elms do not show foliage symptoms for at least 1 >'ear following infection, while many Dutch elm disease-affected elms show foliage symptoms in the year when infection occurs. Drought conditions that prevailed from 1952 through 1955, especially in 1953 and 1954, may ha\e caused a re- duction in the elm leafhopper popula- tion, since this insect is adversely af- fected by such conditions. Precipitation for the 5-month growing season. May through September, was inches [ mm] below normal in 1953 and inches [ mm] below normal in 1954. Total precipitation was inches [ mm] below normal in 1953 and inches [ mm] be- low normal in 1954 (Carter 1955:40). Collection records of the Section of Faunistic Sur\eys and Insect Identifica- tion, Illinois Natural History Survey, show that the populations of leafhop- pers in general were drastically reduced during these drought years and that the elm leafhopper has never been col- lected in abundance in Illinois. The increase in the incidence of phloem necrosis from 1957 through 1959 may have been influenced mainly by an increase in the elm leafhopper population, which might be expected in years of near-normal rainfall. Following 1959 the incidence of phloem necrosis decreased to very low levels during the period of rapid decline in the residual elm population. Only 7 per- cent of the original elm population remained by 1960. Dutch Elm Disease A rapid increase in the number of elms affected by Dutch elm disease started in 1953, 2 years after the disease first appeared in Urbana in a single elm. The number of elms affected an- nually by Dutch elm disease increased until 1957, when 2,116 trees were af- fected. During the 5-year period 1955- 1959, the annual loss of elms to Dutch elm disease was 1,770-2,11
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