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Infection of Austrian and Ponderosa Pines by Dothistroma pini in Eastern Nebraska. Glenn W. Peterson, Plant Pathologist, Rocky Mountain Forest and Range Experiment Station, Forest Service, USDA, Lincoln, Nebraska 68503; Phytopathology 63:1060-1063. Accepted for publication 26 February 1973. DOI: 10.1094/Phyto-63-1060.

Conidia of Dothistroma pini were found in stromata on needles of Austrian and ponderosa pines as early as 23 April; conidia were trapped as early as 15 May, and the dissemination period extended into October. Conidia were disseminated only during periods with rain. No evidence of long-distance dissemination was obtained; conidia were not caught in Hirst spore traps located within 152 cm (5 feet) of infected trees. Current-year needles of the two species did not become infected before 13 July. Infection of the previous seasons’ needles occurred as early as 20-27 May and continued into September. First symptoms were observed as early as 7 September and as late as early November. The time between initial infection and first appearance of symptoms varied from 11 to 16 weeks. Amount of rainfall during the 4-month period (June-September) is a good indicator of amount of infection to be expected; the amount of rain in May is not a good indicator. Infection was extremely light in the two years when rainfall during the indicated 4-month period was approximately 28 cm (11 inches), but infection was heavy in 6 years when rainfall ranged from 38.1 to 58.4 cm (15 to 23 inches). Periods in which rainfall is sufficient to disseminate large numbers of conidia may not be accompanied by conditions which result in infection. Results provide a sound basis for determining when protective fungicides should be applied in eastern Nebraska.