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Factors Important in Artificial Inoculation of Pinus strobus with Cronartium ribicola . Everett M. Hansen, Former Research Assistant, Department of Plant Pathology, University of Wisconsin, Madison, WS 53706, Present address of senior author: Department of Botany and Plant Pathology, Oregon State University, Corvallis, OR 97331; Robert F. Patton, Professor, Department of Plant Pathology, University of Wisconsin, Madison, WS 53706. Phytopathology 67:1108-1112. Accepted for publication 18 March 1977. Copyright 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-1108.

Primary needles of eastern white pine were inoculated with basidiospores in an airstream apparatus and incubated in controlled-environment chambers to determine optimum conditions for infection. The amount and distribution of water on needle surfaces was critical to successful infection. Excess water caused spore clumping and abnormal germination. Exposure to light during incubation increased infection levels indirectly by improving moisture conditions on the needles. Fluctuating temperatures and temperature shocks did not increase infection levels. Infection levels on inoculated seedlings, incubated in a dark dew chamber for 72 hr increased with spore density from 20 to 3,000 spores mm2. The highest spore densities resulted in 1.2 needle spots mm2 of needle surface, with about 10% of the stomata successfully penetrated. Basidiospore germ tubes grew randomly on needles.

Additional keywords: white pine blister rust, epifluorescence microscopy, infection efficiency.