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Powdery Mildew Development on Soybeans with Adult-Plant Resistance. Julia S. Mignucci, Department of Crop Protection, University of Puerto Rico, Mayaguez 00708; S. M. Lim, Agricultural Research, Science and Education Administration, U.S. Department of Agriculture, Urbana, IL 61801. Phytopathology 70:919-921. Accepted for publication 18 March 1980. Copyright The American Phytopathological Society. DOI: 10.1094/Phyto-70-919.

Growth of Microsphaera diffusa on individual leaves of soybean, Glycine max (cultivars Clark, Cloud, Cutler, Hawkeye, Mukden, and Custer), at early seedling stage was followed by remission of fungal growth at later stages of plant development. Variable symptoms of green and yellow ‘islands’, interveinal necrosis, necrotic specks, and crinkling of the leaf blade appeared at time of remission of fungal growth. Mycelial and symptom development were affected by the cultivar, position and age of the leaf, and age of the plants at the time of inoculation. Leaf symptoms were almost absent on young seedlings when mycelia were abundant or on plants that were first inoculated at 50 days of age and which developed negligible infections. On plants inoculated at 8 days, remission of fungal growth occurred on four cultivars by 28 days after inoculation, and on two other cultivars only 18 days were required. When plants were first inoculated at 29 days of age, all six cultivars exhibited remission of fungal growth by 18 days after inoculation. Development of visible mycelia and symptoms were negligible if plants were inoculated at the fourth trifoliolate stage (50 days of age). Designation of adult plant resistance of G. max to M. diffusa requires several examinations of the interactions during the life span of the host.