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Effects of Light on Pycnidium Formation, Sporulation, and Tropism by Septoria nodorum. L. Calpouzos, Department of Plant Pathology, University of Minnesota, St. Paul 55101; D. B. Lapis, Department of Plant Pathology, University of Minnesota, St. Paul 55101. Phytopathology 60:791-794. Accepted for publication 1 December 1969. DOI: 10.1094/Phyto-60-791.

Four isolates of Septoria nodorum formed fertile pycnidia under continuous light, or diurnal light-dark cycles, but not in constant darkness. Under light, the isolates formed pycnidia more frequently on potato-dextrose agar (PDA) than on Czapek’s or malt agar, and more frequently at 15 than at 25 C. A single 24-hr exposure properly timed induced the cultures to form pycnidia; however, some of these pycnidia were sterile. As the duration of irradiation increased from 1 to 14 days, the total number of pycnidia increased as well as the percentage of fertile pycnidia. Light intensities ranging from 80 to 8,000 ergs/cm2 per sec were sufficient to induce pycnidia in two isolates; however, at 80 ergs/cm2 per sec, one of the isolates formed only sterile pycnidia. Negative phototropism was exhibited by hyphae of all our isolates. Only wavelengths in the near ultra-violet shorter than 350 nm induced pycnidia. Phototropism was also induced by wavelengths shorter than 350 nm as well as those lying somewhere between 350 and 510 nm.