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Peronospora trifoliorum Sporangium Development and Effects of Humidity and Light on Discharge and Germination. P. M. Fried, Former Graduate Research Assistant, Department of Plant Pathology, Kansas State University, Manhattan, KS 66506, Senior authorís present address: Department of Plant Pathology, Pennsylvania State University, University Park, PA 16802; D. L. Stuteville, Research Plant Pathologist, Department of Plant Pathology, Kansas State University, Manhattan, KS 66506. Phytopathology 67:890-894. Accepted for publication 17 January 1977. Copyright © 1977 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-67-890.

Peronospora trifoliorum produced sporangia on alfalfa in darkness at ≤ 97% relative humidity (RH). The predominant stages of sporangium development on 11-day-old seedlings observed at 2-hr intervals after induction of sporulation were as follows: at 4 hr sporangiophores were unbranched, at 6 hr sporangiophores had branched, at 8 hr sporangia had formed, and at 10 hr all sporangia were morphologically mature. Sporangium viability was greatest (82%) 12 hr after induction of sporulation (about 2 hr after morphological maturity), decreased slowly to 78% by 16 hr, and then dropped sharply to 52% by 20 hr. Sporangia harvested in darkness 15 hr after induction of sporulation and exposed to 5,400 lux of fluorescent lighting for 0 min, 10 min, 1 hr, or 24 hr followed by darkness for the remainder of a 24-hr period, germinated 43, 72, 69, and 85%, respectively. Light intensities between 540 and 10,800 lux affected germination equally. The viability of sporangia discharged from infected seedlings exposed to drying by laboratory air (about 48% RH) for 0.5 and 3 hr was 31 and 9%, respectively, as compared with 68 and 56%, respectively, for sporangia that remained attached. Nondesiccated sporangia were 81% viable.