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Ecology and Epidemiology

Effects of Soybean Seed Coat Cracks on Seed Exudation and Seedling Quality in Soil Infested with Pythium ultimum. Robert L. Schlub, Former Graduate Research Associate, Department of Plant Pathology, Ohio Agricultural Research and Development Center and The Ohio State University, Wooster, OH 44691, Present address of senior author: Department of Botany and Plant Pathology, Michigan State University, East Lansing, MI 48824; A. F. Schmitthenner, professor, Department of Plant Pathology, Ohio Agricultural Research and Development Center and The Ohio State University, Wooster, OH 44691. Phytopathology 68:1186-1191. Accepted for publication 2 March 1978. Copyright 1978 The American Phytopathological Society, 3340 Pilot Knob Road, St. Paul, MN 55121. All rights reserved.. DOI: 10.1094/Phyto-68-1186.

Nonblemished soybean seeds with intact seed coats exuded 5.3 μg glucose equivalents per hour per seed when buried in glass beads and leached by dripping distilled water at 3 ml/hr for 24 hr. Anthrone-positive leachates of 17.5 μg glucose equivalents per hour per seed were collected after soaking nonblemished seeds in 1 ml water for 3 hr. Exudation from soaking seeds was not influenced by cracks in the hypodermal seed coat layer, but scarifying through the remaining layers increased exudation to 217 μg/hr. Exudation of ninhydrin-positive substances from leached intact seeds was 1.7 μg glycine equivalents per hour per seed. This was increased two-fold by soaking and an additional five-fold by scarification. Germination of washed sporangia of Pythium ultimum was low in distilled water or in a sucrose-asparagine solution containing 1 μg of carbon/ml but increased to 48% or more with 5 μg or more carbon/ml. Spermosphere effects could be detected up to 5 mm from intact and 7.5 mm from scarified seeds after 5 hr in soil at 24 1 C and 0.3 bar matric water potential. Lower quality seedlings resulted from scarified seeds than from seeds with intact or cracked seed coats when planted in saturated soil or soil infested with sporangia of P. ultimum (130 propagules/g dry soil) at 15 C, but addition of sucrose (12.7 mg) to each seed had no added effect. A high population of P. ultimum at 24 C reduced quality of seedlings produced from both intact and scarified seeds to the same level. Apparently adequate sugar is exuded from nondamaged soybean seed for optimum development of P. ultimum and scarification effects on seedling quality are not the result of additional stimulation of P. ultimum by exudates.