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Competition Between Pyrenophora tritici-repentis and Septoria nodorum in the Wheat Leaf as Measured with de Wit Replacement Series. S. R. Adee, Former graduate research assistant, Department of Plant Pathology, Throckmorton Hall, Current address: Agrigenetics, Madison, WI; W. F. Pfender(2), and D. C. Hartnett(3). (2)Associate professor, Department of Plant Pathology, Throckmorton Hall; (3)Assistant professor, Division of Biology, Ackert Hall, Kansas State University, Manhattan 66506-5502. Phytopathology 80:1177-1182. Accepted for publication 8 June 1990. Copyright 1990 The American Phytopathological Society. DOI: 10.1094/Phyto-80-1177.

Competitive interactions between Pyrenophora tritici-repentis and Septoria nodorum were studied quantitatively by a modified plant ecology technique known as ?de Wit replacement series.? P. tritici-repentis and S. nodorum were inoculated alone and in various proportions together on wheat plants at anthesis. Mature leaves were harvested and incubated in moist chambers, after which fungal sporocarps were counted. For each inoculation mixture, the relative yield of each fungus (ratio of its sporocarp yield in the mixed inoculation to its sporocarp yield in single inoculation) was plotted against the proportion of that fungus in the inoculation mixture. The relationship between inoculation ratios and relative yields was compared statistically to a hypothetical noncompetition model. Measurable competition occurred between P. tritici-repentis and S. nodorum. P. tritici-repentis was the better competitor because its relative yields were less affected by the presence of S. nodorum than vice-versa. The de Wit replacement series is a useful tool for quantitatively examining competitive interactions between plant pathogens.

Additional keywords: Leptosphaeria nodorum, Triticum aestivum.