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Alfalfa Seedling Resistance to Phytophthora megasperma. J. A. G. Irwin, Research assistant, Department of Plant Pathology, University of Wisconsin, Madison, 53706; S. A. Miller(2), and D. P. Maxwell(3). (2)(3)Research assistant, and professor, respectively, Department of Plant Pathology, University of Wisconsin, Madison, 53706. Phytopathology 69:1051-1055. Accepted for publication 2 February 1979. Copyright 1979 The American Phytopathological Society. DOI: 10.1094/Phyto-69-1051.

A reliable seedling assay is described which allows rapid differentiation of alfalfa cultivars resistant or susceptible to Phytophthora root rot (caused by Phytophthora megasperma). Fifty-nine of 62 clones, selected as resistant 7 days after the sowing of pregerminated seeds in infested soil, were resistant at maturity. Seedling resistance depended on inoculum concentration and duration of environmental conditions conducive to disease development. At 0.58 g dry weight of mycelium per kilogram dry weight of planting mix, almost all seedlings of cultivars Agate (resistant) and Vernal (susceptible) were killed within 3 days after saturation of the planting mix with water 4 days after sowing. When the inoculum concentration was lowered to 0.080.12 g dry weight of fungus per kilogram dry weight of mix, nearly all seedlings of susceptible cultivars were killed within 34 days after the planting mix was saturated, but a significantly higher percentage of seedlings from resistant cultivars survived more than 7 days under the same conditions. More seedlings of resistant cultivars than of susceptible cultivars survived 5 days after inoculation at the full cotyledon growth stage when zoospores (3 103 to 3 105 zoospores per container) were used as inoculum. The seedling assay was used to screen 20 alfalfa cultivars and breeding lines for which reaction to the fungus previously had been determined by assay of mature plants. Disease ratings of all seedlings tested were similar to the disease ratings of the mature plants. Possible use of the method in alfalfa breeding is discussed.