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Field and Greenhouse Evaluations of Stem Canker Resistance in Soybean

Dep. of Agronomy and Soils, 202 Funchess Hall

E. F. Smith and P. A. Backman

Dep. of Plant Pathology, 139 Funchess Hall, Auburn University, Auburn, AL 36849

^* Corresponding author.

Greenhouse screening using infested toothpicks was compared to field evaluation of soybean [Glycine max (L.) Merr.) breeding lines for resistance to stem canker disease, caused by southern strains ofDiaporthe phaseolorum (Cke. & E11. (Sacc.) var. caulivora Athow and Caldwell (Dpc). Field screenings are reliable indicators of resistance, but often the disease does not develop naturally in field screening nurseries. Thirty-seven random F_4:6 lines from the cross ‘Hutton’ (susceptible ) x ‘Tracy M’ (resistant) were evaluated for their reaction to Dpc in the field (two locations, 2 yr) under natural infestation and infection conditions, and in the greenhouse (three experiments) with artificial inoculation using infested toothpicks. Our objectives were to compare field and greehouse screening and to determine the usefulness of greenhouse inoculation in predicting the yield and disease reaction of breeding lines when these lines were subjected to natural field infection conditions. Field screening based on symptoms and yield was highly effective in identifying resistant genotypes. Heritabilities for yield and disease ratings in the field were 87 and 92%, respectively. The toothpick inoculation procedure used in the greenhouse was effective with each of three Dpc isolates (different in geographic origin from the field location) in identifying the genotypes that showed highest levels of disease resistance in the field. Phenotypic correlations between greenhouse ratings and yield in the infested field ranged from –0.71 to –0.61. Results indicated that selection based on greenhouse screening can be an effective alternative to field screening when resistance is derived from Tracy M.

Key Words: Glycine max L. • Correlated response to selection • Genetic resistance • Heritability • Response to selection

Received for publication September 22, 1987.