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Responses of Alfalfa to Stem-Tip Inoculations with Five Isolates of Sclerotinia trifoliorum

Dep. of Agronomy, Univ. of Sriwijaya, Kampus Unsri Indralaya (OKI), Sumsel, Indonesia

D. E. Rowe^* and R. G. Pratt

USDA-ARS, Crop Science Res. Lab., Forage Res. Unit, P.O. Box 5367, Mississippi State, MS 39762

^* Corresponding author (derfru@ra.msstate.edu).

Effective breeding for economically important resistance to Sclerotinia trifoliorum Eriks in alfalfa (Medicago saliva L.) requires that the plant breeder have an understanding of any genetic basis for variability in measurements of resistance and the likelihood that selection for resistance to one isolate confers resistance to other isolates. Fifty alfalfa plants were repeatedly evaluated for their resistance to five isolates of S. trifoliorum. Tips of excised stems were inoculated with a S. trifoliorum isolate and stem necrosis was measured after 14 d. The inoculation of five stems from each of 50 plants with a single isolate was repeated five times. For the length of stem necrosis, the interaction of pathogen isolate and alfalfa plant was not significant but differences among alfalfa plants and among isolates were significant. The plants that, on average, developed larger necrotic regions did not have more or less variation in their replicated measurements of necrosis than those plants which averaged much smaller necrotic regions. The genotype of the alfalfa plant had a greater effect on the variability in replicate measurements of disease than the genotype of pathogen isolate. Resistance to different isolates were significantly correlated (r = 0.48–0.60) and selection of some plants for resistance to one isolate appeared to improve resistance to other isolates.

Received for publication August 13, 1997.