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Field and Greenhouse Selection for Phytophthora Root Rot Resistance in Alfalfa¹

We developed a field nursery to screen for Phytophthora root rot resistance in alfalfa (Medicago sativa L.). The critical steps included scattering Phytophthora megasperma Drechs.-infested soil over the area, leveling the soil before seeding, and irrigating the soil so that it remained wet for 2 to 3 weeks. In the greenhouse, we tested seedlings in inoculated steamed sand in tanks where the water levels were controlled by subirrigation. Disease severity in field and greenhouse tests was significantly correlated in two sets of comparisons (r = 0.990 and r = 0.949, 8 df). The degree of resistance was significantly correlated with both forage and root production in wet soil. An example of progress in breeding for resistance was the increase in resistant plants in a population from less than 10% to about 50% after 2 cycles of selection and to about 63% after 3 cycles of selection. Other populations have responded similarly.

Key Words: Disease resistance • Phytophthora megasperma

² Research Plant Pathologist and Research Geneticist, ARS, USDA, in the Department of Plant Pathology and the Department of Agronomy and Plant Genetics, respectively. University of Minnesota, St. Paul, MN 55101.

Received for publication June 15, 1973.