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Published in Crop Sci 27:225-228 (1987)
© 1987 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Inheritance of Phytophthora Root Rot Resistance in Two Diploid Alfalfa Species1

M. J. Havey and D. P. Maxwell2

Phytophthora root rot [caused by Phytophthora megasperma Drechs. f. sp. medicaginis (Pmm)] is a major cause of decline of alfalfa (Medicago sativa L.) grown in poorly drained soils. Diploid (2n = 2x= 16) Medicago species have been described as sources of resistance to diseases of cultivated tetraploid (2n=4x=32) alfalfa and are a potential source of resistance to Pmm. The objective of this project was to study the inheritance of resistance to Pmm in plant introductions of diploid M. falcata L. (PI 410970) and M. sativa subsp, x varia (Martyn) Arcangeli (PI 325408). Plants were grown in the greenhouse in pots containing a peat-sand mix (1:1, v/v) and inoculated at 8 weeks with a chopped mycelial suspension of Pmm. Plants resistant and susceptible to Pmm were retained from both diploid Medicago species for use in inheritance studies. Segregations of 3:1 (resistance:susceptibility) in F1 families and backcrosses to the susceptible parent indicated that resistance to Pmm in M. falcata was conditioned by a dominant allele at either of two independently segregating loci, designated as Pm3 and Pm4. Resistance to Pmm in M. sativa subsp, x varia was heritable, but no mode of inheritance is proposed because unexplained segregation ratios were observed in the backcross to the resistant parent. These results demonstrate that M. falcata is a source of a simply inherited resistance to Pmm.

Key Words: Phytophthora megasperma Drechs. f. sp. medicaginis • Medicago falcata L. • Medicago sativa subsp. x varia (Martyn) Arcangeli • Dominant • Simply inherited resistance

1 Contribution from the Dep. of Plant Pathology, Wisconsin Agric. Exp. Stn., Madison. Research supported by College of Agric. and Life Sciences, Univ. of Wisconsin-Madison, Hatch Project 5312, and Environmental Protection Agency/USDA funds as subcontract L200048.

2 Formeresearch assistant (present address: Washington State Univ., Pullman, WA 99164-6421) and professor, Dep. of Plant Pathology, Univ. of Wisconsin-Madison, Madison, WI 53706.

Received for publication March 12, 1986.




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