Transfer of Disease Resistance from Diploid to Tetraploid Alfalfa by Unreduced Female Gametes. M. J. Havey, Department of Plant Pathology, University of Wisconsin, Madison 53706. D. P. Maxwell, Department of Plant Pathology, University of Wisconsin, Madison 53706. Plant Dis. 72:603-604. Accepted for publication 4 February 1988. Copyright 1988 The American Phytopathological Society. DOI: 10.1094/PD-72-0603.

Diploid (2n = 2x = 16) Medicago spp. have been described as sources of resistance to diseases of cultivated tetraploid (2n = 4x = 32) alfalfa (M. sativa). The objective of this research was to transfer sexually to the 4x level germ plasm selected and bred at the 2x level for resistance to Phytophthora root rot, caused by Phytophthora megasperma f. sp. medicaginis. Direct 2x-4x crosses using a susceptible 4x male parent transferred resistance to P. m. f. sp. medicaginis in 2x M. falcata to the 4x level. One M. falcata plant, MfR-28, produced unreduced (2n) female gametes at a high frequency (13 4x plants per 100 flowers pollinated) and allowed for efficient sexual transfer of 2x M. falcata germ plasm to the 4x level. Direct 2x-4x crosses were also successful in achieving tetraploidization of 2x plants resistant to P. m. f. sp. medicaginis and bred by crossing resistant M. falcata plants with resistant diploids derived from the 4x plant M193 by interploid bridge crosses. Two derived diploid by M. falcata hybrids produced 2n female gametes at a frequency equal to or greater than that observed for MfR-28, thus facilitating transfer of resistance to P. m. f. sp. medicaginis from 2x to 4x alfalfa. Our results demonstrate that direct interploid crosses can efficiently transfer disease resistance from the 2x to 4x level and that genes conditioning resistance to P. m. f. sp. medicaginis selected at the 2x level are expressed in 4x alfalfa. Interploid crosses also allow for combination of different disease resistances during tetraploidization.