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Red clover (Trifolium pratense L.) is known to be a moderate host for Aphanomyces euteiches Drechs. Recently, an isolate of A. euteiches (Ae‐572) was observed to be highly virulent on red clover. This research was conducted to determine inheritance of, and select for, resistance to A. euteiches in red clover using an inoculated seedling assay. Resistant and susceptible red clover plants were intercrossed to study the inheritance of resistance. Frequency distributions of resistant and susceptible F1 plants indicated that resistance to A. euteiches is inherited as a quantitative trait. Three cycles of selection for resistance to A. euteiches in red clover resulted in significant improvement in resistance from the original selected population (Cycle 0 = 3.64 mean disease severity index (DSI); Cycle 3 = 2.43 mean DSI). Narrow‐sense heritability estimates were high enough ((h2n = 51% for Cycle 3) to assure that a recurrent selection program utilizing family selection could be used to improve red clover populations for resistance to A. euteiches.

An interest in increasing heterozygosity in tetraploid (2n = 4x = 28) red clover (Trifolium pratense L.) has led to the use of 2x‐2x (bilateral) crosses for tetraploid production. This study cytologically examined F2 bilaterally derived tetraploid red clover plants (Bi4x) for aneuploid frequency and meiotic chromosome pairing. Cytological results on Bi4x were compared to colchicine derived, N2O derived, and 2x‐4x (unilateral) derived tetraploid populations. In addition, dry pollen morphology of 90 Bi4x plants was examined. The frequency of aneuploids in Bi4x was 23%, with chromosome numbers of 25, 26, 27, and 30. Hypoploids (2n < 28; frequency = 90.48%) were more frequent than hyperploids (2n > 28; frequency = 9.52%). Chromosome associations in euploid pollen mother cells averaged 0.35 univalents, 5.96 bivalents, 0.14 trivalents, and 3.80 quadrivalents. The difference between euploid chromosome associations and aneuploids was nonsignificant (x2 = 2.36; P > 0.50). Observations on dry pollen morphology indicated 78.6% of the Bi4x population produced more than 80% tetrahedron shaped pollen (typical of n = 2x = 14 pollen). The frequency of cylindrical shaped pollen (typical of n = x = 7 pollen) ranged from 1% to 90%. The Bi4* plants with a high frequency of cylindrical pollen were crossed to diploid and tetraploid plants. The 2x ✕ Bi4x crosses produced no seed, whereas, the 4x ✕ Bi4x crosses produced seed (18.13% fertility) suggesting cylindrically shaped pollen in the Bi4x population reverted to morphology typical of pollen from diploid plants (n = 2x = 14). Fiftyfour Bi4x plants were selfed and seven seeds were obtained from 6177 hand pollinations indicating a high level of self incompatability.