YIELDS OF ALFALFA VARIETIES SELECTED FOR APHANOMYCES RESISTANCE IN KENTUCKY

Field tests were conducted over 4 Sr to investigate whether alfalfa (M edicago sativa L.) varieties and experimental populations selected for resistance to aphanomyces root rot (caused by Aphanomyces euteiches D rechsler) (here termed aphanomyces-selected varieties) would collectiv ely outyield varieties not selected for resistance (unselected varieti es). All entries were resistant to phytophthora root rot (caused by Ph ytophthora medicaginis E.M. Hans & Maxwell). At the conclusion of the tests, the mean cumulative dry matter yield of aphanomyces-selected va rieties was higher (4.2% increase, P = 0.063) than that of unselected varieties in a site naturally infested with A. euteiches and P. medica ginis, However, total yields of certain unselected varieties equated o r even exceeded (P = 0.10) those of certain aphanomyces-selected varie ties, and there was no correlation among varieties between yield and p ercentage of Aphanomyces-resistant plants. No significant difference i n mean cumulative yield was observed between aphanomyces-selected and unselected varieties in two sites infested with A. euteiches alone nor in four sites not infested with either pathogen, Since outbreaks of s eedling diseases were minor in all tests, the yield effects observed m ay be due to differences in plant health manifested after stand establ ishment, Our data are consistent with the hypothesis that selection fo r Aphanomyces resistance contributes to an overall improvement of elit e alfalfa germplasm for cultivation in poorly drained soils infested w ith both A. euteiches and P. medicaginis. However, considered alone, s election for Aphanomyces resistance in individual varieties was not al ways predictive of high yield in Aphanomyces-infested sites. These tes ts do not rule out the possibility of a substantial yield benefit to A phanomyces resistance under conditions unusually conducive to seedling disease induced by A. euteiches.