COMBINING ABILITY ANALYSIS OF RESISTANCE TO SOUTHERN ROOT-KNOT NEMATODE IN RED-CLOVER

Production of red clover, Trifolium pratense L., is reduced in areas i nfested with root-knot nematodes (RKN) in the southeastern USA. Althou gh progress has been made in improving RKN resistance in red clover th rough recurrent selection, little information is available on the mode of inheritance of red clover resistance to any of the RKN species. Th is study was conducted from 1993 to 1995 in Florida to determine the m ode of inheritance and relative importance of general combining abilit y (GCA) and specific combining ability (SCA) effects in controlling re sistance to Meloidogyne incognita (Kofoid & White) Chitwood Race 3 in red clover. By means of a breeding population selected for low root ga lling and egg mass production when infested with M. incognita, four re d clover plants were identified as resistant, three as intermediate, a nd two additional susceptible individuals were selected from 'Kenstar' . These individuals were cloned and used as parents in a diallel genet ic study. Forty-eight progeny from each of 36 F-1 crosses, without rec iprocals, were evaluated under greenhouse conditions during 1995 for r esponse to inoculation with 1500 M. incognita eggs per plant. Eight we eks after inoculation, the number of galls and egg masses on roots of each plant were counted under low power magnification (10-20 x). Resis tant genotype 119 produced progeny with the least M. incognita reprodu ction and galling. Crosses involving the two susceptible genotypes, N1 and K4, resulted in the highest numbers of galls and egg masses. The GCA effects were significant for both traits, whereas SCA effects were not significant. These results indicate that for this set of parents, breeding procedures that utilize GCA effects to maximize exploitation of additive gene action should be most effective in improving resista nce to M. incognita galling and reproduction.