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Progress in recurrent selection and in crossing cultivars with white clover resistant to the clover root‐knot nematode Meloidogyne trifoliophila

Abstract New Zealand pastures are generally deficient in clover content due in part to root‐invading nematodes. A recurrent selection programme for resistance to the clover root‐knot nematode (Meloidogyne trifoliophila) assessed individual white clover (Trifolium repens) genotypes for the number of root galls and root dry weight. Low galling (resistant) and a few high galling (susceptible) genotypes were crossed to provide divergent selections and progeny screened. In the sixth generation, the resistant selections (n = 581) averaged 45% of the susceptible selections (n = 146) for number of galls per gram of root dry weight. The seventh generation equivalent figure improved to 38% (n = 112 resistant and n = 81 susceptible genotypes). In the seventh generation, survival of the resistant group of lines was poorer than other groups. Resistant genotypes were crossed with established cultivars and progeny screened and crossed. The resistant × cultivar progeny averaged 83% of the susceptible × cultivar progeny's galls per gram in the first cross (ns), 58% (P < 0.001) in the second, and 44% (P < 0.001) in the third cross. Progeny of Sustain × resistant crosses had fewer galls per gram than progeny of crosses with other cultivars. Foliage dry weights were assessed every 3 weeks for 6 months from M. trifoliophila‐infected and nematode‐free pairs of resistant and susceptible white clover lines. Yields of all four lines were reduced by the invasion of the M. trifoliophila second‐stage juveniles but the two resistant lines outyielded the susceptible ones. There was little yield from infected susceptible lines from 127 to 191 days after inoculation. More M. trifoliophila eggs were recovered from one resistant line mainly because it survived and grew better than the other lines.