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The clover cyst nematode (Heterodera trifolii) infects white clover (Trifolium repens) in pasture throughout New Zealand, reducing plant growth and nutrition. The preferred control method for both economic and ecological reasons is to breed resistant cultivars, with resistance measured as a reduction in cysts/g of root dry weight. Many New Zealand and overseas white clover seed lines and cultivars were screened for resistance. There were highly significant differences between lines for number of cysts, root weight, and cysts/g of root dry weight. Selected genotypes were rescreened using five stolon tips per genotype, and the results were correlated with the first screening. There were high correlations between the number of cysts/plant and cysts/g of root dry weight for all screenings. Two Italian Ladino cultivars were generally among the most resistant lines, as was a line bred for H. trifolii resistance in the Netherlands. There were no immune plants, but the results and the calculated broad sense heritabilities indicate that breeding for resistance is possible.

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Stem nematode (Ditylenchus dipsaci)infection of white clover variety G49 was compared to that of other white clover lines in two experiments. The G49 parent lines were also screened for infection in a third experiment. Symptoms of stem nematode infection were assessed at 8, 14, and 21 days after inoculation. The lower infection rate of G49 by D. dipsaci was not sufficient, as a stand-alone trait, to distinguish G49 from other existing lines. However, G49 had a significantly lower infection rate than Pitau so these can be distinguished for the purposes of a Plant Variety Right application. The variation among the parent lines of G49 indicated scope for further selection to improve resistance. Three techniques for applying inoculum to seedlings were compared; the best one uses 1 ml of nematode suspension placed over a 3-day-old seedling lying on the surface of peat-based potting mix which is then lightly covered.