Selection for persistence in red clover: Influence of root disease and stem nematode

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Abstract
The cause of death of red clover plants (Trifolium pratense L.), and attributes associated with persistence of survivors, were investigated in two field plot experiments on land previously used by plant breeders for evaluating persistence of different red clover genotypes. In the first experiment, using ''Grasslands Hamua'' red clover, most plants developed foliage symptoms of stem nematode (Ditylenchus dipsaci (Kuhn) Filipjev) infestation and/or developed severe cortical rot or stele necrosis in their tap roots, during the first year. Cylindrocladium scoparium Morgan, Trichocladium basicola (Berk. and Br.) Carmichael, Verticillium dahliae Kleb., Fusarium solani (Mart.) Sacc., and F. oxysporum Schlecht. were isolated from diseased roots. An internal breakdown of the upper tap root occurred following necrosis of pith tissue. The proportion of dead plants increased rapidly during the second year, and only 12.5% of plants had survived 2 years after sowing. Most plants which persisted for 2 years had developed vigorous, deeply penetrating adventitious roots. In the second experiment, plants from a mixed sowing of the Swiss ''Changins'' were more persistent than plants of the New Zealand cultivars ''Grasslands Hamua'', ''Grasslands Turoa'', and ''Grasslands Pawera''. The Swiss cultivars developed significantly less root rot and stele necrosis, and yielded fewer fungal colonies from stele tissue. In a nationwide survey of fungi in red clover tap roots from 24 sites, V. dahliae was found in two seed crops in Blenheim, and T. basicola and F. solani, in pasture at several sites in the northern North Island where root rot was present. Stele tissue of apparently healthy roots from pasture often yielded many fungal isolates, particularly Phoma medicaginis Mal. and Roum., other Phoma spp., Gliocladium roseum Bain., and Acremonium spp.

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