The relationship of insects, fungi and nematodes to the deterioration of roots of Trifolium hybridum L.*

Abstract

SUMMARYTo evaluate the relative importance and possible interrelationships of insect, fungus, and nematode injury to the deterioration of roots of alsike clover plants, Trifolium hybridum L., heptachlor, chloropicrin, and ethylene dibromide were applied singly and in combinations to the soil prior to seeding. The most effective treatment was an insecticide plus a soil fumigant (heptachlor plus chloropicrin or ethylene dibromide), which improved the uniformity of stand, plant height and flowering, and reduced the density of weeds. In the second year this treatment greatly increased plant survival and yields of forage. All the plants died in the untreated control plots during the third year of the experiment.There was a significant relationship between the occurrence of vascular decay and the incidence of injury by the clover root curculio, Sitona hispidula (Fabr.). There was a less marked relationship between insect injury and cortical decay; but no apparent relationship between insect injury and rootlet rot. Although root lesion nematode, Pratylenchus sp., was present, no direct relationship between nematode injury and root rots was established.The fungi most frequently isolated from alsike clover roots were Fusarium oxysporum Schl. sensu Snyd. and Han., a species of Pyrenochaeta and an unidentified fungus. Verticillium dahliae Kleb. was also present. Tests with F. oxysporum, the Pyrenochaeta sp., and V. dahliae confirmed their pathogenicity on alsike clover roots. In laboratory tests, plants with severe rootlet rot rapidly regenerated feeder rootlets in the presence of adequate soil moisture.