The significance of biological races of Ditylenchus dipsaci and their hybrids

Pure cultures of six races of Ditylenchus dipsaci derived from a single female, namely, lucerne race (LR), red clover race (RCR), white clover race (WCR), narcissus race (NR), tulip race (TR) and oat race (OR) were inoculated into eight plant species growing in pots. Onion was host to all six races, and tulip to all except RCR, whereas lucerne was susceptible only to LR and slightly to WCR. Hyacinth was not very susceptible to any race. The TR and OR were the most polyphagous. Severe symptoms were not necessarily associated with large nematode populations. Parthenogenesis did not occur. Ten fertile hybrids between races were produced and the host range of five of these was tested. On average the hybrids multiplied less than their parent races and their host ranges showed no relationship to those of their parent races. RCR and TR inoculated together into tulip produced significantly fewer nematodes than did TR alone and more than RCR alone. Mixed populations of races occur in nature which are probably a mixture of parental races, their hybrids and back-crosses. Repeated back-crossing of the hybrids with parental types and the slower multiplication of the hybrids is an explanation for the variation in host range of known races and the failure to record new races.