Some Problems Posed by the Heterodera avenae Complex1

Cyst nematodes are sedentary parasites with limited host ranges and co-evolution with circumscribed host groups is believed to result in species complexes. A scheme for this process is given. The Heterodera avenae group is such a complex parasitising cereals and grasses. Although the group includes ten nominal species, only H. mani, H. avenae and H. avenae pathotype 3 present taxonomic problems of immediate relevance in agriculture. Application of a multivariate technique, principal co-ordinate analysis, to second-stage juvenile characters indicates that H. avercae pathotype 3 is as distinct from H. avenae as H. arenaria, an accepted species in the H. avenae group. In contrast there are difficulties with juvenile and cyst morphology in distinguishing H. mani from H. avenae.