Resistance of Cereals to Heterodera avenae: Methods of Investigation, Sources and Inheritance of Resistance1

Abstract

Techniques used in cereal breeding programmes for selecting for resistance to cereal cyst nematodes are described. Routine screening is done in field nurseries and in pots of infested soil in glasshouse and controlled environments. Other investigations of resistance use a laboratory technique which permits identification of the penetration sites of individual nematodes. Erosion of resistance, as indicated by 1) the development of only one or two females or 2) the occasional development of more females on plants with resistance genes, is discussed in relation to variation in nematode virulence and to environmental, especially temperature effects. Identification of tolerance is more difficult unless the differences are great, or the experimentation carefully controlled and replicated. Direct selection for tolerance in segregating generations is not presently practicable. Indirectly selecting for mechanisms of tolerance may eventually prove possible but has not yet been exploited. Available sources of resistance are described in relation to genetic studies and complementary studies of nematode pathotype and host range. There are three distinct resistances in barley which may have been responsible for selecting present pathotypes. Two, Rha1 and Rha2, are useful in breeding and linkage and allelism at these loci are described. In oats, as in wheat, only one locus has been used by breeders. There are probably additional loci in some of the other resistance sources but polyploidy of the host and different virulence gene frequencies in unselected nematode populations make it more difficult to identify genetic relationships in oats and wheat.

Variation in number of cysts formed on susceptible potato cultivars with various populations of Globodera rostochiensis and G. pallida

“…Stelter (1983) and Dale & Phillips (1985) reported extensively on differences in multiplication of PCN between susceptible potato cultivars . Similar differences in the multiplication of Heterodera avenae were found between oat cultivars (Cook & York, 1982) and wheat cultivars (Fisher, 1982), but not in the multiplication of Heterodera schachtii between sugarbeet cultivars (Heijbroek, 1977) . Partial resistance in S. tuberosum ssp .…”

The Cyst Nematodes

“…In routine selection for wellcharacterized, effective resistance, it is often safe to rely on the counts of new white females on the root ball of pot-grown plants (Cook and York, 1982a). Frequently, the resistance threshold has been less than 5% as many females as the control.…”

Forward genetic analysis of plant-parasitic nematode-host interactions.

“…Extensive variations in pathogenicity have been reported within these pathotypes, however, and it is clear that there is more genetic variation within this nematode species than is represented by the current schemes. The use of an alternative host plant, such as oat or wheat, further complicates the picture (Cook and York, 1982). H. avenae populations from Europe and Australia exhibit extensive variation in their relative abilities to parasitize resistant cultivars, indicating the presence of multiple genes conferring parasitic ability on a given host genotype (Triantaphyllou, 1987).…”