Infestations of Russian wheat aphid (Diuraphis noxia) cause significant damage to modern barley cultivars. Several sources of high-level resistance have been identified in the USDA-ARS National Small Grains Collection held at Aberdeen, Idaho, USA. Selections were made within the accession PI 355450 and released as STARS-9301B (Mornhinweg et al., 1995, Crop Sci. 35:603). The agronomic performance of this line, and in lines presumably containing its two resistance genes (Mornhinweg et al., 1995, Crop Sci. 35:1368-1371), remains excellent even under severe infestation. The objectives of this study was to determine the utility of these genes in protecting malting quality characteristics, and to examine the importance of the timing of infestation (early versus late in crop development.

We embarked on a cooperative breeding program using this accession (PI355450/STARS-9301B) in 1991. Single plant selections for resistant BC₁F₂ seedlings were made and advanced through head and head-row selections, with further RWA-infestations made in the greenhouse to identify homozygous resistant lines. Further selections were made among progeny of BC₁ F ₄-derived lines in several agronomically-elite cultivars, including backgrounds with good malting quality. In 1998, several of these lines were grown with their parents in Fort Collins, Colorado, and in Aberdeen, Idaho, in replicated trials. In each replicate, each cultivar or line received one of three treatments: early infestation (at approximately the 5 leaf stage), late infestation (at jointing), or no infestation. Insecticides were used as necessary to control infestations in uninfested and late-infested plots.

The Aberdeen location had a relatively light infestation of RWAs, whereas the level of RWA infestation at Fort Collins was much more severe, These data show that RWA infestation of the susceptible parents caused significant losses in yield and percentage plump kernels, and significant and undesirable changes in important determinants of malting quality such as malt extract, barley protein, and -glucan (Tables 1-6). Early infestation caused severe damage, particularly in Fort Collins. Late infestation did not cause significant damage at Aberdeen; at Fort Collins damage was significant but less than that incurred by the early-infested plots.

In contrast, STARS-9301B and the backcross-derived lines were relatively undamaged, even under the severe infestation conditions experienced at Fort Collins. The level of resistance in several of the backcross-derived lines was at least as good as that of STARS-9301B, although several experience mild to severe damage under early infestation at Fort Collins. All of the backcross-derived lines have been rated as highly-resistant based on greenhouse testing, with the exception of the moderately-resistant 95RWA96. Moderate resistance did not provide significant protection of agronomic and malting characteristics; 95RWA96 suffered significant damage even at Aberdeen where aphid infestations were relatively light.

In conclusion, that data show that the resistance alleles in STARS-9301B can confer excellent resistance to the RWA, and that resistance as measured by agronomic performance is correlated with resistance as measured by malting quality. The data suggest that moderate levels of resistance do not confer good resistance under heavy aphid pressure.

Table 2. Malting Quality of Uninfested and Infested Garnet-derived Lines at Aberdeen, 1998