Russian wheat aphid [RWA; Diuraphis noxia (Mordvilko)] is a worldwide insect pest of barley (Hordeum vulgare L.), causing crop losses each year. Previously identified resistant barley lines do not show variable reactions to the eight United States RWA biotypes identified by wheat reactions. However, additional RWA isolates have been identified outside the United States. The objective of this research was to determine whether additional RWA biotypes that impact resistance in barley exist and whether resistance to these biotypes is present in the resistant barley germplasm and cultivars that have been developed by the USDA‐ARS. A total of 166 barley lines were rated for chlorosis and leaf rolling in response to infestation with RWA United States’ biotype 1 and to individual isolates from Hungary, Chile, Mexico, and Iran. Each barley line was genotyped for 5912 single nucleotide polymorphism (SNP) markers, and the combined data were used for association mapping of loci associated with resistance to each RWA isolate. A total of 40 loci were associated with resistance to the five RWA isolates. Biotype differences were evident, with only 23 lines showing resistance to all five biotypes. Loci with large effects on mean chlorosis and rolling differences were found for resistance to biotypes from the United States, Hungary, and Chile, while the loci for resistance to biotypes from Mexico and Iran had small effects on these traits, which may pose problems for developing resistant germplasm. The 23 lines resistant to all biotypes included 10 unadapted germplasm accessions and 13 improved germplasm lines and provide useful germplasm for developing new barley cultivars with resistance to multiple RWA biotypes.
Abstract. Recent host records for Gephyraulus raphanistri (Kieffer), a flower-gall midge, show restriction to Raphanus raphanistrum throughout Europe. Gephyraulus raphanistri has never been reported infesting commercially grown Brassica crops. Historical records showing a broad host range appear to have resulted from confusion with new or as yet undescribed Gephyraulus spp. and Contarinia nasturtii (Kieffer), a known gall-former of Brassica and other related genera. This study tested host specificity of G. raphanistri in the field in Europe by manipulating host plant phenology of actual and potential hosts in the genera Raphanus and Brassica as part of a risk assessment of the insect as a potential biological control agent of R. raphanistrum, one of the most important weeds of crops in Australia. Raphanus raphanistrum raphanistrum (wild radish), R. raphanistrum landra (coastal wild radish), Raphanus sativus (radish) and Brassica napus (oilseed rape cultivar) were phenologically synchronised for initial flowering and planted out in a flowering time and species block design near a natural population of R. r. landra hosting a natural population of G. raphanistri. Three generation peaks in gall formation were observed in the experiment, with galls developing on all test plants with an apparent preference for R. r. landra. The high field specificity of this gall midge is driven by the synchrony of oviposition and flower availability, not host physiological incompatibility or behavioural unacceptability. Commercially grown Brassica spp. are not suitable hosts for G. raphanistri because in the field they differ in flowering phenology from Raphanus raphanistrum. The overlap in the flowering phenology of the crop and weed in Australia makes this insect unsuitable as a biological control agent.
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