The bird cherry-oat aphid (BCOA, Rhopalosiphum padi ) and the Russian wheat aphid (RWA, Diuraphis noxia ) are major pests of barley and wheat, frequently posing a threat to crop yields. These aphids are managed through chemical pesticides and cultivars bred for resistance. This study evaluated morphological and physiological responses of susceptible and resistant barley (Morex, BCOR001) and wheat (Custer, Dn7) genotypes to individual and combined infestations of BCOA and two biotypes of RWA: RWA1, and RWA2. Plants were assessed weekly for one month to assess morphological responses, and CO 2 assimilation responses were measured using A/Ci curves 14 days after infestation. Aphid infestation significantly reduced several growth parameters from the first week of evaluation, with susceptible genotypes showing more pronounced reductions. In contrast, resistant genotypes—particularly BCOR001—maintained stability in root biomass and aerial dry weight under most aphid treatments. Gas-exchange analyses revealed that non-infested plants had the highest assimilation rates, while aphid-infested plants, especially under attack by aphids feeding on susceptible hosts exhibited notable declines. Interestingly, BCOR001 maintained high assimilation rates under BCOA infestation, suggesting potential tolerance mechanisms. Co-infestations by multiple aphid species did not produce additional damage, suggesting that thresholds based on aphid number are sufficient, regardless of species composition. These results emphasize the importance of evaluating host resistance under co-infestation scenarios and highlight physiological traits that may contribute to enhanced tolerance in resistant genotypes.
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