Resistance of lucerne lines at different stages of plant growth to spotted alfalfa aphid and blue-green aphid

Abstract

The relative resistances of lucerne (Medicago sativa) lines to the spotted alfalfa aphid (SAA) ( Therioaphis trifolii (Monell) f. maculata) and the blue-green aphid (BGA) (Acyrthosiphon kondoi Shinji) are generally assessed as seedlings and not as mature plants. To determine if the orders of resistances dre maintained, two separate glasshouse experiments assessed the relative resistances of 11 and 12 lucerne lines after infestation by SAA and BGA respectively at these stages: seedling, regrowth after cutting, and mature growth. Relative resistance between growth stages was compared by measuring aphid populations, percentage plant mortality and percentage yield reduction of surviving plants, and correlating the ranking for those indices. There was no change in the order of resistance to SAA between growth stages. Plants of both mature-age growth stages were, however, more tolerant of SAA than were seedlings. The technique employed in the experiment did not discriminate for resistance to BGA, though the yield of all lines was reduced by BGA. The implications of these findings are briefly discussed.