Field tests for evaluating spotted alfalfa aphid resistance in lucerne cultivars: a comparison with glasshouse studies

Abstract

Aphid development, dry matter production and seedling and plant survival data were collected for 22 lucerne cultivars growing at Tamworth, New South Wales. These data were used to investigate whether or not the field performance of these cultivars reflected the range of spotted alfalfa aphid (SAA) resistances determined in published reports of percentage seedling survival in glasshouse screening tests. Numbers of SAA were assessed on seedlings and established plants of each cultivar during heavy, natural field infestations (10.6-23.0 SAA/seedling and 4.2-325.3 SAA/stem). Mean total numbers of SAA/seedling did not differ significantly between cultivars, but compared with the two SAA-susceptible cultivars, Hunter River and 77-6-1 545, lower numbers ( P < 0.01) of large nymphs were found on all SAA resistant cultivars except Matador, Resistador II, DeKalb Brand 167, AS 13R, Amador, Mesilla and Falkiner. However, dry matter yields of all cultivars at first flower were not significantly different, although percentage seedling survivals of the two SAA susceptible cultivars were lower (P< 0.01) than those of the SAA resistant cultivars. For established plants, the SAA susceptible cultivars 77-6-1545 and Hunter River had higher total numbers of SAA and SAA large nymphs (P<0.01) than the SAA resistant cultivars. Within the resistant cultivars, Condura 73 Brand, AS 13R, Amador and Falkiner had higher total numbers of SAA (P<0.01) and Condura 73 Brand, AS 13R, Amador, Lahontan, Lahontan Cycle 4 and Falkiner produced more large nymphs ( P< 0.01) than the other cultivars. Despite these differences in aphid numbers, differences in dry matter yields of established plants of SAA resistant cultivars were generally not significant. However, mean yield of SAA susceptible cultivars was 53% lower than that of resistant cultivars, and mean plant survival of SAA susceptible cultivars was 12% compared with 57.6% for resistant cultivars. With the levels of SAA infestations and the temperatures experienced during our field experiments (17¦C and 28¦C), neither total aphid numbers per seedling nor dry matter yield indicated the range of SAA resistance determined for these cultivars in glasshouse seedling tests. However, rankings of the total number of SAA and SAA large nymphs per stem generally reflected the ranking of SAA resistances found in glasshouse tests and so these may be used to identify SAA resistant and susceptible plants in the field. Overall, glasshouse screening tests were a poor indicator of the yield and survival of seedlings and established plants of the resistant cultivars tested.