Comparison of lucernes when grown on a heavy clay soil infected by Phytophthora, and their response to attack by the spotted alfalfa aphid (Therioaphis trifolii f. maculata)

Abstract

Eighteen lucerne cultivars were sown at Deniliquin, New South Wales, in 1976. The aim of the trial was to assess these cultivars for their performance on an irrigated heavy clay soil infected by P. megasperma. The unforeseen arrival of the spotted alfalfa aphid (SAA) in December 1977 complicated the original assessment, but provided an opportunity of assessing the combined effects of SAA and P. megasperma. Twelve of the cultivars were introduced from the USA, and six were Australian lines. The American cultivars were AS1 3, AS1 3R, AS49, AS49R, WL451, WL501 R, WL508, WL512, DeKalb 167, DeKalb 185, Resistador II and Lahontan. None of these was damaged by SAA, and all except WL508 appeared to be well adapted to the heavy clay soil. The Australian cultivars Hunter River, Paravivo and Siro Peruvian were all badly affected by the aphid, and in Siro Peruvian the loss in production was accentuated by Phytophthora root rot. The Australian cultivar Falkiner and its parent line, Lahontan Cycle 4, carried large populations of aphids, but the effect on their production was only slight and temporary. Another Australian line, C3 Composite, yielded well until the aphid infestation, when its yield was temporarily reduced, but recovered after the aphids disappeared. Neither Falkiner nor C3 Composite was affected by root rot.